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FEATURES 12-Bit Monolithic A/D Converter 66 kHz Throughput Rate 12 s Conversion Time 3 s On-Chip Track/Hold Amplifier Low Power Power Save Mode: 2 mW typ Normal Operation: 25 mW typ 70 dB SNR Fast Data Access Time: 57 ns Small 24-Lead SOIC and 0.3" DIP Packages APPLICATIONS Battery Powered Portable Systems Digital Signal Processing Speech Recognition and Synthesis High Speed Modems Control and Instrumentation
LC2MOS Single +5 V Supply, Low Power, 12-Bit Sampling ADC AD7880
FUNCTIONAL BLOCK DIAGRAM
VDD R VINA VINB VREF AGND CS CLKIN CONVST RD BUSY THREE STATE BUFFERS DB11 DB0 CONTROL LOGIC R SAMPLING COMPARATOR + -
LOW POWER CONTROL CIRCUIT
MODE
12-BIT DAC
SAR + COUNTER
AD7880
DGND
GENERAL DESCRIPTION
PRODUCT HIGHLIGHTS
The AD7880 is a high speed, low power, 12-bit A/D converter which operates from a single +5 V supply. It consists of a 3 s track/hold amplifier, a 12 s successive-approximation ADC, versatile interface logic and a multiple-input-range circuit. The part also includes a power save feature. An internal resistor network allows the part to accept both unipolar and bipolar input signals while operating from a single +5 V supply. Fast bus access times and standard control inputs ensure easy interfacing to modern microprocessors and digital signal processors. The AD7880 features a total throughput time of 15 s and can convert full power signals up to 33 kHz with a sampling frequency of 66 kHz. In addition to the traditional dc accuracy specifications such as linearity, full-scale and offset errors, the AD7880 is also fully specified for dynamic performance parameters including harmonic distortion and signal-to-noise ratio. The AD7880 is fabricated in Analog Devices' Linear Compatible CMOS (LC2MOS) process, a mixed technology process that combines precision bipolar circuits with low power CMOS logic. The part is available in a 24-pin, 0.3 inch-wide, plastic or hermetic dual-in-line package (DIP) as well as a small 24-lead SOIC package.
1. Fast Conversion Time. 12 s conversion time and 3 s acquisition time allow for large input signal bandwidth. This performance is ideally suited for applications in areas such as telecommunications, audio, sonar and radar signal processing. 2. Low Power Consumption. 2 mW power consumption in the power-down mode makes the part ideally suited for portable, hand held, battery powered applications. 3. Multiple Input Ranges. The part features three user-determined input ranges, 0 V to +5 V, 0 V to 10 V and 5 V. These unipolar and bipolar ranges are achieved with a 5 V only power supply.
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Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
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, AGND = DGND O = AD7880-SPECIFICATIONS (V = +5 V 5%, V =AllVSpecifications T =to TV, f unless2.5 MHz, MODE = V unless otherwise noted. otherwise noted.)
DD REF DD CLKIN MIN MAX
DD
Parameter DYNAMIC PERFORMANCE Signal-to-Noise Ratio3 (SNR) Total Harmonic Distortion (THD) Peak Harmonic or Spurious Noise Intermodulation Distortion (IMD) Second Order Terms Third Order Terms DC ACCURACY Resolution Integral Nonlinearity Differential Nonlinearity Full-Scale Error Bipolar Zero Error Unipolar Offset Error ANALOG INPUT Input Voltage Ranges
2
B Versions1 70 -80 -80 -80 -80 12 1 1 15 10 5 0 to VREF 0 to 2 VREF VREF 10 5/12 5/12 5 1.5 2.5/VDD
C Versions1 70 -80 -80 -80 -80 12 1 1 5 5 5 0 to VREF 0 to 2 VREF VREF 10 5/12 5/12 5 1.5 2.5/VDD
Units dB min dB typ dB typ dB typ dB typ Bits LSB max LSB max LSB max LSB max LSB max Volts Volts Volts M min k min/max k min/max V mA max V min/max
Test Conditions/Comments Typically SNR Is 72 dB VIN = 1 kHz Sine Wave, fSAMPLE = 66 kHz VIN = 1 kHz Sine Wave, fSAMPLE = 66 kHz VIN = 1 kHz, fSAMPLE = 66 kHz fa = 0.983 kHz, fb = 1.05 kHz, fSAMPLE = 66 kHz fa = 0.983 kHz, fb = 1.05 kHz, fSAMPLE = 66 kHz All DC ACCURACY Specifications Apply for the Three Analog Input Ranges Guaranteed Monotonic
Input Resistance
See Figure 5 See Figure 6 See Figure 7 0 to VREF Range 8 k typical: 0 to 2 VREF Range 8 k typical: VREF Range 5%: Normally VREF = VDD (See Reference Input Section) See Figure 3 for Degradation in Performance Down to 2.5 V
REFERENCE INPUT VREF (For Specified Performance) IREF Nominal Reference Range LOGIC INPUTS CONVST, RD, CS, CLKIN Input High Voltage, VINH Input Low Voltage, VINL Input Current, IIN Input Capacitance, CIN4 MODE INPUT Input High Voltage, VINH Input Low Voltage, VINL Input Current, IIN Input Capacitance, CIN4 LOGIC OUTPUTS DB11-DB0, BUSY Output High Voltage, VOH Output Low Voltage, VOL DB11-DB0 Floating-State Leakage Current Floating-State Output Capacitance4 CONVERSION Conversion Time Track/Hold Acquisition Time POWER REQUIREMENTS VDD IDD Normal Power Mode @ +25C TMIN to TMAX Power Save Mode @ +25C TMIN to TMAX Power Dissipation Normal Power Mode @ +25C TMIN to TMAX Power Save Mode @ +25C TMIN to TMAX
2.4 0.8 10 10 4 1 125 10
2.4 0.8 10 10 4 1 125 10
V min V max A max pF max V min V max A max pF max
VIN = 0 V or VDD
VIN = 0 V or VDD
4.0 0.4 10 10 12 3 +5 7.5 10 750 1 37.5 50 3.75 5
4.0 0.4 10 10 12 3 +5 7.5 10 750 1 37.5 50 3.75 5
V min V max A max pF max s max s max V nom mA max mA max A max mA max mW max mW max mW max mW max
ISOURCE = 400 A ISINK = 1.6 mA
fCLKIN = 2.5 MHz
5% for Specified Performance Typically 4 mA; MODE = VDD Typically 5 mA; MODE = VDD Logic Inputs @ 0 V or VDD; MODE = 0 V Logic Inputs @ 0 V or VDD; MODE = 0 V VDD = 5 V: Typically 20 mW; MODE = VDD VDD = 5 V: Typically 25 mW; MODE = VDD VDD = 5 V: Typically 2 mW; MODE = 0 V VDD = 5 V: Typically 2.5 mW; MODE = 0 V
NOTES 1 Temperature ranges are as follows: B/C Versions, -40C to +85C. 2 VIN = 0 to VREF 3 SNR calculation includes distortion and noise components. 4 Sample tested @ +25C to ensure compliance. Specifications subject to change without notice.
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AD7880 TIMING CHARACTERISTICS1 (V
Parameter t1 t2 t3 t4 t5 t6 t7 2 t8 3 Limit at +25 C (All Versions) 50 130 0 0 0 60 57 5 50
DD
= +5 V
5%, VREF = VDD, AGND = DGND = 0 V)
Units ns min ns min ns min ns min ns min ns min ns max ns min ns max Conditions/Comments CONVST Pulse Width CONVST to BUSY Falling Edge BUSY to CS Setup Time CS to RD Setup Time CS to RD Hold Time RD Pulse Width Data Access Time after RD Bus Relinquish Time after RD
Limit at TMIN, TMAX (All Versions) 50 130 0 0 0 75 70 5 50
NOTES 1 Timing specifications in bold print are 100% production tested. All other times are sample tested at +25 C to ensure compliance. All input signals are specified with tr = tf = 5 ns (10% to 90% of 5 V) and timed from a voltage level of 1.6 V. 2 t7 is measured with the load circuit of Figure 2 and defined as the time required for an output to cross 0.8 V or 2.4 V. 3 t8 is derived from the measured time taken by the data outputs to change by 0.5 V when loaded with the circuit of Figure 2. The measured number is then extrapolated back to remove the effects of charging the 50 pF capacitor. This means that the time, t 8, quoted in the timing characteristics is the true bus relinquish time of the part and as such is independent of external bus loading capacitances.
t1
CONVST TRACK/HOLD GOES INTO HOLD
Table I. Truth Table
t2 t CONVERT
BUSY
CS 1 1 0 0
t4 t6 t5
CONVST 1 j 1 1
RD X 1 0 1
Function Not Selected Start Conversion g Enable ADC Data Data Bus Three Stated
t3
CS
ABSOLUTE MAXIMUM RATINGS*
t8
RD
t7
THREE-STATE DB0 - DB11
DATA VALID
Figure 1. Timing Diagram
1.6mA
TO OUTPUT PIN 50pF 200A
+ 2.1V
VDD to AGND . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 V to +7 V VDD to DGND . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 V to +7 V AGND to DGND . . . . . . . . . . . . . . . . . -0.3 V to VDD + 0.3 V VINA, VINB to AGND (Figure 5) . . . . . . -0.3 V to VDD + 0.3 V VINA to AGND (Figure 6) . . . . . . . . . -0.6 V to 2 VDD + 0.6 V VINA to AGND (Figure 7) . . . . . -VDD - 0.3 V to VDD + 0.3 V VREF to AGND . . . . . . . . . . . . . . . . . . . . . . . . . . 0.3 V to VDD Digital Inputs to DGND . . . . . . . . . . . -0.3 V to VDD + 0.3 V Digital Outputs to DGND . . . . . . . . . . -0.3 V to VDD + 0.3 V Operating Temperature Range Industrial (B, C Versions) . . . . . . . . . . . . . -40C to +85C Storage Temperature Range . . . . . . . . . . . -65C to +150C Lead Temperature (Soldering, 10 secs) . . . . . . . . . . . . +300C Power Dissipation (Any Package) to +75C . . . . . . . . 450 mW Derates above +75C by . . . . . . . . . . . . . . . . . . . . . 10 mW/C
*Stresses above those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
Figure 2. Load Circuit for Access and Relinquish Time
CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the AD7880 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
WARNING!
ESD SENSITIVE DEVICE
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AD7880
ORDERING GUIDE Bipolar Zero Error Package (LSBs) Option* 10 10 5 5 10 5 N-24 Q-24 N-24 Q-24 R-24 R-24 PIN CONFIGURATION
VINA 1 VINB 2 AGND 3 VREF 4 CS 5 CONVST 6 RD 7 BUSY 8 CLKIN 9 DGND 10 DB0 11 DB1 12 24 VDD 23 MODE 22 DB11 21 DB10
Model AD7880BN AD7880BQ AD7880CN AD7880CQ AD7880BR AD7880CR
Temperature Range -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C
Full-Scale Error (LSBs) 15 15 5 5 15 5
AD7880
TOP VIEW (Not to Scale)
20 DB9 19 DB8 18 DB7 17 DB6 16 DB5 15 DB4 14 DB3 13 DB2
*N = Plastic DIP; Q = Cerdip; R = SOIC (Small Outline Integrated Circuit).
PIN FUNCTION DESCRIPTION
Pin No. 1 2 3 4 5 6 7 8 9 10 11 . . . 22 23 24
Pin Mnemonic Function Analog Input. Analog Input. Analog Ground. Voltage Reference Input. This is normally tied to VDD. Chip Select. Active Low Logic input. The device is selected when this input is active. Convert Start. A low to high transition on this input puts the track/hold into hold mode and starts conversion. This input is asynchronous to the CLKIN and is independent of CS and RD. RD Read. Active Low Logic Input. This input is used in conjunction with CS low to enable data outputs. BUSY Active Low Logic Output. This status line indicates converter status. BUSY is low during conversion. CLKIN Clock Input. TTL-compatible logic input. Used as the clock source for the A/D converter. The mark/ space ratio of the clock can vary from 40/60 to 60/40. DGND Digital Ground. DB0-DB11 Three-State Data Outputs. These become active when CS and RD are brought low. MODE MODE Input. This input is used to put the device into the power save mode (MODE = 0 V). During normal operation, the MODE input will be a logic high (MODE = VDD). VDD Power Supply. This is nominally +5 V. VINA VINB AGND VREF CS CONVST
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AD7880
CIRCUIT INFORMATION
The AD7880 is a +5 V single supply 12-bit A/D converter. The part requires no external components apart from a 2.5 MHz external clock and power supply decoupling capacitors. It contains a 12-bit successive approximation ADC based on a fast-settling voltage-output DAC, a high speed comparator and SAR, as well as the necessary control logic. The charge balancing comparator used in the AD7880 provides the user with an inherent trackand-hold function. The ADC is specified to work with sampling rates up to 66 kHz.
CONVERTER DETAILS
R VINA + VINB R -
VDAC
Figure 4. AD7880 Input Circuit
The AD7880 conversion cycle is initiated on the rising edge of the CONVST pulse, as shown in the timing diagram of Figure 1. The rising edge of the CONVST pulse places the track/hold amplifier into "HOLD" mode. The conversion cycle then takes between 26 and 28 clock periods. The maximum specified conversion time is 12 s. This corresponds to a conversion cycle time of 28 clock periods with a CLKIN frequency of 2.5 MHz and also includes internal propagation delays. During conversion the BUSY output will remain low, and the output databus drivers will be three-stated. When a conversion is completed, the BUSY output will go to a high level, and the result of the conversion can be read by bringing CS and RD low. The track/hold amplifier acquires a 12-bit input signal in 3 s. The overall throughput time for the AD7880 is equal to the conversion time plus the track/hold acquisition time. For a 2.5 MHz input clock the throughput time is 15 s.
REFERENCE INPUT
The AD7880 accommodates three separate input ranges, 0 to VREF, 0 to 2 VREF and VREF. The input configurations corresponding to these ranges are shown in Figures 5, 6 and 7. With VREF = VDD and using a nominal VDD of +5 V, the input ranges are 0 V to 5 V, 0 V to 10 V and +5 V, as shown in Table II.
Table II. Analog Input Ranges
Analog Input Range 0 V to +5 V 0 V to +10 V 5 V
VREF VDD VDD VDD
Input Connections VINA VINB VIN VIN VIN VIN AGND VREF
Connection Diagram Figure 5 Figure 6 Figure 7
R VIN = 0 TO VREF V INA VINB VREF VREF AGND R
SAMPLING COMPARATOR 0 TO VREF + -
For specified performance, it is recommended that the reference input be tied to VDD. The part, however, will operate with a reference down to 2.5 V though with reduced performance specifications. Figure 3 shows a graph of signal-to-noise ratio (SNR) versus VREF. VREF must not be allowed to go above VDD by more than 100 mV.
74 FS = 51.2kHz F = 2.525kHz IN T = 25 C A 70
SNR - dBs
12-BIT DAC
Figure 5. 0 to VREF Unipolar Input Configuration
R VIN = 0 TO 2VREF VINA VINB R SAMPLING COMPARATOR 0 TO VREF + -
72
68
VREF
VREF AGND
12-BIT DAC
66
64
62
Figure 6. 0 to 2 VREF Unipolar Input Configuration
R VINA VINB VREF AGND R SAMPLING COMPARATOR 0 TO VREF + -
60
2
3 V REF - Volts
4
5
VIN = VREF
Figure 3. SNR vs. VREF
ANALOG INPUT
VREF
12-BIT DAC
The AD7880 has two analog input pins, VINA and VINB. Figure 4 shows the input circuitry to the ADC sampling comparator. The on-board attenuator network, made up of equal resistors, allows for various input ranges. REV. 0 -5-
Figure 7. VREF Bipolar Input Configuration
AD7880
The AD7880 has two unipolar input ranges, 0 V to 5 V and 0 V to 10 V. Figure 5 shows the analog input for the 0 V to 5 V range. The designed code transitions occur midway between successive integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, 5/2 LSBs . . . FS -3/2 LSBs). The output code is straight binary with 1 LSB = FS/4096 = 5 V/4096 = 1.22 mV. The same applies for the 0 V to 10 V range, as shown in Figure 6, except that the LSB size is bigger. In this case 1 LSB = FS/4096 = 10 V/4096 = 2.44 mV. The ideal input/output transfer characteristic for both these unipolar ranges is shown in Figure 8.
OUTPUT CODE
CLOCK INPUT
The AD7880 is specified to operate with a 2.5 MHz clock connected to the CLKIN input pin. This pin may be driven directly by CMOS or TTL buffers. The mark/space ratio on the clock can vary from 40/60 to 60/40. As the clock frequency is slowed down, it can result in slightly degraded accuracy performance. This is due to leakage effects on the hold capacitor in the internal track-and-hold amplifier. Figure 10 is a typical plot of accuracy versus clock frequency for the ADC.
2.5
NORMALIZED LINEARITY ERROR
1LSB = FS 4096
111...111 111...110 111...101 111...100
2.0
1.5
1.0
0.5
000...011 000...010 000...001 000...000 0V 1LSB VIN INPUT VOLTAGE
0.0 0.5
1.5
2.5
3.5
CLOCK FREQUENCY - MHz
+ FS - 1LSB
Figure 10. Normalized Linearity Error vs. Clock Frequency
TRACK/HOLD AMPLIFIER
Figure 8. AD7880 Unipolar Transfer Characteristic
Figure 7 shows the AD7880's 5 V bipolar analog input configuration. Once again the designed code transitions occur midway between successive integer LSB values. The output code is straight binary with 1 LSB = FS/4096 = 10 V/4096 = 2.44 mV. The ideal bipolar input/output transfer characteristic is shown in Figure 9.
OUTPUT CODE
The charge balanced comparator used in the AD7880 for the A/D conversion provides the user with an inherent track/hold function. The track/hold amplifier acquires an input signal to 12-bit accuracy in less than 3 s. The overall throughput time is equal to the conversion time plus the track/hold amplifier acquisition time. For a 2.5 MHz input clock, the throughput time is 15 s. The operation of the track/hold amplifier is essentially transparent to the user. The track/hold amplifier goes from its tracking mode to its hold mode at the start of conversion, i.e., on the rising edge of CONVST as shown in Figure 1.
OFFSET AND FULL-SCALE ADJUSTMENT
111...111 111...110
100...101 100...000 011...111
- FS 2
-1LSB +1LSB + FS - 1LSB 2
011...110 000...001 000...000
FS = 10V 1LSB = FS 4096
In most Digital Signal Processing (DSP) applications, offset and full-scale errors have little or no effect on system performance. Offset error can always be eliminated in the analog domain by ac coupling. Full-scale error effect is linear and does not cause problems as long as the input signal is within the full dynamic range of the ADC. Some applications will require that the input signal range match the maximum possible dynamic range of the ADC. In such applications, offset and full-scale error will have to be adjusted to zero. The following sections describe suggested offset and full-scale adjustment techniques which rely on adjusting the inherent offset of the op amp driving the input to the ADC as well as tweaking an additional external potentiometer as shown in Figure 11.
0V VIN INPUT VOLTAGE
Figure 9. AD7880 Bipolar Transfer Characteristic
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AD7880
R1 10 k V1 R2 500
Signal-to-Noise Ratio (SNR)
+ - R4 10 k VINA
R3 10 k R5 10 k
AD7880*
AGND
SNR is the measured signal-to-noise ratio at the output of the ADC. The signal is the rms magnitude of the fundamental. Noise is the rms sum of all the nonfundamental signals up to half the sampling frequency (FS/2) excluding dc. SNR is dependent upon the number of quantization levels used in the digitization process; the more levels, the smaller the quantization noise. The theoretical signal to noise ratio for a sine wave input is given by: SNR = (6.02 N + 1.76) dB where N is the number of bits. Thus for an ideal 12-bit converter, SNR = 74 dB. (1)
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 11. Offset and Full-Scale Adjust Circuit
Unipolar Adjustments
In the case of the 0 V to 5 V unipolar input configuration, unipolar offset error must be adjusted before full-scale error. Adjustment is achieved by trimming the offset of the op amp driving the analog input of the AD7880. This is done by applying an input voltage of 0.61 mV (1/2 LSB) to V1 in Figure 11 and adjusting the op amp offset voltage until the ADC output code flickers between 0000 0000 0000 and 0000 0000 0001. For full-scale adjustment, an input voltage of 4.9982 V (FS-3/2 LSBs) is applied to V1 and R2 is adjusted until the output code flickers between 1111 1111 1110 and 1111 1111 1111. The same procedure is required for the 0 V to 10 V input configuration of Figure 6. An input voltage of 1.22 mV (1/2 LSB) is applied to V1 in Figure 11 and the op amp's offset voltage is adjusted until the ADC output code flickers between 0000 0000 0000 and 0000 0000 0001. For full-scale adjustment, an input voltage of 9.9963 V (FS-3/2 LSBs) is applied to V1 and R2 is adjusted until the output code flickers between 1111 1111 1110 and 1111 1111 1111.
Bipolar Adjustments
The output spectrum from the ADC is evaluated by applying a sine wave signal of very low distortion to the VIN input which is sampled at a 66 kHz sampling rate. A Fast Fourier Transform (FFT) plot is generated from which the SNR data can be obtained. Figure 12 shows a typical 2048 point FFT plot of the AD7880 with an input signal of 2.5 kHz and a sampling frequency of 61 kHz. The SNR obtained from this graph is 73 dB. It should be noted that the harmonics are taken into account when calculating the SNR.
Bipolar zero and full-scale errors for the bipolar input configuration of Figure 7 are adjusted in a similar fashion to the unipolar case. Again, bipolar zero error must be adjusted before full-scale error. Bipolar zero error adjustment is achieved by trimming the offset of the op amp driving the analog input of the AD7880 while the input voltage is 1/2 LSB below ground. This is done by applying an input voltage of -1.22 mV (1/2 LSB) to V1 in Figure 11 and adjusting the op amp offset voltage until the ADC output code flickers between 0111 1111 1111 and 1000 0000 0000. For full-scale adjustment, an input voltage of 4.9982 V (FS/2-3/2 LSBs) is applied to V1 and R2 is adjusted until the output code flickers between 1111 1111 1110 and 1111 1111 1111.
DYNAMIC SPECIFICATIONS
Figure 12. FFT Plot
Effective Number of Bits
The formula given in Equation 1 relates the SNR to the number of bits. Rewriting the formula, as in Equation 2, it is possible to get a measure of performance expressed in effective number of bits (N). N= SNR - 1.76 6.02 (2)
The AD7880 is specified and tested for dynamic performance specifications as well as traditional dc specifications such as integral and differential nonlinearity. The ac specifications are required for signal processing applications such as speech recognition, spectrum analysis and high speed modems. These applications require information on the ADC's effect on the spectral content of the input signal. Hence, the parameters for which the AD7880 is specified include SNR, harmonic distortion, intermodulation distortion and peak harmonics. These terms are discussed in more detail in the following sections.
The effective number of bits for a device can be calculated directly from its measured SNR. Figure 13 shows a plot of effective number of bits versus input frequency for an AD7880 with a sampling frequency of 61 kHz. The effective number of bits typically remains better than 11.5 for frequencies up to 12 kHz.
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AD7880
12
11.5
11
10.5 SAMPLE FREQUENCY = 61kHz TA = 25 C
Using the CCIF standard where two input frequencies near the top end of the input bandwidth are used, the second and third order terms are of different significance. The second order terms are usually distanced in frequency from the original sine waves, while the third order terms are usually at a frequency close to the input frequencies. As a result, the second and third order terms are specified separately. The calculation of the intermodulation distortion is as per the THD specification where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the fundamental expressed in dBs. In this case, the input consists of two, equal amplitude, low distortion, sine waves. Figure 14 shows a typical IMD plot for the AD7880.
EFFECTIVE NUMBER OF BITS
10
15 INPUT FREQUENCY - kHz
30.5
Figure 13. Effective Number of Bits vs. Frequency
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the rms value of the fundamental. For the AD7880, THD is defined as: THD = 20 log V 2 + V 3 + V 4 + V5 + V 6 V1
2 2 2 2 2
(3)
where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5 and V6 are the rms amplitudes of the second through the sixth harmonic. The THD is also derived from the FFT plot of the ADC output spectrum. Intermodulation Distortion With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities will create distortion products at sum and difference frequencies of mfa nfb where m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for which neither m nor n are equal to zero. For example, the second order terms include (fa + fb) and (fa - fb), while the third order terms include (2fa + fb), (2fa - fb), (fa + 2fb) and (fa - 2fb).
Figure 14. IMD Plot
Peak Harmonic or Spurious Noise Peak harmonic or spurious noise is defined as the ratio of the rms value of the next largest component in the ADC output spectrum (up to FS/2 and excluding dc) to the rms value of the fundamental. Normally, the value of this specification will be determined by the largest harmonic in the spectrum, but for parts where the harmonics are buried in the noise floor the peak will be a noise peak.
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AD7880
MICROPROCESSOR INTERFACING The AD7880 high speed bus timing allows direct interfacing to real time digital signal processors, DSPs, as well as modern high speed, 16-bit microprocessors. Suitable microprocessor interfaces are shown in Figures 15 through 20.
AD7880-ADSP-2100 Interface
TIMER PA2 ADDRESS BUS PA0
ADDR DECODE MEN EN
CONVST CS
Figure 15 shows an interface between the AD7880 and the ADSP-2100. Conversion is initiated using a timer to drive the CONVST input asynchronously to the microprocessor. This allows very accurate control of the sampling instant. When conversion is complete, the AD7880 BUSY line goes high. An inverter on this BUSY output drives the IRQ line low thus providing an interrupt to the ADSP-2100 when conversion is completed. The conversion result is then read from the AD7880 into the ADSP-2100 with the following instruction: MR0 = DM(ADC) where MR0 is the ADSP-2100 MR0 Register and where ADC is the AD7880 address.
TMS32010
DEN INT
AD7880*
RD BUSY DB11 DB0
D15 DATA BUS D0 *ADDITIONAL PINS OMITTED FOR CLARITY
Figure 16. AD7880-TMS32010 Interface
DMA13 ADDRESS BUS DMA0 CONVST ADDR DECODE DMS EN CS TIMER
AD7880-TMS320C25 Interface
ADSP-2100 (ADSP-2101/ ADSP-2102)
DMRD (RD) IRQn RD
AD7880*
BUSY DB11 DB0
Figure 17 shows an interface between the AD7880 and the TMS320C25. As with the two previous interfaces, conversion is initiated with a timer, and the processor is interrupted when the conversion sequence is completed. The TMS320C25 does not have a separate RD output to drive the AD7880 RD input directly. This has to be generated from the processor STRB and R/W outputs with the addition of some logic gates. The RD signal is OR-gated with the MSC signal to provide the one WAIT state required in the read cycle for correct interface timing. Conversion results are read from the AD7880 using the following instruction: IN D,ADC where D is Data Memory Address and where ADC is the AD7880 address.
TIMER A15 A0 ADDRESS BUS
DMD15 DATA BUS DMD0 * ADDITIONAL PINS OMITTED FOR CLARITY
Figure 15. AD7880-ADSP-2100 (ADSP-2101/ADSP-2102) Interface
AD7880-ADSP-2101/ADSP-2102 Interface
IS
ADDR DECODE EN
CONVST CS
The interface outlined in Figure 15 also forms the basis for an interface between the AD7880 and the ADSP-2101/ADSP-2102. The READ line of the ADSP-2101/ADSP-2102 is labeled RD. In this interface, the RD pulse width of the processor can be programmed using the Data Memory Wait State Control Register. The instruction used to read a conversion result is as outlined for the ADSP-2100.
AD7880-TMS32010 Interface
TMS320C25
INTn STRB R/W READY MSC
AD7880*
BUSY
RD
DB11 DB0
An interface between the AD7880 and the TMS32010 is shown in Figure 16. Once again the conversion is initiated using an external timer and the TMS32010 is interrupted when conversion is completed. The following instruction is used to read the conversion result from the AD7880: IN D,ADC where D is Data Memory Address and where ADC is the AD7880 address. REV. 0
D15 DATA BUS D0 *ADDITIONAL PINS OMITTED FOR CLARITY
Figure 17. AD7880-TMS320C25 Interface
Some applications may require that the conversion be initiated by the microprocessor rather than an external timer. One option is to decode the AD7880 CONVST from the address bus so that -9-
AD7880
a write operation starts a conversion. Data is read at the end of the conversion sequence as before. Figure 19 shows an example of initiating conversion using this method. A similar implementation can be used for DSPs. Note that for all interfaces, a read operation should not be attempted during conversion.
AD7880-MC68000 Interface
ADDRESS BUS
8086
ADDR DECODE
CS
An interface between the AD7880 and the MC68000 is shown in Figure 18. As before, conversion is initiated using an external timer. The AD7880 BUSY line can be used to interrupt the processor or, alternatively, software delays can ensure that conversion has been completed before a read to the AD7880 is attempted. Because of the nature of its interrupts, the 68000 requires additional logic (not shown in Figure 18) to allow it to be interrupted correctly. For further information on 68000 interrupts, consult the 68000 users manual. The MC68000 AS and R/W outputs are used to generate a separate RD input signal for the AD7880. CS is used to drive the 68000 DTACK input to allow the processor to execute a normal read operation to the AD7880. The conversion results are read using the following 68000 instruction: MOVE.W ADC, D0 where D0 is the 68000 D0 register where ADC is the AD7880 address
A15 ADDRESS BUS A0 TIMER
ALE WR RD
LATCH
AD7880*
CONVST RD DB11 DB0
AD15 ADDRESS/DATA BUS AD0 *ADDITIONAL PINS OMITTED FOR CLARITY
Figure 19. AD7880-8086 Interface
AD7880-6809 Interface
MC68000
AS DTACK
ADDR DECODE EN CS
CONVST
The AD7880 can also interface quite easily with 8-bit microprocessors. The 12-bit parallel data output from the AD7880 can be read into the microprocessor as an 8+4 byte structure. Figure 20 shows an interface to the MC6809 8-bit microprocessor. As in previous cases, conversion is initiated using an external timer. At the end of conversion, BUSY triggers a one-shot which drives the IRQ interrupt input of the microprocessor. A double read is then performed to two unique addresses. The first read fetches the lower 8 bits (DB0-DB7) and loads the 74HC374 latch with the upper 4 bits (DB8-DB11). The second read fetches these upper 4 bits.
A15 ADDRESS BUS A0 TIMER
AD7880*
R/W
RD DB11 DB0
MC6809
ADDR DECODE
CONVST CS
AD7880*
D15 DATA BUS D0 *ADDITIONAL PINS OMITTED FOR CLARITY
R/W E IRQ ONE SHOT RD BUSY
Figure 18. AD7880-MC68000 Interface
AD7880-8086 Interface
Figure 19 shows an interface between the AD7880 and the 8086 microprocessor. Unlike the previous interface examples, the microprocessor initiates conversion. This is achieved by gating the 8086 WR signal with a decoded address output (different to the AD7880 CS address). Conversion is initiated and the result is read from the AD7880 using the following instruction: MOV AX, ADC where AX is the 8086 accumulator and where ADC is the AD7880 address
OE CLK D3 Q3 D0 Q0 74HC374 D7 DATA BUS D0
DB11 DB8 DB7 DB0
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 20. AD7880-6809 Interface
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APPLICATION HINTS
Good printed circuit board (PCB) layout is as important as the circuit design itself in achieving high speed A/D performance. The AD7880's comparator is required to make bit decisions on an LSB size of 1.22 mV. To achieve this, the designer must be conscious of noise both in the ADC itself and in the preceding analog circuitry. Switching mode power supplies are not recommended, as the switching spikes will feed through to the comparator causing noisy code transitions. Other causes of concern are ground loops and digital feedthrough from microprocessors. These are factors which influence any ADC, and a proper PCB layout which minimizes these effects is essential for best performance.
LAYOUT HINTS
V+ V+
LK2 AB
VDD
ANALOG INPUT SKT1
LK1 + -
C1 10F V+ IC1 V-
C2 0.1F
TO ADC
C3 10F
C4 0.1F
Ensure that the layout for the printed circuit board has the digital and analog signal lines separated as much as possible. Take care not to run digital tracks alongside analog signal tracks. Guard (screen) the analog input with AGND. Establish a single point analog ground (star ground) separate from the logic system ground at the AD7880 AGND pin or as close as possible to the AD7880. Connect all other grounds and the AD7880 DGND to this single analog ground point. Do not connect any other digital grounds to this analog ground point. Low impedance analog and digital power supply common returns are essential to low noise operation of the ADC, so make the foil width for these tracks as wide as possible. The use of ground planes minimizes impedance paths and also guards the analog circuitry from digital noise. The circuit layout of Figures 26 and 27 have both analog and digital ground planes which are kept separated and only joined together at the AD7880 AGND pin.
NOISE
V- AB LK3
Figure 21. Analog Input Buffering
When it is required to drive the AD7880 with the 0 V to 10 V input range, an external supply must be connected to V+ (see Figure 21). In bipolar operation, positive and negative supplies must be connected to V+ and V-. The AD711 is a general purpose op amp which could be used to drive the analog input of the AD7880.
POWER-DOWN CONTROL (MODE INPUT)
Keep the input signal leads to VIN and signal return leads from AGND as short as possible to minimize input noise coupling. In applications where this is not possible, use a shielded cable between the source and the ADC. Reduce the ground circuit impedance as much as possible since any potential difference in grounds between the signal source and the ADC appears as an error voltage in series with the input signal.
ANALOG INPUT BUFFERING
The AD7880 is designed for systems which need to have minimum power consumption. This includes such applications as hand held, portable battery powered systems and remote monitoring systems. As well as consuming minimum power under normal operating conditions, typically 20 mW, the AD7880 can be put into a power-down or sleep mode when not required to convert signals. When in this power-down mode, the AD7880 consumes approximately 2 mW of power. The AD7880 is powered down by bringing the MODE input pin to a Logic Low in conjunction with keeping the RD input control High. The AD7880 will remain in the power-down mode until MODE is brought to a Logic High again. The MODE input should be driven with CD4000 or HCMOS logic levels. It is recommended that one "dummy" conversion be implemented before reading conversion data from the AD7880 after it has been in the power-down mode. This is required to reset all internal logic and control circuitry. In a remote monitoring system where, say, 10 conversions are required to be taken with a sampling interval of 1 second, an additional 11th conversion must be carried out. Figure 22 gives a plot of power consumption
20 POWER CONSUMPTION - mW 2 0 1.65 x 10 - 4 1 TIME - secs 2 CONVERTING CONVERTING
To achieve specified performance, it is recommended that the analog input (VINA, VINB) be driven from a low impedance source. This necessitates the use of an input buffer amplifier. The choice of op amp will be a function of the particular application and the desired analog input range. The data acquisition circuit, described in this data sheet allows for various op amp configurations. Figure 21 shows the analog input buffer circuit. The options available to drive the supply of the op amp are: Single +5 V (derived from PCB 5 V supply) Dual Supply (externally supplied to V+ and V-) 5 V, 12 V or 15 V The simplest configuration is the 0 V to 5 V range of Figure 5. A single supply 5 V op amp is recommended for such an implementation. This will allow for operation of the AD7880 in the 0 V to 5 V unipolar range without supplying an external supply to V+ and V-. The 5 V supply is derived from the systems +5 V VDD supply. REV. 0
POWER-DOWN
POWER-DOWN
Figure 22. Power Consumption for Normal Operation and Power-Down Operation vs. Time
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AD7880
as a function of time for such operation. The total conversion time for each cycle is 11 x 15 s (where 15 s is the time taken for a single conversion) corresponding to 1.65 x 10-4 secs. Hence: Average Power = PowerCONVERTING + PowerPOWER-DOWN = {20 mW x (1.65 x 10-4)/(10)} + {2 mW x (9.9998)/(10)} = 2.029 mW
AD7880 DATA ACQUISITION LAYOUT
Figure 24 shows the AD7880 in a data acquisition circuit. The corresponding printed circuit board (PCB) layout and silkscreen are shown in Figures 25 to 27. The only additional component required for a full data acquisition system is an antialiasing filter. There is a component grid provided near the analog input on the PCB which may be used for such a filter or any other input conditioning circuitry. To facilitate this option there is a shorting link (labeled LK1 on the PCB) on the analog input track. With LK1 in place, the analog input connects to the buffer amplifier driving the AD7880. With LK1 removed, a wire link is needed to connect the analog input to the PCB component grid.
INTERFACE CONNECTIONS
Connects the analog input to a buffer amplifier. The analog input may also be connected to a component grid for signal conditioning. LK2, LK3 Allows for various op amp power supplies to be used to drive the input buffer of the AD7880. External supplies may be connected to V+ and V-. Alternatively, the AD7880's +5 V system supply and AGND can be selected to drive a single supply op amp. LK4 Configures the various analog input ranges, 0 V to 5 V, 0 V to 10 V or 5 V. LK5 Selects reference input to VREF of AD7880. Normally connected to VDD. An external reference could also be wired in. LK6 Selects power-down or sleep mode. The shorting plug is connected to VDD for normal operation. LK7 Connects the AD7880 RD input directly to the RD input of SKT4 or to a decoded STRB and R/W input. This shorting plug setting depends on the microprocessor, e.g., the TMS320C25 requires a decoded RD signal.
R/W RD CS BUSY N/C DB10 DB8 DB6 DB4 DB2 DB0 + 5V GND 1 3 5 7 9 11 13 15 17 19 21 23 25 2 4 6 8 10 12 14 16 18 20 22 24 26 STRB N/C N/C BUSY N/C DB11 DB9 DB7 DB5 DB3 DB1 + 5V GND
LK1
The data acquisition board contains a parallel connection port labeled SKT4. This is a 26-contact IDC Connector and provides for direct microprocessor connection to the board. This connector, the pinout of which is shown in Figure 23, contains all data, control and status signals of the AD7880 (with the exception of the CONVST and the CLKIN inputs both of which are provided via SKT2 and SKT3 respectively). It also contains decoded R/W and STRB inputs which are necessary for interfacing to many microprocessors including the TMS320C25 and the Motorola 68000 series. Link LK7 selects RD directly or alternatively, the decoded version. Note that the AD7880 CS input must be decoded prior to the AD7880 evaluation board. SKT1, SKT2 and SKT3 are three sub-miniature connectors (SMC) which provide input connections for the analog input, the CONVST input and the CLKIN input. Three different input ranges can be accepted by the AD7880 each of which is configured by selecting shorting plug options A, B or C of LK4. Position A corresponds to the 0 V to 5 V unipolar configuration of Figure 5, position B corresponds to the bipolar 5 V configuration of Figure 7 and position C allows for a 0 V to +10 V unipolar range as shown in Figure 6.
POWER SUPPLY CONNECTIONS
Figure 23. SKT4, IDC Connector Pinout
COMPONENT LIST
The PCB requires a single +5 V power supply (labeled VDD). Good decoupling allows this supply to drive the AD7880 VDD which also drives the VREF input as well as the op amp power supply. In circumstances where bipolar 5 V or a unipolar 0 V to 10 V input ranges are required, provision has been allowed for the connection of separate op amp power supplies ( 15 V, 12 V, 5 V, etc.) to V+ and V-. LK2 and LK3 shorting links allow for the selection of user defined op amp power supplies or the on-board single +5 V supply.
LINK OPTIONS
IC1 IC2 IC3 C1, C3, C5 C2, C4, C6, C7 R1, R2 LK1, LK2, LK3 LK4, LK5, LK6 LK7 SKT1, SKT2, SKT3
Op Amp* AD7880 Analog-to-Digital Converter 74HC00 Quad NAND Gate 10 F Capacitors 0.1 F Capacitors 10 k Pull-up Resistors Shorting Links
SKT4
Sub-Miniature Connectors Vendor No: Sealectro 50-051-0000 (Socket) Vendor No: Sealectro 50-007-0000 (Plug) 26-Contact (2 Row) IDC Connector
There are seven link options which must be set before using the board. These are outlined below:
NOTE *See ANALOG INPUT BUFFERING section. .
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Figure 24. Data Acquisition Circuit Using the AD7880
Figure 25. PCB Silkscreen for Figure 24
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Figure 26. PCB Component Side Layout for Figure 24
Figure 27. PCB Solder Side Layout for Figure 24
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OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
24-Lead Plastic DIP (N-24)
1.228 (31.19) 1.226 (31.14)
24 1 13 12
0.260 0.001 (6.61 0.03) 0.32 (8.128) 0.30 (7.62)
PIN 1 0.130 (3.30) 0.128 (3.25) SEATING PLANE
0.011 (0.28) 0.02 (0.5) 0.016 (0.41) 0.11 (2.79) 0.09 (2.28) 0.07 (1.78) 0.05 (1.27) 15 0 0.009 (0.23)
NOTES: 1. LEAD NO. 1 IDENTIFIED BY DOT OR NOTCH. 2. PLASTIC LEADS WILL BE EITHER SOLDER DIPPED OR TIN LEAD PLATED IN ACCORDANCE WITH MIL-M-38510 REQUIREMENTS.
24-Lead Cerdip (Q-24)
24
13
1
12
0.295 (7.493) MAX 0.320 (8.128) 0.291 (7.4) 0.180 (4.572) MAX 0.070 (1.778) 0.012 (0.305) 15 0 0.008 (0.203)
1.290 (32.77) MAX 0.225 (5.715) SEATING MAX PLANE 0.125 (3.175) MIN
0.021 (0.533) 0.015 (0.381) TYP
0.110 (2.794) 0.009 (2.286) TYP
0.065 (1.651) 0.020 (0.508) 0.055 (1.397)
1. LEAD NO. 1 IDENTIFIED BY DOT OR NOTCH. 2. CERDIP LEADS WILL BE EITHER TIN PLATED OR SOLDER DIPPED IN ACCORDANCE WITH MIL-M-38510 REQUIREMENTS.
24-Lead SOIC (R-24)
0.614 (15.6) 0.598 (15.2)
24 13
0.419 (10.65) 0.394 (10.00)
1 12
0.299 (7.6) 0.291 (7.4)
PIN 1
0.104 (2.65) 0.093 (2.35)
0.03 (0.75) 0.01 (0.25)
0.012 (0.3) 0.004 (0.1)
0.05 (1.27) BSC
0.019 (0.49) 0.014 (0.35)
0.013 (0.32) 0.009 (0.23)
8 0
0.05 (1.27) 0.016 (0.40)
REV. 0
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C1414-10-6/90
PRINTED IN U.S.A.

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