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19-1387; Rev 0; 11/98
+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX
General Description
The MAX144/MAX145 low-power, 12-bit analog-todigital converters (ADCs) are available in 8-pin MAX and DIP packages. Both devices operate with a single +2.7V to +5.25V supply and feature a 7.4s successive-approximation ADC, automatic power-down, fast wake-up (2.5s), an on-chip clock, and a high-speed, 3-wire serial interface. Power consumption is only 3.2mW (VDD = +3.6V) at the maximum sampling rate of 108ksps. At slower throughput rates, the automatic shutdown (0.2A) further reduces power consumption. The MAX144 provides 2-channel, single-ended operation and accepts input signals from 0 to VREF. The MAX145 accepts pseudo-differential inputs ranging from 0 to V REF . An external clock accesses datathrough the 3-wire serial interface, which is SPITM, QSPITM, and MICROWIRETM-compatible. Excellent dynamic performance and low power, combined with ease of use and small package size, make these converters ideal for battery-powered and dataacquisition applications, or for other circuits with demanding power-consumption and space requirements. For pin-compatible 10-bit ADCs, see the MAX157 and MAX159.
Features
o Single-Supply Operation (+2.7V to +5.25V) o Two Single-Ended Channels (MAX144) One Pseudo-Differential Channel (MAX145) o Low Power 0.9mA (108ksps, +3V Supply) 100A (10ksps, +3V Supply) 10A (1ksps, +3V Supply) 0.2A (Power-Down Mode) o Internal Track/Hold o 108ksps Sampling Rate o SPI/QSPI/MICROWIRE-Compatible 3-Wire Serial Interface o Space-Saving 8-Pin MAX Package o Pin-Compatible 10-Bit Versions Available
MAX144/MAX145
Ordering Information
PART MAX144ACUA MAX144BCUA MAX144ACPA MAX144BCPA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C 0C to +70C PIN-PACKAGE 8 MAX 8 MAX 8 Plastic DIP 8 Plastic DIP Dice* 8 MAX 8 MAX INL (LSB) 0.5 1 0.5 1 1 0.5 1 0.5 1 0.5 1 0.5 1 0.5 1 1 0.5 1 0.5 1 0.5 1
Applications
Battery-Powered Systems Portable Data Logging Isolated Data Acquisition Process-Control Monitoring Instrumentation Test Equipment Medical Instruments System Supervision
MAX144BC/D 0C to +70C MAX144AEUA -40C to +85C MAX144BEUA -40C to +85C
Pin Configuration
TOP VIEW
VDD 1 CH0 (CH+) 2 CH1 (CH-) 3 8 7 SCLK DOUT CS/SHDN REF
MAX144AEPA -40C to +85C 8 Plastic DIP MAX144BEPA -40C to +85C 8 Plastic DIP MAX144AMJA -55C to +125C 8 CERDIP** MAX144BMJA -55C to +125C MAX145ACUA 0C to +70C MAX145BCUA 0C to +70C MAX145ACPA 0C to +70C MAX145BCPA 0C to +70C MAX145BC/D 0C to +70C MAX145AEUA -40C to +85C MAX145BEUA -40C to +85C MAX145AEPA -40C to +85C MAX145BEPA -40C to +85C 8 CERDIP** 8 MAX 8 MAX 8 Plastic DIP 8 Plastic DIP Dice* 8 MAX 8 MAX 8 Plastic DIP 8 Plastic DIP
MAX144 MAX145
6 5
GND 4
( ) ARE FOR MAX145 ONLY
MAX/DIP
MAX145AMJA -55C to +125C 8 CERDIP** MAX145BMJA -55C to +125C 8 CERDIP** *Dice are specified at TA = +25C, DC parameters only. **Contact factory for availability.
SPI and QSPI are trademarks of Motorola, Inc. MICROWIRE is a trademark of National Semiconductor Corp.
________________________________________________________________ Maxim Integrated Products
1
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+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX MAX144/MAX145
ABSOLUTE MAXIMUM RATINGS
VDD to GND ..............................................................-0.3V to +6V CH0, CH1 (CH+, CH-) to GND ................. -0.3V to (VDD + 0.3V) REF to GND .............................................. -0.3V to (VDD + 0.3V) Digital Inputs to GND. ............................................. -0.3V to +6V DOUT to GND............................................ -0.3V to (VDD + 0.3V) DOUT Sink Current ........................................................... 25mA Continuous Power Dissipation (TA = +70C) MAX (derate 4.1mW/C above +70C) .................... 330mW Plastic DIP (derate 9.09mW/C above +70C) ............727mW CERDIP (derate 8.00mW/C above +70C) . .............. 640mW Operating Temperature Ranges (TA) MAX144/MAX145_C_A .......................................0C to +70C MAX144/MAX145_E_A. ...................................-40C to +85C MAX144/MAX145_M_A ................................ -55C to +125C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10sec) .............................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V DD = +2.7V to +5.25V, V REF = 2.5V, 0.1F capacitor at REF, f SCLK = 2.17MHz, 16 clocks/conversion cycle (108ksps), CH- = GND for MAX145, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER DC ACCURACY (Note 1) Resolution Relative Accuracy (Note 2) Differential Nonlinearity Offset Error Gain Error (Note 3) Gain Temperature Coefficient Channel-to-Channel Offset Matching Channel-to-Channel Gain Matching 0.8 0.05 0.05 RES INL DNL MAX14_A MAX14_B No missing codes over temperature 12 0.5 1 0.75 3 3 Bits LSB LSB LSB LSB ppm/C LSB LSB SYMBOL CONDITIONS MIN TYP MAX UNITS
DYNAMIC SPECIFICATIONS (fIN(sine-wave) = 10kHz, VIN = 2.5Vp-p, 108ksps, fSCLK = 2.17MHz, CH- = GND for MAX145) Signal-to-Noise Plus Distortion Ratio Total Harmonic Distortion (including 5th-order harmonic) Spurious-Free Dynamic Range Channel-to-Channel Crosstalk Small-Signal Bandwidth Full-Power Bandwidth CONVERSION RATE Conversion Time (Note 5) T/H Acquisition Time Aperture Delay Aperture Jitter Serial Clock Frequency 2 fSCLK External clock mode Internal clock mode, for data transfer only 0.1 0 tCONV tACQ 25 <50 2.17 5 External clock, fSCLK = 2.17MHz, 16 clocks/conversion cycle Internal clock 7.4 5 7 2.5 s ns ps MHz s SINAD THD SFDR fIN = 65kHz, VIN = 2.5Vp-p (Note 4) -3dB rolloff 80 -85 2.25 1.0 70 -80 dB dB dB dB MHz MHz
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+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX
ELECTRICAL CHARACTERISTICS (continued)
(V DD = +2.7V to +5.25V, V REF = 2.5V, 0.1F capacitor at REF, f SCLK = 2.17MHz, 16 clocks/conversion cycle (108ksps), CH- = GND for MAX145, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER ANALOG INPUTS Analog Input Voltage Range (Note 6) Multiplexer Leakage Current Input Capacitance EXTERNAL REFERENCE Input Voltage Range (Note 7) Input Current Input Resistance Shutdown REF Input Current DIGITAL INPUTS (CS/SHDN) AND OUTPUT (DOUT) VDD 3.6V Input High Voltage VIH VDD > 3.6V Input Low Voltage Input Hysteresis Input Leakage Current Input Capacitance Output Low Voltage Output High Voltage Three-State Output Leakage Current Three-State Output Capacitance POWER REQUIREMENTS Positive Supply Voltage Positive Supply Current Power-Supply Rejection (Note 9) VDD IDD PSR Operating mode Shutdown, CS/SHDN = GND VDD = 2.7V to 5.25V, VREF = 2.5V, full-scale input 2.7 0.9 0.2 0.15 5.25 2.0 5 V mA A mV COUT VIL VHYS IIN CIN VOL VOH VIN = 0 or VDD (Note 8) ISINK = 5mA ISINK = 16mA ISOURCE = 0.5mA CS/SHDN = VDD CS/SHDN = VDD (Note 8) VDD - 0.5 10 15 0.5 0.2 1 15 0.4 2.0 3.0 0.8 VREF VREF = 2.5V 18 0 100 25 0.01 10 VDD + 50mV 140 V A k A CIN VIN On/off leakage current, VIN = 0 to VDD 0 0.01 16 VREF 1 V A pF SYMBOL CONDITIONS MIN TYP MAX UNITS
MAX144/MAX145
V V V A pF V V A pF
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+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX MAX144/MAX145
TIMING CHARACTERISTICS (Figure 7)
(V DD = +2.7V to +5.25V, V REF = 2.5V, 0.1F capacitor at REF, f SCLK = 2.17MHz, 16 clocks/conversion cycle (108ksps), CH- = GND for MAX145, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Wake-Up Time (Note 10) CS/SHDN Fall to Output Enable CS/SHDN Rise to Output Disable SCLK Fall to Output Data Valid SCLK Clock Frequency SYMBOL tWAKE tDV tTR tDO fSCLK CL = 100pF CL = 100pF, Figure 1 CL = 100pF, Figure 1 External clock Internal clock, SCLK for data transfer only External clock SCLK Pulse Width High tCH Internal clock, SCLK for data transfer only (Note 8) External clock SCLK Pulse Width Low SCLK to CS/SHDN Setup CS/SHDN Pulse Width tCL tSCLKS tCS Internal clock, SCLK for data transfer only (Note 8) 20 0.1 0 215 50 215 50 60 60 ns ns ns ns ns CONDITIONS MIN 2.5 120 120 120 2.17 5 TYP MAX UNITS s ns ns ns MHz
Note 1: Tested at VDD = +2.7V. Note 2: Relative accuracy is the deviation of the analog value at any code from its theoretical value after full-scale range has been calibrated. Note 3: Offset nulled. Note 4: "On" channel is grounded; sine wave applied to "off" channel (MAX144 only). Note 5: Conversion time is defined as the number of clock cycles times the clock period; clock has 50% duty cycle. Note 6: The common-mode range for the analog inputs is from GND to VDD (MAX145 only). Note 7: ADC performance is limited by the converter's noise floor, typically 300Vp-p. Note 8: Guaranteed by design. Not subject to production testing. Note 9: Measured as VFS(2.7V) - VFS(5.25V). Note 10: SCLK must remain stable during this time.
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+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX
Typical Operating Characteristics
(VDD = +3.0V, VREF = 2.5V, 0.1F at REF, fSCLK = 2.17MHz, 16 clocks/conversion cycle (108ksps), CH- = GND for MAX145, TA = +25C, unless otherwise noted.)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX144/5-01
MAX144/MAX145
SUPPLY CURRENT vs. TEMPERATURE
MAX144/5-02
SUPPLY CURRENT vs. SAMPLING RATE
VDD = VREF CL = 20pF CODE = 101010100000
MAX144/5-03
1500
1300 SUPPLY CURRENT (A)
SUPPLY CURRENT (A)
SUPPLY CURRENT (A)
VREF = VDD RL = CL = 50pF CODE = 101010100000
1500 VREF = VDD RL = CL = 50pF CODE = 101010100000
10,000
1000
1250
1100
100
1000
900
10
700
750
1
500 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V)
500 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
0.1 0.1 1 10 100 1k 10k 100k SAMPLING RATE (sps)
SHUTDOWN CURRENT vs. SUPPLY VOLTAGE
MAX144/5-04
SHUTDOWN CURRENT vs. TEMPERATURE
MAX144/5-05
OFFSET ERROR vs. SUPPLY VOLTAGE
MAX144/5-06
1000 VREF = VDD SHUTDOWN CURRENT (nA) 800
1000
VREF = VDD
1.0
SHUTDOWN CURRENT (nA)
800
0.8 OFFSET ERROR (LSB) -60 -40 -20 0 20 40 60 80 100 120 140
600
600
0.6
400
400
0.4
200
200
0.2
0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V)
0 TEMPERATURE (C)
0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V)
OFFSET ERROR vs. TEMPERATURE
MAX144/5-07
GAIN ERROR vs. SUPPLY VOLTAGE
MAX144/5-08
GAIN ERROR vs. TEMPERATURE
0.4 0.3 GAIN ERROR (LSB) 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4
MAX144/5-09
1.0 0.9 0.8 OFFSET ERROR (LSB) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -60 -35 -10 15 40 65 90 TEMPERATURE (C)
0.5 0.4 0.3 GAIN ERROR (LSB) 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4
0.5
115 140
-0.5 2.5
3.0
3.5
4.0 VDD (V)
4.5
5.0
5.5
-0.5 -60
-35
-10
15
40
65
90
115 140
TEMPERATURE (C)
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+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX MAX144/MAX145
Typical Operating Characteristics (continued)
(VDD = +3.0V, VREF = 2.5V, 0.1F at REF, fSCLK = 2.17MHz, 16 clocks/conversion cycle (108ksps), CH- = GND for MAX145, TA = +25C, unless otherwise noted.)
INTEGRAL NONLINEARITY vs. OUTPUT CODE
MAX144/5-10
INTEGRAL NONLINEARITY vs. SUPPLY VOLTAGE
MAX144/5-11
INTEGRAL NONLINEARITY vs. TEMPERATURE
MAX144/5-12
0.20 0.15 0.10 INL (LSB)
0.5
0.5
0.4
0.4
INL (LSB)
0 -0.05 -0.10 -0.15 -0.20 0 1024 2048 OUTPUT CODE 3072 4096
0.2
INL (LSB)
0.05
0.3
0.3
0.2
0.1
0.1
0 2.5 3.0 3.5 4.0 VDD (V) 4.5 5.0 5.5
0 -60 -35 -10 15 40 65 90 115 140 TEMPERATURE (C)
FFT PLOT
MAX144/5-13
0 -20 AMPLITUDE (dB) -40 -60 -80 -100 -120 -140 0 27
EFFECTIVE NUMBER OF BITS
VDD = +2.7V fIN = 10kHz fSAMPLE = 108ksps
VDD = +2.7V 11.8
11.6
11.4
11.2
54
11.0 1 10 FREQUENCY (kHz) 100
FREQUENCY (kHz)
Pin Description
PIN 1 2 3 4 5 6 7 8 6 NAME VDD CH0 (CH+) CH1 (CH-) GND REF CS/SHDN DOUT SCLK Positive Supply Voltage, +2.7V to +5.25V Analog Input: MAX144 = single-ended (CH0); MAX145 = differential (CH+) Analog Input: MAX144 = single-ended (CH1); MAX145 = differential (CH-) Analog and Digital Ground External Reference Voltage Input. Sets the analog voltage range. Bypass with a 100nF capacitor close to the device. Active-Low Chip-Select Input/Active-High Shutdown Input. Pulling CS/SHDN high puts the device into shutdown with a maximum current of 5A. Serial Data Output. Data changes state at SCLK's falling edge. High impedance when CS/SHDN is high. Serial Clock Input. DOUT changes on the falling edge of SCLK. FUNCTION
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MAX144/5-14
20
EFFECTIVE NUMBER OF BITS vs. FREQUENCY
12.0
+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX MAX144/MAX145
VDD DOUT DOUT 6k GND a) HIGH-Z TO V0H, V0L TO V0H, AND VOH TO HIGH-Z CL CL GND b) HIGH-Z TO V0L, V0H TO V0L, AND VOL TO HIGH-Z 6k
Figure 1. Load Circuits for Enable and Disable Time
_______________Detailed Description
The MAX144/MAX145 analog-to-digital converters (ADCs) use a successive-approximation conversion (SAR) technique and on-chip track-and-hold (T/H) structure to convert an analog signal to a serial 12-bit digital output data stream. This flexible serial interface provides easy interface to microprocessors (Ps). Figure 2 shows a simplified functional diagram of the internal architecture for both the MAX144 (2 channels, single-ended) and the MAX145 (1 channel, pseudo-differential).
CS/SHDN SCLK INTERNAL CLOCK CONTROL LOGIC CH0 (CH+) CH1 (CH-) REF ANALOG INPUT MUX (2 CHANNEL) T/H OUTPUT REGISTER DOUT
SCLK 12-BIT IN SAR OUT ADC
MAX144 MAX145
Analog Inputs: Single-Ended (MAX144) and Pseudo-Differential (MAX145)
The sampling architecture of the ADC's analog comparator is illustrated in the equivalent input circuit of Figure 3. In single-ended mode (MAX144), both channels CH0 and CH1 are referred to GND and can be connected to two different signal sources. Following the power-on reset, the ADC is set to convert CH0. After CH0 has been converted, CH1 will be converted and the conversions will continue to alternate between channels. Channel switching is performed by toggling the CS/SHDN pin. Conversions can be performed on the same channel by toggling CS/SHDN twice between conversions. If only one channel is required, CH0 and CH1 may be connected together; however, the output data will still contain the channel identification bit (before the MSB). For the MAX145, the input channels form a single differential channel pair (CH+, CH-). This configuration is pseudo-differential to the effect that only the signal at IN+ is sampled. The return side IN- must remain stable within 0.5LSB (0.1LSB for optimum results) with respect to GND during a conversion. To accomplish this, connect a 0.1F capacitor from IN- to GND. During the acquisition interval, the channel selected as the positive input (IN+) charges capacitor CHOLD. The acquisition interval spans from when CS/SHDN falls to the falling edge of the second clock cycle (external
( ) ARE FOR MAX145
Figure 2. Simplified Functional Diagram
12-BIT CAPACITIVE DAC REF CH0 (CH+) CH1 (CH-) CHOLD 16pF ZERO RIN 9k CSWITCH TRACK HOLD T/H
MAX144 MAX145
COMPARATOR TO SAR
INPUT MUX
GND SINGLE-ENDED MODE: CH0, CH1 = IN+; GND = INDIFFERENTIAL-ENDED MODE: CH+ = IN+; CH- = IN-
CONTROL LOGIC ( ) ARE FOR MAX145
Figure 3. Analog Input Channel Structure
clock mode) or from when CS/SHDN falls to the first falling edge of SCLK (internal clock mode). At the end of the acquisition interval, the T/H switch opens, retaining charge on CHOLD as a sample of the signal at IN+. The conversion interval begins with the input multiplexer switching CHOLD from the positive input (IN+) to the negative input (IN-). This unbalances node ZERO at the comparator's positive input.
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+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX MAX144/MAX145
The capacitive digital-to-analog converter (DAC) adjusts during the remainder of the conversion cycle to restore node ZERO to 0V within the limits of 12-bit resolution. This action is equivalent to transferring a 16pF * [(VIN+) - (VIN-)] charge from CHOLD to the binary-weighted capacitive DAC, which in turn forms a digital representation of the analog input signal. Higher source impedances can be used if a 0.01F capacitor is connected to the individual analog inputs. Together with the input impedance, this capacitor forms an RC filter, limiting the ADC's signal bandwidth.
Input Bandwidth
The MAX144/MAX145 T/H stage offers a 2.25MHz small-signal and a 1MHz full-power bandwidth, which make it possible to use the parts for digitizing highspeed transients and measuring periodic signals with bandwidths exceeding the ADCs sampling rate by using undersampling techniques. To avoid high-frequency signals being aliased into the frequency band of interest, anti-alias filtering is recommended. Most aliasing problems can be fixed easily with an external resistor and a capacitor. However, if DC precision is required, it is usually best to choose a continuous or switched-capacitor filter, such as the MAX7410/ MAX7414 (Figure 4). Their Butterworth characteristic generally provides the best compromise (with regard to rolloff and attenuation) in filter configurations, is easy to design, and provides a maximally flat passband response.
Track/Hold (T/H)
The ADC's T/H stage enters its tracking mode on the falling edge of CS/SHDN. For the MAX144 (singleended inputs), IN- is connected to GND and the converter samples the positive ("+") input. For the MAX145 (pseudo-differential inputs), IN- connects to the negative input ("-") and the difference of [(VIN+) - (VIN-)] is sampled. At the end of the conversion, the positive input connects back to IN+ and CHOLD charges to the input signal. The time required for the T/H stage to acquire an input signal is a function of how fast its input capacitance is charged. If the input signal's source impedance is high, the acquisition time lengthens, and more time must be allowed between conversions. The acquisition time, tACQ, is the maximum time the device takes to acquire the signal, and is also the minimum time required for the signal to be acquired. Calculate this with the following equation: tACQ = 9(RS + RIN)CIN where RS is the source impedance of the input signal, RIN (9k) is the input resistance, and CIN (16pF) is the input capacitance of the ADC. Source impedances below 1k have no significant impact on the AC performance of the MAX144/MAX145.
VDD
Analog Input Protection
Internal protection diodes, which clamp the analog input to VDD and GND, allow each input channel to swing within GND - 300mV to VDD + 300mV without damage. However, for accurate conversions, both inputs must not exceed VDD + 50mV or be less than GND - 50mV. If an off-channel analog input voltage exceeds the supplies, limit the input current to 4mA.
4 VDD
SHDN
7 470**
0.1F
2
CH0
1 VDD
REF
5
EXTERNAL REFERENCE
2 fC = 15kHz
IN
MAX7410 MAX7414
OUT 5 8 CLK
MAX144
3 0.01F** 8 SCLK CS/SHDN 4 1.5MHz OSCILLATOR 6 P/C CH1 DOUT 7
COM 1 0.01F
OS 6
GND 3
GND
**USED TO ATTENUATE SWITCHED-CAPACITOR FILTER CLOCK NOISE
Figure 4. Analog Input with Anti-Aliasing Filter Structure
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+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX
Selecting Clock Mode
To start the conversion process on the MAX144/ MAX145, pull CS/SHDN low. At CS/SHDN's falling edge, the part wakes up and the internal T/H enters track mode. In addition, the state of SCLK at CS/SHDN's falling edge selects internal (SCLK = high) or external (SCLK = low) clock mode.
Internal Clock (fSCLK < 100kHz or fSCLK > 2.17MHz) In internal clock mode, the MAX144/MAX145 run from an internal, laser-trimmed oscillator to within 20% of the 2MHz specified clock rate. This releases the system microprocessor from running the SAR conversion clock and allows the conversion results to be read back at the processor's convenience, at any clock rate from 0 to 5MHz. Operating the MAX144/MAX145 in internal clock mode is necessary for serial interfaces operating with clock frequencies lower than 100kHz or greater than 2.17MHz. Select internal clock mode (Figure 5), by holding SCLK high during a high/low transition of CS/SHDN. The first SCLK falling edge samples the data and initiates a conversion using the integrated on-chip oscillator. After the conversion, the oscillator shuts off and DOUT goes high, signaling the end of conversion (EOC). Data can then be read out with SCLK.
External Clock (fSCLK = 100kHz to 2.17MHz) The external clock mode (Figure 6) is selected by transitioning CS/SHDN from high to low while SCLK is low. The external clock signal not only shifts data out, but also drives the analog-to-digital conversion. The input is sampled and conversion begins on the falling edge of the second clock pulse. Conversion must be completed within 140s to prevent degradation in the conversion results caused by droop on the T/H capacitors. External clock mode provides the best throughput for clock frequencies between 100kHz and 2.17MHz.
MAX144/MAX145
Output Data Format
Table 1 illustrates the 16-bit, serial data stream output format for both the MAX144 and MAX145. The first three bits are always logic high (including the EOC bit for internal clock mode), followed by the channel identification (CHID = 0 for CH0, CHID = 1 for CH1, CHID = 1 for the MAX145), and then 12 bits of data in MSB-first format. After the last bit has been read out, additional SCLK pulses will clock out trailing zeros. DOUT transitions on the falling edge of SCLK. The output remains high-impedance when CS/SHDN is high.
ACTIVE
POWER DOWN tCS
ACTIVE
CS/SHDN SCLK HIGH-Z DOUT
tWAKE (tACQ)
tCONV
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16 HIGH-Z
EOC SAMPLING INSTANT
1
1 CHID MSB D10 D9
D8
D7
D6
D5
D4
D3
D2
D1
D0
Figure 5. Internal Clock Mode Timing
ACTIVE
POWER DOWN tCS
ACTIVE SAMPLING INSTANT
ACTIVE
POWER DOWN
CS/SHDN SCLK HIGH-Z DOUT
tWAKE (tACQ)
1
2
3
4
5
6
7
8 D8
9 D7
10 D6
11 D5
12 D4
13 D3
14 D2
15 D1
16 D0 HIGH-Z
CHID MSB D10 D9
Figure 6. External Clock Mode Timing
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+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX MAX144/MAX145
Table 1. Serial Output Data Stream for Internal and External Clock Mode
SCLK CYCLE DOUT (Internal Clock) DOUT (External Clock) 1 EOC 1 2 1 1 3 1 1 4 5 6 D10 D10 7 D9 D9 8 D8 D8 9 D7 D7 10 D6 D6 11 D5 D5 12 D4 D4 13 D3 D3 14 D2 D2 15 D1 D1 16 D0 D0 CHID D11 CHID D11
External Reference
An external reference is required for both the MAX144 and the MAX145. At REF, the DC input resistance is a minimum of 18k. During a conversion, a reference must be able to deliver 250A of DC load current and have an output impedance of 10 or less. Use a 0.1F bypass capacitor for best performance. The reference input structure allows a voltage range of 0 to VDD + 50mV, although noise levels will decrease effective resolution at lower reference voltages.
Effective Number of Bits (ENOB)
ENOB indicates the global accuracy of an ADC at a specific input frequency and sampling rate. An ideal ADC's error consists only of quantization noise. With an input range equal to the full-scale range of the ADC, the effective number of bits can be calculated as follows: ENOB = (SINAD - 1.76) / 6.02
Total Harmonic Distortion (THD)
THD is the ratio of the RMS sum of the first five harmonics of the input signal to the fundamental itself. This is expressed as: 2 2 2 2 V2 + V3 + V4 + V5 log V1
Automatic Power-Down Mode
Whenever the MAX144/MAX145 are not selected (CS/SHDN = V DD ), the parts enter their shutdown mode. In shutdown all internal circuitry turns off, reducing supply current to typically less than 0.2A. With an external reference stable to within 1LSB, the wake-up time is 2.5s. If the external reference is not stable within 1LSB, the wake-up time must be increased to allow the reference to stabilize.
THD = 20
__________Applications Information
Signal-to-Noise Ratio (SNR)
For a waveform perfectly reconstructed from digital samples, the theoretical maximum SNR is the ratio of full-scale analog input (RMS value) to the RMS quantization error (residual error). The ideal, theoretical minimum analog-to-digital noise is caused by quantization error only and results directly from the ADC's resolution (N bits): SNR(MAX) = (6.02 * N + 1.76)dB In reality, there are other noise sources besides quantization noise: thermal noise, reference noise, clock jitter, etc. Therefore, SNR is computed by taking the ratio of the RMS signal to the RMS noise which includes all spectral components minus the fundamental, the first five harmonics, and the DC offset.
where V1 is the fundamental amplitude, and V2 through V5 are the amplitudes of the 2nd- through 5th-order harmonics.
Spurious-Free Dynamic Range (SFDR)
SFDR is the ratio of RMS amplitude of the fundamental (maximum signal component) to the RMS value of the next largest spurious component, excluding DC offset.
Connection to Standard Interfaces
The MAX144/MAX145 interface is fully compatible with SPI, QSPI, and MICROWIRE standard serial interfaces. If a serial interface is available, establish the CPU's serial interface as master so that the CPU generates the serial clock for the MAX144/MAX145. Select a clock frequency from 100kHz to 2.17MHz (external clock mode). 1) Use a general-purpose I/O line on the CPU to pull CS/SHDN low while SCLK is low. 2) Wait for the minimum wake-up time (tWAKE) specified before activating SCLK. 3) Activate SCLK for a minimum of 16 clock cycles. The serial data stream of three leading ones, the channel identification, and the MSB of the digitized input signal begin at the first falling clock edge. DOUT transitions on SCLK's falling edge and is available in MSB-first format. Observe the SCLK to
Signal-to-Noise Plus Distortion (SINAD)
SINAD is the ratio of the fundamental input frequency's RMS amplitude to RMS equivalent of all other ADC output signals: SignalRMS SINAD(dB) = 20 log (Noise + Distortion)RMS
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+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX MAX144/MAX145
DOUT valid timing characteristic. Data should be clocked into the P on SCLK's rising edge. 4) Pull CS/SHDN high at or after the 16th falling clock edge. If CS/SHDN remains low, trailing zeros will be clocked out after the LSB. 5) With CS/SHDN high, wait at least 60ns (tCS) before starting a new conversion by pulling CS/SHDN low. A conversion can be aborted by pulling CS/SHDN high before the conversion ends; wait at least 60ns before starting a new conversion. Data can be output in two 8-bit sequences or continuously. The bytes will contain the result of the conversion padded with three leading ones and the channel identification before the MSB. If the serial clock hasn't been idled after the last LSB and CS/SHDN is kept low, DOUT sends trailing zeros.
SPI and MICROWIRE Interface When using SPI (Figure 8a) or MICROWIRE (Figure 8b) interfaces, set CPOL = 0 and CPHA = 0. Conversion begins with a falling edge on CS/SHDN (Figure 8c). Two consecutive 8-bit readings are necessary to obtain the entire 12-bit result from the ADC. DOUT data transitions on the serial clock's falling edge and is clocked into the P on SCLK's rising edge. The first 8-bit data stream contains three leading ones, the channel identi-
CS/SHDN
***
tSCLKS
tCL
tCH
tCS
SCLK
***
tDV DOUT HIGH-2 ***
tDO
tTR HIGH-2
Figure 7. Detailed Serial-Interface Timing Sequence
I/O SCK SPI MISO VDD
CS/SHDN SCLK DOUT MICROWIRE
I/O SK SI
CS/SHDN SCLK DOUT
SS
MAX144 MAX145
MAX144 MAX145
Figure 8a. SPI Connections
8b. MICROWIRE Connections
1ST BYTE READ SCLK CS/SHDN 1 2 3 4 5 6 7 8 9 10 11
2ND BYTE READ 12 13 14 15 16
HIGH-Z DOUT* CHID D11 MSB D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 LSB SAMPLING INSTANT *WHEN CS/SHDN IS HIGH, DOUT = HIGH-Z
Figure 8c. SPI/MICROWIRE Interface Timing Sequence (CPOL = CPHA = 0)
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+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX MAX144/MAX145
fication, and the first four data bits starting with the MSB. The second 8-bit data stream contains the remaining bits, D7 through D0.
QSPI Interface Using the high-speed QSPI interface with CPOL = 0 and CPHA = 0, the MAX144/MAX145 support a maximum fSCLK of 2.17MHz. The QSPI circuit in Figure 9a can be programmed to perform a conversion on each of the two channels for the MAX144. Figure 9b shows the QSPI interface timing.
CS SCK MISO QSPI VDD
CS/SHDN SCLK DOUT
SS
MAX144 MAX145
PIC16 with SSP Module and PIC17 Interface The MAX144/MAX145 are compatible with a PIC16/ PIC17 controller (C), using the synchronous serial-port (SSP) module. To establish SPI communication, connect the controller as shown in Figure 10a and configure the PIC16/PIC17 as system master by initializing its synchronous serialport control register (SSPCON) and synchronous serialport status register (SSPSTAT) to the bit patterns shown in Tables 2 and 3. In SPI mode, the PIC16/PIC17 Cs allow 8 bits of data to be synchronously transmitted and received simultaneously. Two consecutive 8-bit readings (Figure 10b) are necessary to obtain the entire 12-bit result from the ADC. DOUT data transitions on the serial clock's falling edge and is clocked into the C on SCLK's rising edge. The first 8-bit data stream contains three leading ones, the channel identification, and the first four data bits starting with the MSB. The second 8-bit data stream contains the remaining bits, D7 through D0.
Figure 9a. QSPI Connections
SCLK CS/SHDN
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
HIGH-Z DOUT CHID D11 MSB D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 LSB SAMPLING INSTANT *WHEN CS/SHDN IS HIGH, DOUT = HIGH-Z
Figure 9b. QSPI Interface Timing Sequence (CPOL = CPHA = 0)
Table 2. Detailed SSPCON Register Contents
CONTROL BIT WCOL SSPOV SSPEN CKP SSPM3 SSPM2 SSPM1 SSPM0 BIT7 BIT6 BIT5 BIT4 BIT3 BIT2 BIT1 BIT0 MAX144/MAX145 SETTINGS X X 1 0 0 0 0 1 Synchronous Serial-Port Mode Select Bit. Sets SPI master mode and selects fCLK = fOSC / 16. SYNCHRONOUS SERIAL-PORT CONTROL REGISTER (SSPCON) Write Collision Detection Bit Receive Overflow Detect Bit Synchronous Serial-Port Enable Bit. 0: Disables serial port and configures these pins as I/O port pins. 1: Enables serial port and configures SCK, SDO and SCI pins as serial port pins. Clock Polarity Select Bit. CKP = 0 for SPI master mode selection.
X = Don't care
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+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX
Table 3. Detailed SSPSTAT Register Contents
CONTROL BIT SMP CKE D/A P S R/W UA BF BIT7 BIT6 BIT5 BIT4 BIT3 BIT2 BIT1 BIT0 MAX144/MAX145 SETTINGS 0 1 X X X X X X SYNCHRONOUS SERIAL-PORT STATUS REGISTER (SSPSTAT) SPI Data Input Sample Phase. Input data is sampled at the middle of the data output time. SPI Clock Edge Select Bit. Data will be transmitted on the rising edge of the serial clock. Data Address Bit Stop Bit Start Bit Read/Write Bit Information Update Address Buffer Full Status Bit
MAX144/MAX145
X = Don't care
Layout, Grounding, and Bypassing
For best performance, use printed circuit boards (PCBs). Wire-wrap configurations are not recommended, since the layout should ensure proper separation of analog and digital traces. Run analog and digital lines anti-parallel to each other, and don't lay out digital signal paths underneath the ADC package. Use separate analog and digital PCB ground sections with only one star-point (Figure 11) connecting the two ground systems
VDD VDD
(analog and digital). For lowest-noise operation, ensure the ground return to the star ground's power supply is low impedance and as short as possible. Route digital signals far away from sensitive analog and reference inputs. High-frequency noise in the power supply VDD could influence the proper operation of the ADC's fast comparator. Bypass VDD to the star ground with a network of two parallel capacitors (0.1F and 1F) located as close as possible to the power supply pin of MAX144/ MAX145. Minimize capacitor lead length for best supply-noise rejection and add an attenuation resistor (10) if the power supply is extremely noisy.
SCLK DOUT CS/SHDN
SCK SDI I/O
MAX144 MAX145
GND
PIC16/17
GND
Figure 10a. SPI Interface Connection for a PIC16/PIC17 Controller
1ST BYTE READ SCLK CS/SHDN 1 2 3 4 5 6 7 8 9 10 11
2ND BYTE READ 12 13 14 15 16
HIGH-Z DOUT* CHID D11 MSB D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 LSB SAMPLING INSTANT *WHEN CS/SHDN IS HIGH, DOUT = HIGH-Z
Figure 10b. SPI Interface Timing with PIC16/PIC17 in Master Mode (CKE = 1, CKP = 0, SMP = 0, SSPM3-SSPM0 = 0001)
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+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX MAX144/MAX145
POWER SUPPLIES +3V R* = 10 1F +3V GND
0.1F VDD GND +3V DGND
MAX144 MAX145
* OPTIONAL FILTER RESISTOR
DIGITAL CIRCUITRY
Figure 11. Power-Supply Bypassing and Grounding
Chip Information
TRANSISTOR COUNT: 2,058 SUBSTRATE CONNECTED TO GND
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+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX
Package Information
8LUMAXD.EPS
MAX144/MAX145
______________________________________________________________________________________
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+2.7V, Low-Power, 2-Channel, 108ksps, Serial 12-Bit ADCs in 8-Pin MAX MAX144/MAX145
Package Information (continued)
PDIPN.EPS
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 1998 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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