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Final Electrical Specifications
LTC2401/LTC2402 1-/2-Channel 24-Bit Power No Latency TMADC in MSOP-10
FEATURES
s s s s
DESCRIPTIO
January 2000
s s s s s s s
s s
s s
24-Bit ADC in Tiny MSOP-10 Package 1- or 2-Channel Inputs Automatic Channel Selection (Ping-Pong) (LTC2402) Zero Scale and Full Scale Set for Reference and Ground Sensing 4ppm INL, No Missing Codes 4ppm Full-Scale Error 0.5ppm Offset 0.6ppm Noise Internal Oscillator--No External Components Required 110dB Min, 50Hz/60Hz Notch Filter Single Conversion Settling Time for Multiplexed Applications Reference Input Voltage: 0.1V to VCC Live Zero--Extended Input Range Accommodates 12.5% Overrange and Underrange Single Supply 2.7V to 5.5V Operation Low Supply Current (200A) and Auto Shutdown
The LTC(R)2401/LTC2402 are 1- and 2-channel 2.7V to 5.5V micropower 24-bit analog-to-digital converters with an integrated oscillator, 4ppm INL and 0.6ppm RMS noise. These ultrasmall devices use delta-sigma technology and a new digital filter architecture that settles in a single cycle. This eliminates the latency found in conventional converters and simplifies multiplexed applications. Through a single pin, the LTC2401/LTC2402 can be configured for better than 110dB rejection at 50Hz or 60Hz 2%, or can be driven by an external oscillator for a user defined rejection frequency in the range 1Hz to 120Hz. The internal oscillator requires no external frequency setting components. These converters accept an external reference voltage from 0.1V to VCC. With an extended input conversion range of -12.5% VREF to 112.5% VREF (VREF = FSSET - ZSSET), the LTC2401/LTC2402 smoothly resolve the offset and overrange problems of preceding sensors or signal conditioning circuits. The LTC2401/LTC2402 communicate through a 2- or 3-wire digital interface that is compatible with SPI and MICROWIRETM protocols.
, LTC and LT are registered trademarks of Linear Technology Corporation. No Latency is a trademark of Linear Technology Corporation. MICROWIRE is a trademark of National Semiconductor Corporation.
APPLICATIO S
s s s s s s s
Weight Scales Direct Temperature Measurement Gas Analyzers Strain-Gage Transducers Instrumentation Data Acquisition Industrial Process Control
TYPICAL APPLICATIO
2.7V TO 5.5V 1F 1 VCC LTC2402 REFERENCE VOLTAGE ZSSET + 0.1V TO VCC ANALOG INPUT RANGE -0.12VREF TO 1.12VREF (VREF = FSSET - ZSSET) 0V TO FSSET - 100mV 2 3 4 5 FSSET CH1 CH0 ZSSET SCK SDO CS GND FO
Pseudo Differential Bridge Digitizer
2.7V TO 5.5V
VCC
10
= INTERNAL OSC/50Hz REJECTION = EXTERNAL CLOCK SOURCE = INTERNAL OSC/60Hz REJECTION
9
8 7 6
3-WIRE SPI INTERFACE
24012 TA01
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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1 2 4 3 5 VCC LTC2402 FSSET 9 SCK CH0 CH1 ZSSET GND 6 SDO CS FO 8 7 10
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3-WIRE SPI INTERFACE
INTERNAL OSCILLATOR 60Hz REJECTION
24012TA02
1
LTC2401/LTC2402
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (VCC) to GND .......................- 0.3V to 7V Analog Input Voltage to GND ....... - 0.3V to (VCC + 0.3V) Reference Input Voltage to GND .. - 0.3V to (VCC + 0.3V) Digital Input Voltage to GND ........ - 0.3V to (VCC + 0.3V) Digital Output Voltage to GND ..... - 0.3V to (VCC + 0.3V)
PACKAGE/ORDER INFORMATION
ORDER PART NUMBER
TOP VIEW VCC FSSET VIN NC ZSSET 1 2 3 4 5 10 9 8 7 6 FO SCK SDO CS GND TOP VIEW
LTC2401CMS LTC2401IMS MS10 PART MARKING LTMB LTMC
MS10 PACKAGE 10-LEAD PLASTIC MSOP TJMAX = 125C, JA = 130C/W
Consult factory for Military grade parts.
CONVERTER CHARACTERISTICS The q denotes specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25C. VREF = FSSET - ZSSET. (Notes 3, 4)
PARAMETER Resolution No Missing Codes Resolution Integral Nonlinearity Offset Error Offset Error Drift Full-Scale Error Full-Scale Error Drift Total Unadjusted Error Output Noise Normal Mode Rejection 60Hz 2% Normal Mode Rejection 50Hz 2% Power Supply Rejection, DC Power Supply Rejection, 60Hz 2% Power Supply Rejection, 50Hz 2% 0.1V FSSET VCC, ZSSET = 0V (Note 5) FSSET = 2.5V, ZSSET = 0V (Note 6) FSSET = 5V, ZSSET = 0V (Note 6) 2.5V FSSET VCC, ZSSET = 0V 2.5V FSSET VCC, ZSSET = 0V 2.5V FSSET VCC, ZSSET = 0V 2.5V FSSET VCC, ZSSET = 0V FSSET = 2.5V, ZSSET = 0V FSSET = 5V, ZSSET = 0V VIN = 0V (Note 13) (Note 7) (Note 8) FSSET = 2.5V, ZSSET = 0V, VIN = 0V FSSET = 2.5V, ZSSET = 0V, VIN = 0V, (Note 7) FSSET = 2.5V, ZSSET = 0V, VIN = 0V, (Note 8)
q q q
CONDITIONS
q q q q q
2
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WW
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(Notes 1, 2)
Operating Temperature Range LTC2401/LTC2402C ................................ 0C to 70C LTC2401/LTC2402I ............................ - 40C to 85C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
ORDER PART NUMBER
VCC FSSET CH1 CH0 ZSSET 1 2 3 4 5 10 9 8 7 6 FO SCK SDO CS GND
LTC2402CMS LTC2402IMS MS10 PART MARKING LTMD LTME
MS10 PACKAGE 10-LEAD PLASTIC MSOP TJMAX = 125C, JA = 130C/W
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MIN 24 24
TYP
MAX
UNITS Bits Bits
2 4 0.5 0.01 4 0.04 5 10 3 110 110 130 130 100 110 110
10 15 2 10
ppm of VREF ppm of VREF ppm of VREF ppm of VREF/C ppm of VREF ppm of VREF/C ppm of VREF ppm of VREF VRMS dB dB dB dB dB
LTC2401/LTC2402
A ALOG I PUT A D REFERE CE
SYMBOL VIN FSSET ZSSET CS(IN) CS(REF) IIN(LEAK) IREF(LEAK) PARAMETER Input Voltage Range Full-Scale Set Range Zero-Scale Set Range Input Sampling Capacitance Reference Sampling Capacitance Input Leakage Current Reference Leakage Current CS = VCC
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VREF = FSSET - ZSSET. (Note 3)
CONDITIONS (Note 14)
q q q
VREF = 2.5V, CS = VCC
DIGITAL I PUTS A D DIGITAL OUTPUTS
SYMBOL VIH VIL VIH VIL IIN IIN CIN CIN VOH VOL VOH VOL IOZ PARAMETER High Level Input Voltage CS, FO Low Level Input Voltage CS, FO High Level Input Voltage SCK Low Level Input Voltage SCK Digital Input Current CS, FO Digital Input Current SCK Digital Input Capacitance CS, FO Digital Input Capacitance SCK High Level Output Voltage SDO Low Level Output Voltage SDO High Level Output Voltage SCK Low Level Output Voltage SCK High-Z Output Leakage SDO (Note 9) IO = - 800A IO = 1.6mA IO = - 800A (Note 10) IO = 1.6mA (Note 10) CONDITIONS 2.7V VCC 5.5V 2.7V VCC 3.3V 4.5V VCC 5.5V 2.7V VCC 5.5V
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 3)
MIN
q q q q q q
2.7V VCC 5.5V (Note 9) 2.7V VCC 3.3V (Note 9) 4.5V VCC 5.5V (Note 9) 2.7V VCC 5.5V (Note 9) 0V VIN VCC 0V VIN VCC (Note 9)
POWER REQUIRE E TS
SYMBOL VCC ICC PARAMETER Supply Voltage Supply Current Conversion Mode Sleep Mode
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 3)
CONDITIONS
q
CS = 0V (Note 12) CS = VCC (Note 12)
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MIN - 0.125 * VREF 0.1 + ZSSET 0
TYP
MAX 1.125 * VREF VCC FSSET - 0.1
UNITS V V V pF pF
10 15
q q
-10 - 12
1 1
10 12
nA nA
TYP
MAX
UNITS V V
2.5 2.0 0.8 0.6 2.5 2.0 0.8 0.6 -10 -10 10 10 10 10
V V V V V V A A pF pF V
q q q q q
VCC - 0.5 0.4 VCC - 0.5 0.4 -10 10
V V V A
MIN 2.7
TYP
MAX 5.5
UNITS V A A
q q
200 20
300 30
3
LTC2401/LTC2402 TI I G CHARACTERISTICS
SYMBOL fEOSC tHEO tLEO tCONV PARAMETER External Oscillator Frequency Range External Oscillator High Period External Oscillator Low Period Conversion Time FO = 0V FO = VCC External Oscillator (Note 11) Internal Oscillator (Note 10) External Oscillator (Notes 10, 11) (Note 10) (Note 9) (Note 9) (Note 9) Internal Oscillator (Notes 10, 12) External Oscillator (Notes 10, 11) (Note 9)
q q q q q q q q
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 3)
CONDITIONS
q q q q q q
fISCK DISCK fESCK tLESCK tHESCK tDOUT_ISCK tDOUT_ESCK t1 t2 t3 t4 tKQMAX tKQMIN t5 t6
Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. Note 2: All voltage values are with respect to GND. Note 3: VCC = 2.7 to 5.5V unless otherwise specified. Input source resistance = 0. Note 4: Internal Conversion Clock source with the FO pin tied to GND or to VCC or to external conversion clock source with fEOSC = 153600Hz unless otherwise specified. Note 5: Guaranteed by design, not subject to test. Note 6: Integral nonlinearity is defined as the deviation of a code from a straight line passing through the actual endpoints of the transfer curve. The deviation is measured from the center of the quantization band. Note 7: FO = 0V (internal oscillator) or fEOSC = 153600Hz 2% (external oscillator). Note 8: FO = VCC (internal oscillator) or fEOSC = 128000Hz 2% (external oscillator).
4
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MIN 2.56 0.5 0.5
TYP
MAX 307.2 390 390
UNITS kHz s s ms ms ms kHz kHz
130.66 133.33 136 156.80 160 163.20 20480/fEOSC (in kHz) 19.2 fEOSC/8 45 250 250 1.64 1.67 1.70 256/fEOSC (in kHz) 32/fESCK (in kHz) 0 0 0 50 200 15 50 50 150 150 150 55 2000
Internal SCK Frequency Internal SCK Duty Cycle External SCK Frequency Range External SCK Low Period External SCK High Period Internal SCK 32-Bit Data Output Time External SCK 32-Bit Data Output Time CS to SDO Low Z CS to SDO High Z CS to SCK CS to SCK SCK to SDO Valid SDO Hold After SCK SCK Set-Up Before CS SCK Hold After CS
% kHz ns ns ms ms ms ns ns ns ns ns ns ns ns
(Note 10) (Note 9) (Note 5)
q q q q q q
Note 9: The converter is in external SCK mode of operation such that the SCK pin is used as digital input. The frequency of the clock signal driving SCK during the data output is fESCK and is expressed in kHz. Note 10: The converter is in internal SCK mode of operation such that the SCK pin is used as digital output. In this mode of operation, the SCK pin has a total equivalent load capacitance CLOAD = 20pF. Note 11: The external oscillator is connected to the FO pin. The external oscillator frequency, fEOSC, is expressed in kHz. Note 12: The converter uses the internal oscillator. FO = 0V or FO = VCC. Note 13: The output noise includes the contribution of the internal calibration operations. Note 14: For reference voltage values VREF > 2.5V, the extended input of - 0.125 * VREF to 1.125 * VREF is limited by the absolute maximum rating of the Analog Input Voltage pin (Pin 3). For 2.5V < VREF 0.267V + 0.89 * VCC, the input voltage range is - 0.3V to 1.125 * VREF. For 0.267V + 0.89 * VCC < VREF VCC, the input voltage range is - 0.3V to VCC + 0.3V.
LTC2401/LTC2402
PIN FUNCTIONS
VCC (Pin 1): Positive Supply Voltage. Bypass to GND (Pin 4) with a 10F tantalum capacitor in parallel with 0.1F ceramic capacitor as close to the part as possible. FSSET (Pin 2): Full-Scale Set Input. This pin defines the full-scale input value. When VIN = FSSET, the ADC outputs full scale (FFFFFH). The total reference voltage is FSSET - ZSSET. CH0, CH1 (Pins 4, 3): Analog Input Channels. The input voltage range is - 0.125 * VREF to 1.125 * VREF. For VREF > 2.5V, the input voltage range may be limited by the absolute maximum rating of - 0.3V to VCC + 0.3V. Conversions are performed alternately between CH0 and CH1 for the LTC2402. Pin 4 is a No Connect (NC) on the LTC2401. ZSSET (Pin 5): Zero-Scale Set Input. This pin defines the zero-scale input value. When VIN = ZSSET, the ADC outputs zero scale (00000H). GND (Pin 6): Ground. Shared pin for analog ground, digital ground, reference ground and signal ground. Should be connected directly to a ground plane through a minimum length trace or it should be the single-point-ground in a single-point grounding system. CS (Pin 7): Active LOW Digital Input. A LOW on this pin enables the SDO digital output and wakes up the ADC. Following each conversion, the ADC automatically enters the Sleep mode and remains in this low power state as long as CS is HIGH. A LOW on CS wakes up the ADC. A LOW-to-HIGH transition on this pin disables the SDO digital output. A LOW-to-HIGH transition on CS during the Data Output transfer aborts the data transfer and starts a new conversion. SDO (Pin 8): Three-State Digital Output. During the data output period, this pin is used for serial data output. When the chip select CS is HIGH (CS = VCC), the SDO pin is in a high impedance state. During the Conversion and Sleep periods, this pin can be used as a conversion status output. The conversion status can be observed by pulling CS LOW. SCK (Pin 9): Bidirectional Digital Clock Pin. In the Internal Serial Clock Operation mode, SCK is used as digital output for the internal serial interface clock during the data output period. In the External Serial Clock Operation mode, SCK is used as digital input for the external serial interface. An internal pull-up current source is automatically activated in Internal Serial Clock Operation mode. The Serial Clock mode is determined by the level applied to SCK at power up and the falling edge of CS. FO (Pin 10): Frequency Control Pin. Digital input that controls the ADC's notch frequencies and conversion time. When the FO pin is connected to VCC (FO = VCC), the converter uses its internal oscillator and the digital filter's first null is located at 50Hz. When the FO pin is connected to GND (FO = 0V), the converter uses its internal oscillator and the digital filter's first null is located at 60Hz. When FO is driven by an external clock signal with a frequency fEOSC, the converter uses this signal as its clock and the digital filter first null is located at a frequency fEOSC/2560.
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LTC2401/LTC2402
APPLICATIO S I FOR ATIO
Output Data Format
The LTC2401/LTC2402 serial output data stream is 32 bits long. The first 4 bits represent status information indicating the sign, selected channel, input range and conversion state. The next 24 bits are the conversion result, MSB first. The remaining 4 bits are sub LSBs beyond the 24-bit level that may be included in averaging or discarded without loss of resolution. Bit 31 (first output bit) is the end of conversion (EOC) indicator. This bit is available at the SDO pin during the conversion and sleep states whenever the CS pin is LOW. This bit is HIGH during the conversion and goes LOW when the conversion is complete. Bit 30 (second output bit) is LOW if the last conversion was performed on CH0 and HIGH for CH1. Bit 29 (third output bit) is the conversion result sign indicator (SIG). If VIN is >0, this bit is HIGH. If VIN is <0, this bit is LOW. The sign bit changes state during the zero code. Bit 28 (forth output bit) is the extended input range (EXR) indicator. If the input is within the normal input range 0 VIN VREF, this bit is LOW. If the input is outside the normal input range, VIN > VREF or VIN < 0, this bit is HIGH. The function of these bits is summarized in Table 1.
CS
BIT 31 SDO Hi-Z EOC
BIT 30 CH0/CH1
BIT 29 SIG
SCK
1 SLEEP
2
Figure 1. Output Data Timing
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Table 1. LTC2401/LTC2402 Status Bits
Input Range VIN > VREF 0 < VIN VREF VIN = 0+/0 - VIN < 0 Bit 31 EOC 0 0 0 0 Bit 30 CH0/CH1 0/1 0/1 0/1 0/1 Bit 29 SIG 1 1 1/0 0 Bit 28 EXR 1 0 0 1
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Bit 27 (fifth output bit) is the most significant bit (MSB). Bits 27-4 are the 24-bit conversion result MSB first. Bit 4 is the least significant bit (LSB). Bits 3-0 are sub LSBs below the 24-bit level. Bits 3-0 may be included in averaging or discarded without loss of resolution. Data is shifted out of the SDO pin under control of the serial clock (SCK), see Figure 1. Whenever CS is HIGH, SDO remains high impedance and any SCK clock pulses are ignored by the internal data out shift register. In order to shift the conversion result out of the device, CS must first be driven LOW. EOC is seen at the SDO pin of the device once CS is pulled LOW. EOC changes real time from HIGH to LOW at the completion of a conversion. This signal may be used as an interrupt for an external microcontroller. Bit 31 (EOC) can be captured on the first rising edge of SCK. Bit 30 is shifted out of the device on the first
BIT 28 EXT
BIT 27 MSB
BIT 4 LSB24
BIT 0
3
4
5
27
28
32 CONVERSION
24012 F01
DATA OUTPUT
LTC2401/LTC2402
APPLICATIO S I FOR ATIO
falling edge of SCK. The final data bit (Bit 0) is shifted out on the falling edge of the 31st SCK and may be latched on the rising edge of the 32nd SCK pulse. On the falling edge of the 32nd SCK pulse, SDO goes HIGH indicating a new conversion cycle has been initiated. This bit serves as EOC (Bit 31) for the next conversion cycle. Table 2 summarizes the output data format. As long as the voltage on the VIN pin is maintained within the - 0.3V to (VCC + 0.3V) absolute maximum operating range, a conversion result is generated for any input value from - 0.125 * VREF to 1.125 * VREF. For input voltages greater than 1.125 * VREF, the conversion result is clamped to the value corresponding to 1.125 * VREF. For input voltages below - 0.125 * VREF, the conversion result is clamped to the value corresponding to - 0.125 * VREF. Single Ended Half-Bridge Digitizer with Reference and Ground Sensing Sensors convert real world phenomena (temperature, pressure, gas levels, etc.) into a voltage. Typically, this voltage is generated by passing an excitation current
Table 2. LTC2401/LTC2402 Output Data Format
Input Voltage VIN > 9/8 * VREF 9/8 * VREF VREF + 1LSB VREF 3/4VREF + 1LSB 3/4VREF 1/2VREF + 1LSB 1/2VREF 1/4VREF + 1LSB 1/4VREF 0+/0 - -1LSB -1/8 * VREF VIN < -1/8 * VREF Bit 31 EOC 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Bit 30 CH SELECT CH0/CH1 CH0/CH1 CH0/CH1 CH0/CH1 CH0/CH1 CH0/CH1 CH0/CH1 CH0/CH1 CH0/CH1 CH0/CH1 CH0/CH1 CH0/CH1 CH0/CH1 CH0/CH1 Bit 29 SIG 1 1 1 1 1 1 1 1 1 1 1/0** 0 0 0 Bit 28 EXR 1 1 1 0 0 0 0 0 0 0 0 1 1 1 Bit 27 MSB 0 0 0 1 1 1 1 0 0 0 0 1 1 1
*The sub LSBs are valid conversion results beyond the 24-bit level that may be included in averaging or discarded without loss of resolution. **The sign bit changes state during the 0 code.
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through the sensor. The wires connecting the sensor to the ADC form parasitic resistors RP1 and RP2. The excitation current also flows through parasitic resistors RP1 and RP2, as shown in Figure 2. The voltage drop across these parasitic resistors leads to systematic offset and full-scale errors. In order to eliminate the errors associated with these parasitic resistors, the LTC2401/LTC2402 include a fullscale set input (FSSET) and a zero-scale set input (ZSSET). As shown in Figure 3, the FSSET pin acts as a zero input full-scale sense input. Errors due to parasitic resistance RP1 in series with the half-bridge sensor are
RP1
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+ V - FULL-SCALE ERROR + SENSOR OUTPUT -
IEXCITATION
SENSOR
RP2
+ V - OFFSET ERROR
24012 F02
Figure 2. Errors Due to Excitation Currents
Bit 26 0 0 0 1 1 0 0 1 1 0 0 1 1 1
Bit 25 0 0 0 1 0 1 0 1 0 1 0 1 1 1
Bit 24 1 1 0 1 0 1 0 1 0 1 0 1 0 0
Bit 23 1 1 0 1 0 1 0 1 0 1 0 1 0 0
... ... ... ... ... ... ... ... ... ... ... ... ... ... ...
Bit 4 LSB 1 1 0 1 0 1 0 1 0 1 0 1 0 0
Bit 3-0 SUB LSBs* X X X X X X X X X X X X X X
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LTC2401/LTC2402
APPLICATIO S I FOR ATIO
removed by the FSSET input to the ADC. The absolute fullscale output of the ADC (data out = FFFFFFHEX ) will occur at VIN = VB = FSSET, see Figure 4. Similarly, the offset errors due to RP2 are removed by the ground sense input ZSSET. The absolute zero output of the ADC (data out = 000000HEX) occurs at VIN = VA = ZSSET. Parasitic resistors RP3 to RP5 have negligible errors due to the 1nA (typ) leakage current at pins FSSET, ZSSET and VIN. The wide dynamic input range (- 300mV to 5.3V) and low noise (0.6ppm RMS) enable the LTC2401 or the LTC2402 to directly digitize the output of the bridge sensor.
RP1 VB
VCC IDC = 0 RP3 IDC = 0 2 LTC2401 FSSET SCK VIN SDO CS 5 6 ZSSET GND FO 10
24012 F03
9 8 7 3-WIRE SPI INTERFACE
IEXCITATION RP4 IDC = 0 VA RP2 RP5
3
ADC DATA OUT
1
Figure 3. Half-Bridge Digitizer with Zero-Scale and Full-Scale Sense
12k COLD JUNCTION THERMISTOR 100
+
THERMOCOUPLE
Figure 5. Isolated Temperature Measurement
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The LTC2402 is ideal for applications requiring continuous monitoring of two input sensors. As shown in Figure 5, the LTC2402 can monitor both a thermocouple temperature probe and a cold junction temperature sensor. Absolute temperature measurements can be performed with a variety of thermocouples using digital cold junction compensation. The selection between CH0 and CH1 is automatic. Initially, after power-up, a conversion is performed on CH0. For each subsequent conversion, the input channel selection
12.5% EXTENDED RANGE FFFFFH 00000H 12.5% EXTENDED RANGE ZSSET VIN
24012 F04
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FSSET
Figure 4. Transfer Curve with Zero-Scale and Full-Scale Set
2.7V TO 5.5V LTC2402 1 2 3 4 5 VCC FSSET CH1 CH0 ZSSET FO SCK SDO CS GND 10 9 8 7 6 PROCESSOR
24012 F05
-
ISOLATION BARRIER
LTC2401/LTC2402
APPLICATIO S I FOR ATIO
is alternated. Embedded within the serial data output is a status bit indicating which channel corresponds to the conversion result. If the conversion was performed on CH0, this bit (Bit 30) is LOW and is HIGH if the conversion was performed on CH1 (see Figure 6). There are no extra control or status pins required to perform the alternating 2-channel measurements. The LTC2402 only requires two digital signals (SCK and SDO). This simplification is ideal for isolated temperature measurements or systems where minimal control signals are available. Pseudo Differential Applications Generally, designers choose fully differential topologies for several reasons. First, the interface to a 4- or 6-wire bridge is simple (it is a differential output). Second, they require good rejection of line frequency noise. Third, they
SCK
***
SDO EOC CH1
CH1 DATA OUT EOC CH0
Figure 6. Embedded Selected Channel Indicator
350
350
Figure 7. Pseudo Differential Strain Guage Application
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typically look at a small differential signal sitting on a large common mode voltage; they need accurate measurements of the differential signal independent of the common mode input voltage. Many applications currently using fully differential analog-to-digital converters for any of the above reasons may migrate to a pseudo differential conversion using the LTC2402. Direct Connection to a Full Bridge The LTC2402 interfaces directly to a 4- or 6-wire bridge, as shown in Figure 7. Like the LTC2401, the LTC2402 includes a FSSET and a ZSSET for sensing the excitation voltage directly across the bridge. This eliminates errors due to excitation currents flowing through parasitic resistors. The LTC2402 also includes two single ended input channels which can tie directly to the differential output of the bridge. The two conversion results may be digitally subtracted yielding the differential result.
***
CH0 DATA OUT
24012 F06
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IEXCITATION IDC = 0 2
5V 1 VCC
350 3 4 350 IDC = 0 5
FSSET LTC2402 9 SCK CH1 CH0 SDO CS FO ZSSET GND
24012 F07
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3-WIRE SPI INTERFACE
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LTC2401/LTC2402
APPLICATIO S I FOR ATIO
The LTC2402's single ended rejection of line frequencies (2%) and harmonics is better than 110dB. Since the device performs two independent single ended conversions each with > 110dB rejection, the overall common mode and differential rejection is much better than the 80dB rejection typically found in other differential input delta-sigma converters. In addition to excellent rejection of line frequency noise, the LTC2402 also exhibits excellent single ended noise rejection over a wide range of frequencies due to its 4th order sinc filter. Each single ended conversion independently rejects high frequency noise (> 60Hz). Care must be taken to insure noise at frequencies below 15Hz and at multiples of the ADC sample rate (15,600Hz) are not present. For this application, it is recommended the LTC2402 is placed in close proximity to the bridge sensor in order to reduce the noise injected into the ADC input. By performing three successive conversions (CH0-CH1-CH0), the drift and low frequency noise can be measured and compensated for digitally. The absolute accuracy (less than 10 ppm total error) of the LTC2402 enables extremely accurate measurement of small signals sitting on large voltages. Each of the two pseudo differential measurements performed by the LTC2402 is absolutely accurate independent of the common mode voltage output from the bridge. The pseudo differential result obtained from digitally subtracting the two single ended conversion results is accurate to within
IEXCITATION = 200A
+ Pt VRTD 100 -
IEXCITATION = 200A
Figure 8. RTD Remote Temperature Measurement
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the noise level of the device (3VRMS) divided by square root of 2, independent of the common mode input voltage. Typically, a bridge sensor outputs 2mV/V full scale. With a 5V excitation, this translates to a full-scale output of 10mV. Divided by the RMS noise of 4.2V(= 3V * 1.414), this circuit yields 2,300 counts with no averaging or amplification. If more counts are required, several conversions may be averaged (the number of effective counts is increased by a factor of square root of 2 for each doubling of averages). An RTD Temperature Digitizer RTDs used in remote temperature measurements often have long lead lengths between the ADC and RTD sensor. These long lead lengths lead to voltage drops due to excitation current in the interconnect to the RTD. This voltage drop can be measured and digitally removed using the LTC2402 (see Figure 8). The excitation current (typically 200A) flows from the ADC through a long lead length to the remote temperature sensor (RTD). This current is applied to the RTD, whose resistance changes as a function of temperature (100 to 400 for 0C to 800C). The same excitation current flows back to the ADC ground and generates another voltage drop across the return leads. In order to get an accurate measurement of the temperature, these voltage drops must be measured and removed from the conversion result. Assuming the resistance is approximately the same
5V 1 2 VCC FSSET LTC2402 4 SCK CH0 SDO CS CH1 FO ZSSET GND
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R1 3
IDC = 0 R2
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LTC2401/LTC2402
APPLICATIO S I FOR ATIO
for the forward and return paths (R1 = R2), the auxiliary channel on the LTC2402 can measure this drop. These errors are then removed with simple digital correction. The result of the first conversion on CH0 corresponds to an input voltage of VRTD + R1 * IEXCITATION. The result of the second conversion (CH1) is - R1 * IEXCITATION. Note, the LTC2402's input range is not limited to the supply rails, it has underrange capabilities. The device's input range is - 300mV to VREF + 300mV. Adding the two conversion results together, the voltage drop across the RTD's leads are cancelled and the final result is VRTD. An Isolated, 24-Bit Data Acquisition System The LTC1535 is useful for signal isolation. Figure 9 shows a fully isolated, 24-bit differential input A/D converter implemented with the LTC1535 and LTC2402. Power on the isolated side is regulated by an LT1761-5.0 low noise, low dropout micropower regulator. Its output is suitable for driving bridge circuits and for ratiometric applications.
1/2 BAT54C
T1
1/2 BAT54C "SDO" RO ST1 RE DE DI VCC1 10F 10V TANT ST2 LTC1535 G1 G2 VCC2 A B Y Z
"SCK" LOGIC 5V
+
1
1
1
2
ISOLATION BARRIER
T1 = COILTRONICS CTX02-14659 OR SIEMENS B78304-A1477-A3
Figure 9. Complete, Isolated 24-Bit Data Acquisition System
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During power-up, the LTC2402 becomes active at VCC = 2.3V, while the isolated side of the LTC1535 must wait for VCC2 to reach its undervoltage lockout threshold of 4.2V. Below 4.2V, the LTC1535's driver outputs Y and Z are in a high impedance state, allowing the 1k pull-down to define the logic state at SCK. When the LTC2402 first becomes active, it samples SCK; a logic "0" provided by the 1k pull-down invokes the external serial clock mode. In this mode, the LTC2402 is controlled by a single clock line from the nonisolated side of the barrier, through the LTC1535's driver output Y. The entire power-up sequence, from the time power is applied to VCC1 until the LT1761's output has reached 5V, is approximately 1ms. Data returns to the nonisolated side through the LTC1535's receiver at RO. An internal divider on receiver input B sets a logic threshold of approximately 3.4V at input A, facilitating communications with the LTC2402's SDO output without the need for any external components.
LT1761-5
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+
10F 16V TANT 1F
IN SHDN
OUT 10F BYP
+
GND
10F 10V TANT
2
+
2
10F 10V TANT LTC2402 FO SCK SDO CS GND VCC FSSET CH1 CH0 ZSSET
10F CERAMIC
2
1k
1 2
= LOGIC COMMON = FLOATING COMMON
2 2
24012 F09
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LTC2401/LTC2402
PACKAGE I FOR ATIO
0.007 (0.18) 0.021 0.006 (0.53 0.015)
* DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE ** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
RELATED PARTS
PART NUMBER LT1019 LT1025 LTC1043 LTC1050 LT1236A-5 LTC1391 LT1460 LT1461-2.5 LTC2400 LTC2404/LTC2408 DESCRIPTION Precision Bandgap Reference, 2.5V, 5V Micropower Thermocouple Cold Junction Compensator Dual Precision Instrumentation Switched Capacitor Building Block Precision Chopper Stabilized Op Amp Precision Bandgap Reference, 5V 8-Channel Multiplexer Micropower Series Reference Precision Micropower Voltage Reference 24-Bit, No Latency ADC in SO-8 4-/8-Channel, 24-Bit, No Latency ADC COMMENTS 3ppm/C Drift, 0.05% Max 80A Supply Current, 0.5C Initial Accuracy Precise Charge, Balanced Switching, Low Power No External Components 5V Offset, 1.6VP-P Noise 0.05% Max, 5ppm/C Drift Low RON: 45, Low Charge Injection Serial Interface 0.075% Max, 10ppm/C Max Drift, 2.5V, 5V and 10V Versions, MSOP, PDIP, SO-8, SOT-23 and TO-92 Packages 50A Supply Current, 3ppm/C Drift 4ppm INL, 10ppm Total Unadjusted Error, 200A 4ppm INL, 10ppm Total Unadjusted Error, 200A
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
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Dimensions in inches (millimeters) unless otherwise noted.
MS10 Package 10-Lead Plastic MSOP
(LTC DWG # 05-08-1661)
0.118 0.004* (3.00 0.102)
10 9 8 7 6
0.193 0.006 (4.90 0.15)
0.118 0.004** (3.00 0.102)
12345
0.040 0.006 (1.02 0.15) 0 - 6 TYP SEATING PLANE 0.009 (0.228) REF
0.034 0.004 (0.86 0.102)
0.0197 (0.50) BSC
0.006 0.004 (0.15 0.102)
MSOP (MS10) 1098
24012i LT/TP 0100 4K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 2000

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