LTC2493 24-Bit 2-/4-Channel DS ADC with Easy Drive Input Current Cancellation and I2C Interface Description Features Up to 2 Differential or 4 Single-Ended Inputs ■ Easy DriveTM Technology Enables Rail-to-Rail Inputs with Zero Differential Input Current ■ Directly Digitizes High Impedance Sensors with Full Accuracy ■ 2-Wire I2C Interface with 9 Addresses Plus One Global Address for Synchronization ■ 600nV RMS Noise ■ Integrated High Accuracy Temperature Sensor ■ GND to V CC Input/Reference Common Mode Range ■ Programmable 50Hz, 60Hz or Simultaneous 50Hz/ 60Hz Rejection Mode ■ 2ppm INL, No Missing Codes ■ 1ppm Offset and 15ppm Full-Scale Error ■ 2x Speed/Reduced Power Mode (15Hz Using Internal Oscillator and 80µA at 7.5Hz Output) ■ No Latency: Digital Filter Settles in a Single Cycle, Even After a New Channel Is Selected ■ Single Supply 2.7V to 5.5V Operation (0.8mW) ■ Internal Oscillator ■ Tiny 4mm × 3mm DFN Package ■ Applications ■ ■ ■ ■ Direct Sensor Digitizer Direct Temperature Measurement Instrumentation Industrial Process Control The LTC®2493 is a 4-channel (2-channel differential), 24-bit, No Latency DSTM ADC with Easy Drive technology and a 2-wire, I2C interface. The patented sampling scheme eliminates dynamic input current errors and the shortcomings of on-chip buffering through automatic cancellation of differential input current. This allows large external source impedances and rail-to-rail input signals to be directly digitized while maintaining exceptional DC accuracy. The LTC2493 includes a high accuracy temperature sensor and an integrated oscillator. This device can be configured to measure an external signal (from combinations of 4 analog input channels operating in singleended or differential modes) or its internal temperature sensor. The integrated temperature sensor offers 1/30th °C resolution and 2°C absolute accuracy. The LTC2493 allows a wide common mode input range (0V to VCC), independent of the reference voltage. Any combination of single-ended or differential inputs can be selected and the first conversion, after a new channel is selected, is valid. Access to the multiplexer output enables optional external amplifiers to be shared between all analog inputs and auto calibration continuously removes their associated offset and drift. , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. No Latency DS and Easy Drive are trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Typical Application Data Acquisition System with Temperature Compensation Integrated High Performance Temperature Sensor 5 2.7V TO 5.5V 4 4-CHANNEL MUX IN+ TEMPERATURE SENSOR 1.7k 24-BIT ∆Σ ADC WITH EASY-DRIVE IN– CH3 COM REF + 10µF REF – SDA SCL 2-WIRE I2C INTERFACE CA1 CA0 9-PIN SELECTABLE ADDRESSES FO OSC 2493 vTA01a 3 ABSOLUTE ERROR (°C) CH2 0.1µF VCC CH0 CH1 2 1 0 –1 –2 –3 –4 –5 –55 –30 –5 20 45 70 TEMPERATURE (°C) 95 120 2493 TA02 2493fa LTC2493 Absolute Maximum Ratings pIN CONFIGURATION (Notes 1, 2) Supply Voltage (VCC).................................... –0.3V to 6V Analog Input Voltage (CH0 to CH3, COM)...................–0.3V to (VCC + 0.3V) REF+, REF–. ...................................–0.3V to (VCC + 0.3V) Digital Input Voltage.......................–0.3V to (VCC + 0.3V) Digital Output Voltage....................–0.3V to (VCC + 0.3V) Operating Temperature Range LTC2493C................................................. 0°C to 70°C LTC2493I.............................................. –40°C to 85°C Storage Temperature Range.................... –65°C to 150°C FO 1 14 REF– CA0 2 13 REF+ CA1 3 SCL 4 SDA 5 10 CH2 GND 6 9 CH1 COM 7 8 CH0 12 VCC 15 11 CH3 DE PACKAGE 14-LEAD (4mm × 3mm) PLASTIC DFN TJMAX = 125°C, θJA = 37°C/W EXPOSED PAD (PIN 15) IS GND, MUST BE SOLDERED TO PCB order information LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC2493CDE#PBF LTC2493IDE#PBF LTC2493CDE#TRPBF LTC2493IDE#TRPBF 2493 2493 14-Lead (4mm × 3mm) Plastic DFN 14-Lead (4mm × 3mm) Plastic DFN 0°C to 70°C –40°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on non-standard lead based finish parts. *For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ 2493fa LTC2493 Electrical Characteristics (Normal Speed) The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4) PARAMETER CONDITIONS Resolution (No Missing Codes) 0.1V ≤ VREF ≤ VCC, –FS ≤ VIN ≤ +FS (Note 5) MIN TYP MAX UNITS Integral Nonlinearity 5V ≤ VCC ≤ 5.5V, VREF = 5V, VIN(CM) = 2.5V (Note 6) 2.7V ≤ VCC ≤ 5.5V, VREF = 2.5V, VIN(CM) = 1.25V (Note 6) ● ● 2 1 10 Offset Error 2.5V ≤ VREF ≤ VCC, GND ≤ IN+ = IN– ≤ VCC (Note 13) ● 0.5 2.5 Offset Error Drift 2.5V ≤ VREF ≤ VCC, GND ≤ IN+ = IN– ≤ VCC Positive Full-Scale Error 2.5V ≤ VREF ≤ VCC, IN+ = 0.75VREF , IN– = 0.25VREF Positive Full-Scale Error Drift 2.5V ≤ VREF ≤ VCC, IN+ = 0.75VREF , IN– = 0.25VREF Negative Full-Scale Error 2.5V ≤ VREF ≤ VCC, IN+ = 0.25VREF , IN– = 0.75VREF Negative Full-Scale Error Drift 2.5V ≤ VREF ≤ VCC, IN+ = 0.25VREF , IN– = 0.75VREF 0.1 ppm of VREF/°C Total Unadjusted Error 5V ≤ VCC ≤ 5.5V, VREF = 2.5V, VIN(CM) = 1.25V 5V ≤ VCC ≤ 5.5V, VREF = 5V, VIN(CM) = 2.5V 2.7V ≤ VCC ≤ 5.5V, VREF = 2.5V, VIN(CM) = 1.25V 15 15 15 ppm of VREF ppm of VREF ppm of VREF Output Noise 2.7V < VCC < 5.5V, 2.5V ≤ VREF ≤ VCC, GND ≤ IN+ = IN– ≤ VCC (Note 12) 0.6 µVRMS Internal PTAT Signal TA = 27°C (Note 13) 24 Bits ppm of VREF ppm of VREF µV 10 nV/°C 25 ● 0.1 ppm of VREF ppm of VREF/°C 25 ● 27.8 28.0 Internal PTAT Temperature Coefficient ppm of VREF 28.2 mV 93.5 µV/°C Electrical Characteristics (2x Speed) The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4) PARAMETER CONDITIONS Resolution (No Missing Codes) 0.1V ≤ VREF ≤ VCC, –FS ≤ VIN ≤ +FS (Note 5) MIN TYP MAX UNITS Integral Nonlinearity 5V ≤ VCC ≤ 5.5V, VREF = 5V, VIN(CM) = 2.5V (Note 6) 2.7V ≤ VCC ≤ 5.5V, VREF = 2.5V, VIN(CM) = 1.25V (Note 6) ● Offset Error 2.5V ≤ VREF ≤ VCC, GND ≤ IN+ = IN– ≤ VCC (Note 13) ● Offset Error Drift 2.5V ≤ VREF ≤ VCC, GND ≤ IN+ = IN– ≤ VCC Positive Full-Scale Error 2.5V ≤ VREF ≤ VCC, IN+ = 0.75VREF , IN– = 0.25VREF Positive Full-Scale Error Drift 2.5V ≤ VREF ≤ VCC, IN+ = 0.75VREF , IN– = 0.25VREF Negative Full-Scale Error 2.5V ≤ VREF ≤ VCC, IN+ = 0.25VREF , IN– = 0.75VREF Negative Full-Scale Error Drift 2.5V ≤ VREF ≤ VCC, IN+ = 0.25VREF , IN– = 0.75VREF 0.1 ppm of VREF/°C Output Noise 5V ≤ VCC ≤ 5.5V, VREF = 5V, GND ≤ IN+ = IN– ≤ VCC 0.85 µVRMS 24 Bits 2 1 10 0.2 2 ppm of VREF ppm of VREF mV 100 nV/°C 25 ● 0.1 ppm of VREF ppm of VREF/°C 25 ● ppm of VREF Converter Characteristics The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3) PARAMETER CONDITIONS Input Common Mode Rejection DC 2.5V ≤ VREF ≤ VCC, GND ≤ IN+ = IN– ≤ VCC (Note 5) MIN , GND ≤ IN+ = IN– ≤ V TYP MAX UNITS ● 140 dB Input Common Mode Rejection 50Hz ±2% 2.5V ≤ VREF ≤ VCC CC (Notes 5, 7) ● 140 dB Input Common Mode Rejection 60Hz ±2% 2.5V ≤ VREF ≤ VCC, GND ≤ IN+ = IN– ≤ VCC (Notes 5, 8) ● 140 dB 110 120 dB 110 120 dB , GND ≤ IN+ = IN– ≤ V Input Normal Mode Rejection 50Hz ±2% 2.5V ≤ VREF ≤ VCC CC (Notes 5, 7) ● Input Normal Mode Rejection 60Hz ±2% 2.5V ≤ VREF ≤ VCC, GND ≤ IN+ = IN– ≤ VCC (Notes 5, 8) ● , GND ≤ IN+ = IN– ≤ V Input Normal Mode Rejection 50Hz/60Hz ±2% 2.5V ≤ VREF ≤ VCC Reference Common Mode Rejection DC 2.5V ≤ VREF ≤ VCC, GND ≤ IN+ = IN– ≤ VCC (Note 5) CC (Notes 5, 9) = 2.5V, IN+ = IN– = GND Power Supply Rejection DC VREF Power Supply Rejection, 50Hz ±2%, 60Hz ±2% VREF = 2.5V, IN+ = IN– = GND (Notes 7, 8, 9) ● 87 ● 120 dB 140 dB 120 dB 120 dB 2493fa LTC2493 Analog Input and Reference The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3) SYMBOL PARAMETER IN+ Absolute/Common Mode IN+ Voltage (IN+ Corresponds to the Selected Positive Input Channel) CONDITIONS GND – 0.3V MIN VCC + 0.3V V IN– Absolute/Common Mode IN– Voltage (IN– Corresponds to the Selected Negative Input Channel) GND – 0.3V VCC + 0.3V V VIN Input Differential Voltage Range (IN+ – IN–) ● –FS +FS V FS Full Scale of the Differential Input (IN+ – IN–) ● 0.5VREF LSB Least Significant Bit of the Output Code ● FS/224 REF+ Absolute/Common Mode REF+ Voltage ● 0.1 VCC V REF– Absolute/Common Mode REF– Voltage ● GND REF+ – 0.1V V VREF Reference Voltage Range (REF+ – REF–) ● 0.1 CS(IN+) IN+ Sampling Capacitance 11 pF CS(IN–) IN– Sampling Capacitance 11 pF CS(VREF) VREF Sampling Capacitance 11 pF IDC_LEAK(IN+) IDC_LEAK(IN–) IDC_LEAK(REF+) IDC_LEAK(REF–) IN+ DC Leakage Current Sleep Mode, IN+ = GND ● –10 1 10 nA IN– DC Leakage Current Sleep Mode, IN– = GND ● –10 1 10 nA REF+ DC Leakage Current Sleep Mode, REF+ = V ● –100 1 100 nA REF– DC Leakage Current Sleep Mode, REF– = GND ● –100 1 100 nA tOPEN MUX Break-Before-Make QIRR MUX Off Isolation CC VIN = 2VP-P DC to 1.8MHz TYP MAX UNITS V VCC V 50 ns 120 dB I 2C Inputs And Digital Outputs The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3) SYMBOL PARAMETER VIH High Level Input Voltage CONDITIONS ● MIN TYP MAX VIL Low Level Input Voltage ● 0.3VCC V VIHA Low Level Input Voltage for Address Pins CA0, CA1 ● 0.05VCC V VILA High Level Input Voltage for Address Pins CA0, CA1 ● RINH Resistance from CA0, CA1 to VCC to Set Chip Address Bit to 1 ● 10 kW RINL Resistance from CA0, CA1 to GND to Set Chip Address Bit to 0 ● 10 kW RINF Resistance from CA0, CA1 to GND or VCC to Set Chip Address Bit to Float ● 2 II Digital Input Current (fO) ● –10 VHYS Hysteresis of Schmitt Trigger Inputs (Note 5) ● 0.05VCC VOL Low Level Output Voltage (SDA) I = 3mA ● tOF Output Fall Time VIH(MIN) to VIL(MAX) Bus Load CB 10pF to 400pF (Note 14) ● IIN Input Leakage (SDA, SCL) 0.1VCC ≤ VIN ≤ 0.9VCC CCAX External Capacitative Load on Chip Address Pins (CA0, CA1) for Valid Float 0.7VCC UNITS V 0.95VCC V MW 10 µA V 0.4 V 250 ns ● 1 µA ● 10 pF 20 + 0.1CB 2493fa LTC2493 Power Requirements The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3) SYMBOL PARAMETER VCC Supply Voltage ICC Supply Current CONDITIONS MIN Conversion Current (Note 11) Temperature Measurement (Note 11) Sleep Mode (Note 11) TYP 2.7 ● 160 200 1 ● ● ● MAX UNITS 5.5 V 275 300 2 µA µA µA Digital Inputs And Digital Outputs The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3) SYMBOL PARAMETER CONDITIONS MIN fEOSC External Oscillator Frequency Range (Note 16) tHEO External Oscillator High Period tLEO External Oscillator Low Period tCONV_1 Conversion Time for 1x Speed Mode 50Hz Mode 60Hz Mode Simultaneous 50Hz/60Hz Mode External Oscillator (Note 10) tCONV_2 Conversion Time for 2x Speed Mode 50Hz Mode 60Hz Mode Simultaneous 50Hz/60Hz Mode External Oscillator (Note 10) TYP MAX UNITS ● 10 4000 kHz ● 0.125 50 µs ● 0.125 50 µs ● ● ● 157.2 131 144.1 160.3 133.6 146.9 41036/fEOSC (in kHz) 163.5 136.3 149.9 ms ms ms ms ● ● ● 78.7 65.6 72.2 80.3 66.9 73.6 81.9 68.2 75.1 ms ms ms ms 20556/fEOSC (in kHz) I 2C TIMING CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3, 15) SYMBOL PARAMETER fSCL SCL Clock Frequency CONDITIONS ● MIN 0 TYP MAX UNITS 400 kHz tHD(STA) Hold Time (Repeated) Start Condition ● 0.6 µs tLOW Low Period of the SCL Pin ● 1.3 µs tHIGH High Period of the SCL Pin ● 0.6 µs tSU(STA) Set-Up Time for a Repeated Start Condition ● 0.6 µs 0 tHD(DAT) Data Hold Time ● tSU(DAT) Data Set-Up Time ● 100 tr Rise Time for SDA Signals (Note 14) ● 20 + 0.1CB 300 ns tf Fall Time for SDA Signals (Note 14) ● 20 + 0.1CB 300 ns tSU(STO) Set-Up Time for Stop Condition ● 0.6 Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: All voltage values are with respect to GND. Note 3: Unless otherwise specified: VCC = 2.7V to 5.5V VREFCM = VREF/2, FS = 0.5VREF VIN = IN+ – IN–, VIN(CM) = (IN+ – IN–)/2, where IN+ and IN– are the selected input channels. Note 4: Use internal conversion clock or external conversion clock source with fEOSC = 307.2kHz 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. 0.9 µs ns µs Note 7: 50Hz mode (internal oscillator) or fEOSC = 256kHz ±2% (external oscillator). Note 8: 60Hz mode (internal oscillator) or fEOSC = 307.2kHz ±2% (external oscillator). Note 9: Simultaneous 50Hz/60Hz mode (internal oscillator) or fEOSC = 280kHz ±2% (external oscillator). Note 10: The external oscillator is connected to the FO pin. The external oscillator frequency, fEOSC, is expressed in kHz. Note 11: The converter uses its internal oscillator. Note 12: The output noise includes the contribution of the internal calibration operations. Note 13: Guaranteed by design and test correlation. Note 14: CB = capacitance of one bus line in pF (10pF ≤ CB ≤ 400pF). Note 15: All values refer to VIH(MIN) and VIL(MAX) levels. Note 16: Refer to Applications Information section for Performance vs Data Rate graphs. 2493fa LTC2493 Typical Performance Characteristics –45°C 2 25°C 0 85°C –1 1 –45°C, 25°C, 85°C 0 –1 –2 –2 –3 –2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 INPUT VOLTAGE (V) 2 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) 4 0 25°C Total Unadjusted Error (VCC = 5V, VREF = 2.5V) 12 VCC = 5V VREF = 2.5V VIN(CM) = 1.25V FO = GND 8 85°C TUE (ppm of VREF) TUE (ppm of VREF) 8 –45°C –4 –8 2 85°C –45°C 0 –4 12 12 NUMBER OF READINGS (%) 4 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) –4 –0.25 0.25 0.75 INPUT VOLTAGE (V) –3 –2.4 –1.8 –1.2 –0.6 0 0.6 OUTPUT READING (µV) VCC = 5V TA = 25°C VREF = 5V RMS NOISE = 0.60µV VIN = 0V VIN(CM) = 2.5V 4 3 4 0 1.25 Long-Term ADC Readings 6 0 2493 G07 –0.75 2493 G06 5 8 2 1.8 85°C –45°C –12 –1.25 1.25 10,000 CONSECUTIVE READINGS RMS = 0.59µV VCC = 2.7V AVERAGE = –0.19µV VREF = 2.5V 10 VIN = 0V TA = 25°C 2 1.2 25°C 0 Noise Histogram (7.5sps) 14 6 1.25 2493 G03 –8 Noise Histogram (6.8sps) NUMBER OF READINGS (%) 4 2493 G05 14 8 –0.25 0.25 0.75 INPUT VOLTAGE (V) VCC = 2.7V VREF = 2.5V VIN(CM) = 1.25V FO = GND 8 4 –12 –1.25 2.5 10,000 CONSECUTIVE READINGS RMS = 0.60µV VCC = 5V AVERAGE = –0.69µV VREF = 5V 10 VIN = 0V TA = 25°C –0.75 Total Unadjusted Error (VCC = 2.7V, VREF = 2.5V) 25°C 2493 G04 –3 –2.4 –1.8 –1.2 –0.6 0 0.6 OUTPUT READING (µV) –1 12 –8 –12 –2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 INPUT VOLTAGE (V) –45°C, 25°C, 85°C 0 2493 G02 Total Unadjusted Error (VCC = 5V, VREF = 5V) VCC = 5V VREF = 5V VIN(CM) = 2.5V FO = GND 1 –3 –1.25 1.25 2493 G01 12 VCC = 2.7V VREF = 2.5V VIN(CM) = 1.25V FO = GND –2 –3 –1.25 2.5 Integral Nonlinearity (VCC = 2.7V, VREF = 2.5V) 2 ADC READING (µV) INL (ppm of VREF) 1 3 VCC = 5V VREF = 2.5V VIN(CM) = 1.25V FO = GND TUE (ppm of VREF) 2 3 VCC = 5V VREF = 5V VIN(CM) = 2.5V FO = GND INL (ppm of VREF) 3 Integral Nonlinearity (VCC = 5V, VREF = 2.5V) INL (ppm of VREF) Integral Nonlinearity (VCC = 5V, VREF = 5V) 2 1 0 –1 –2 –3 –4 1.2 1.8 2493 G08 –5 0 10 30 40 20 TIME (HOURS) 50 60 2493 G09 2493fa LTC2493 Typical Performance Characteristics RMS Noise vs Input Differential Voltage RMS NOISE (µV) 0.8 VCC = 5V VREF = 5V VIN(CM) = 2.5V TA = 25°C FO = GND VCC = 5V VREF = 5V VIN = 0V TA = 25°C FO = GND 0.9 0.8 0.7 0.6 0.6 0.4 2.5 –1 0 2 1 3 5 4 1.0 VREF = 2.5V VIN = 0V VIN(CM) = GND TA = 25°C FO = GND 0.6 0.7 0.6 0.5 3.5 3.9 4.3 VCC (V) 4.7 5.1 0.4 5.5 0 1 2 3 VREF (V) 4 0.1 0.3 OFFSET ERROR (ppm of VREF) OFFSET ERROR (ppm of VREF) 0.2 0 –0.1 –0.2 –0.3 –45 –30 –15 0 15 30 45 60 TEMPERATURE (°C) 75 90 2493 G16 0.2 0.1 0 –0.1 –0.2 –0.3 –1 0 1 3 2 VIN(CM) (V) 5 4 0.2 0.1 Offset Error vs VCC 0.3 REF+ = 2.5V REF– = GND VIN = 0V VIN(CM) = GND TA = 25°C FO = GND 0 Offset Error vs VREF VCC = 5V REF – = GND VIN = 0V VIN(CM) = GND TA = 25°C FO = GND 0.2 0.1 0 –0.1 –0.1 –0.2 –0.3 2.7 6 2493 G15 OFFSET ERROR (ppm of VREF) Offset Error vs Temperature VCC = 5V VREF = 5V VIN = 0V VIN(CM) = GND FO = GND VCC = 5V VREF = 5V VIN = 0V TA = 25°C FO = GND 2493 G14 2493 G13 0.3 5 90 Offset Error vs VIN(CM) 0.3 VCC = 5V VIN = 0V VIN(CM) = GND TA = 25°C FO = GND 0.8 0.5 3.1 75 2493 G12 RMS Noise vs VREF 0.9 0.7 0 15 30 45 60 TEMPERATURE (°C) 2493 G11 RMS Noise vs VCC 0.4 2.7 0.4 –45 –30 –15 6 OFFSET ERROR (ppm of VREF) RMS NOISE (µV) 0.8 0.6 VIN(CM) (V) RMS NOISE (µV) 0.9 0.7 0.5 2493 G10 1.0 0.8 0.5 0.4 –2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 2 INPUT DIFFERENTIAL VOLTAGE (V) VCC = 5V VREF = 5V VIN = 0V VIN(CM) = GND FO = GND 0.9 0.7 0.5 RMS Noise vs Temperature (TA) 1.0 RMS NOISE (µV) 0.9 RMS Noise vs VIN(CM) 1.0 RMS NOISE (µV) 1.0 –0.2 3.1 3.5 3.9 4.3 VCC (V) 4.7 5.1 5.5 2493 G17 –0.3 0 1 2 3 VREF (V) 4 5 2493 G18 2493fa LTC2493 Typical Performance Characteristics On-Chip Oscillator Frequency vs VCC 310 308 308 306 304 VCC = 4.1V VREF = 2.5V VIN = 0V VIN(CM) = GND FO = GND 300 –45 –30 –15 306 304 75 300 90 2.5 –60 –80 3.0 3.5 4.0 VCC (V) 4.5 5.0 –120 –140 30600 30650 30700 30750 FREQUENCY AT VCC (Hz) 1.6 VCC = 5V 1.0 0.8 0.6 VCC = 2.7V 0.4 0 15 30 45 60 TEMPERATURE (°C) 75 90 2493 G25 FO = GND VCC = 5V 160 VCC = 2.7V 140 120 100 –45 –30 –15 30800 0 15 30 45 60 TEMPERATURE (°C) 400 350 2 VCC = 5V VCC = 3V 0 Integral Nonlinearity (2x Speed Mode; VCC = 5V, VREF = 5V) VCC = 5V VREF = 5V VIN(CM) = 2.5V FO = GND 25°C, 85°C –1 200 100 90 1 300 250 75 2493 G24 3 –2 150 0.2 0 –45 –30 –15 VREF = VCC IN+ = GND IN– = GND FO = EXT OSC TA = 25°C 450 SUPPLY CURRENT (µA) SLEEP MODE CURRENT (µA) 500 1M 180 Conversion Current vs Output Data Rate FO = GND 1.2 10k 100k 1k 100 FREQUENCY AT VCC (Hz) 2493 G23 Sleep Mode Current vs Temperature 1.4 10 2493 G21 200 2493 G22 1.8 1 Conversion Current vs Temperature –80 –120 2.0 –140 PSRR vs Frequency at VCC –60 –100 0 20 40 60 80 100 120 140 160 180 200 220 FREQUENCY AT VCC (Hz) 5.5 VCC = 4.1V DC ±0.7V V = 2.5V –20 INREF + = GND – IN = GND –40 FO = GND TA = 25°C –100 –140 –80 CONVERSION CURRENT (µA) REJECTION (dB) –40 0 REJECTION (dB) –20 –60 2493 G20 PSRR vs Frequency at VCC VCC = 4.1V DC ±1.4V VREF = 2.5V IN+ = GND IN– = GND FO = GND TA = 25°C –40 –120 2493 G19 0 VCC = 4.1V DC ±0.7V VREF = 2.5V IN+ = GND IN– = GND FO = GND TA = 25°C –100 302 0 15 30 45 60 TEMPERATURE (°C) PSRR vs Frequency at VCC –20 INL (µV) 302 VREF = 2.5V VIN = 0V VIN(CM) = GND FO = GND TA = 25°C 0 REJECTION (dB) 310 FREQUENCY (kHz) FREQUENCY (kHz) On-Chip Oscillator Frequency vs Temperature 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) 2493 G26 –45°C –3 –2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 INPUT VOLTAGE (V) 2 2.5 2493 G27 2493fa LTC2493 Typical Performance Characteristics 3 3 VCC = 5V VREF = 2.5V VIN(CM) = 1.25V FO = GND 1 2 INL (ppm OF VREF) 2 INL (ppm OF VREF) Integral Nonlinearity (2x Speed Mode; VCC = 2.7V, VREF = 2.5V) 85°C 0 –45°C, 25°C –1 –2 16 VCC = 2.7V VREF = 2.5V VIN(CM) = 1.25V FO = GND 1 85°C 0 –45°C, 25°C –1 –2 –3 –1.25 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) –3 –1.25 1.25 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) 198 196 OFFSET ERROR (µV) RMS NOISE (µV) 0.8 0.6 0.4 VCC = 5V VIN = 0V VIN(CM) = GND FO = GND TA = 25°C 5 2493 G31 183.8 186.2 OUTPUT READING (µV) 194 230 192 190 188 186 180 188.6 2493 G30 240 VCC = 5V VREF = 5V VIN = 0V FO = GND TA = 25°C 220 VCC = 5V VREF = 5V VIN = 0V VIN(CM) = GND FO = GND 210 200 190 180 170 182 4 181.4 Offset Error vs Temperature (2x Speed Mode) 184 3 2 VREF (V) 4 0 179 1.25 OFFSET ERROR (µV) 200 1 6 Offset Error vs VIN(CM) (2x Speed Mode) 1.0 0 8 2493 G29 RMS Noise vs VREF (2x Speed Mode) 0.2 RMS = 0.85µV 10,000 CONSECUTIVE AVERAGE = 0.184mV 14 READINGS VCC = 5V 12 VREF = 5V VIN = 0V T = 25°C 10 A 2 2493 G28 0 Noise Histogram (2x Speed Mode) NUMBER OF READINGS (%) Integral Nonlinearity (2x Speed Mode; VCC = 5V, VREF = 2.5V) –1 0 1 3 VIN(CM) (V) 2 4 5 6 2493 G32 160 –45 –30 –15 0 15 30 45 60 TEMPERATURE (°C) 75 90 2493 G33 2493fa LTC2493 Typical Performance Characteristics 250 240 VREF = 2.5V VIN = 0V VIN(CM) = GND FO = GND TA = 25°C 150 100 50 PSRR vs Frequency at VCC (2x Speed Mode) 0 VCC = 5V VIN = 0V VIN(CM) = GND FO = GND TA = 25°C 230 OFFSET ERROR (µV) 200 OFFSET ERROR (µV) Offset Error vs VREF (2x Speed Mode) 220 –40 210 200 190 2 2.5 3 4 3.5 VCC (V) 4.5 5.5 5 160 1 0 2 4 3 VREF (V) –60 10 10k 100k 1k 100 FREQUENCY AT VCC (Hz) 1M 2493 G36 0 VCC = 4.1V DC ±1.4V REF+ = 2.5V REF– = GND IN+ = GND IN– = GND FO = GND TA = 25°C VCC = 4.1V DC ±0.7V REF+ = 2.5V REF– = GND IN+ = GND –40 IN– = GND FO = GND –60 TA = 25°C –20 –80 –80 –100 –100 –120 –120 –140 1 PSRR vs Frequency at VCC (2x Speed Mode) REJECTION (dB) RREJECTION (dB) –40 –140 5 2493 G35 PSRR vs Frequency at VCC (2x Speed Mode) –20 –80 –120 2493 G34 0 –60 –100 180 170 0 VCC = 4.1V DC ±0.7V REF+ = 2.5V REF– = GND IN+ = GND IN– = GND FO = GND TA = 25°C –20 REJECTION (dB) Offset Error vs VCC (2x Speed Mode) 0 20 40 60 80 100 120 140 160 180 200 220 FREQUENCY AT VCC (Hz) 2493 G37 –140 30600 30650 30700 30750 FREQUENCY AT VCC (Hz) 30800 2493 G38 2493fa 10 LTC2493 Pin Functions FO (Pin 1): Frequency Control Pin. Digital input that controls the internal conversion clock rate. When FO is connected to GND, the converter uses its internal oscillator running at 307.2kHz. The conversion clock may also be overridden by driving the FO pin with an external clock in order to change the output rate and the digital filter rejection null. CA0, CA1 (Pins 2, 3): Chip Address Control Pins. These pins are configured as a three-state (LOW, HIGH, Floating) address control bits for the device’s I2C address. SCL (Pin 4): Serial Clock Pin of the I2C Interface. The LTC2493 can only act as a slave and the SCL pin only accepts an external serial clock. Data is shifted into the SDA pin on the rising edges of the SCL clock and output through the SDA pin on the falling edges of the SCL clock. SDA (Pin 5): Bidirectional Serial Data Line of the I2C Inter- face. In the transmitter mode (Read), the conversion result is output through the SDA pin, while in the receiver mode (Write), the device channel select and configuration bits are input through the SDA pin. The pin is high impedance during the data input mode and is an open drain output (requires an appropriate pull-up device to VCC) during the data output mode. GND (Pin 6): Ground. Connect this pin to a common ground plane through a low impedance connection. COM (Pin 7): The Common Negative Input (IN –) for All Single-Ended Multiplexer Configurations. The voltage on CH0-CH3 and COM pins can have any value between GND – 0.3V to VCC + 0.3V. Within these limits, the two selected inputs (IN+ and IN– ) provide a bipolar input range (VIN = IN+ – IN– ) from –0.5 • VREF to 0.5 • VREF . Outside this input range, the converter produces unique over-range and under-range output codes. CH0 to CH3 (Pin 8-Pin 11): Analog Inputs. May be programmed for single-ended or differential mode. VCC (Pin 12): Positive Supply Voltage. Bypass to GND with a 10µF tantalum capacitor in parallel with a 0.1µF ceramic capacitor as close to the part as possible. REF+, REF – (Pin 13, Pin 14): Differential Reference Input. The voltage on these pins can have any value between GND and VCC as long as the reference positive input, REF+, remains more positive than the negative reference input, REF–, by at least 0.1V. The differential voltage (VREF = REF+ – REF –) sets the full-scale range for all input channels. Exposed Pad (Pin 15): Ground. This pin is ground and must be soldered to the PCB ground plane. For prototyping purposes, this pin may remain floating. 2493fa 11 LTC2493 FUNCTIONAL Block Diagram VCC INTERNAL OSCILLATOR TEMP SENSOR GND CH0 CH1 CH2 CH3 COM FO (INT/EXT) AUTOCALIBRATION AND CONTROL REF + REF – IN+ MUX IN– – + 1.7k DIFFERENTIAL 3RD ORDER ∆Σ MODULATOR I2C INTERFACE SDA SCL CA0 CA1 DECIMATING FIR ADDRESS 2493 BD 2493fa 12 LTC2493 Applications Information CONVERTER OPERATION Converter Operation Cycle The LTC2493 is a multichannel, low power, delta-sigma analog-to-digital converter with a 2-wire, I2C interface. Its operation is made up of four states (see Figure 1). The converter operating cycle begins with the conversion, followed by the sleep state and ends with the data input/output cycle . Initially, at power-up, the LTC2493 performs a conversion. Once the conversion is complete, the device enters the sleep state. While in the sleep state, power consumption is reduced by two orders of magnitude. The part remains in the sleep state as long it is not addressed for a read/write operation. The conversion result is held indefinitely in a static shift register while the part is in the sleep state. The device will not acknowledge an external request during the conversion state. After a conversion is finished, the device is ready to accept a read/write request. Once the LTC2493 is addressed for a read operation, the device begins outputting the conversion result under the control of the serial clock (SCL). There is no latency in the conversion result. The data output is 32 bits long and contains a 24-bit plus sign conversion result. Data is updated on the falling edges of SCL allowing the user to reliably latch data on the rising edge of SCL. A new conversion is initiated by a stop condition following a valid write operation or an incomplete read operation. The conversion automatically begins at the conclusion of a complete read cycle (all 32 bits read out of the device). Ease of Use The LTC2493 data output has no latency, filter settling delay, or redundant data associated with the conversion cycle. There is a one-to-one correspondence between the conversion and the output data. Therefore, multiplexing multiple analog inputs is straightforward. Each conversion, immediately following a newly selected input or mode, is valid and accurate to the full specifications of the device. POWER-ON RESET DEFAULT CONFIGURATION: IN+ = CH0, IN– = CH1 50Hz/60Hz REJECTION 1X OUTPUT CONVERSION SLEEP NO ACKNOWLEDGE YES DATA OUTPUT/INPUT NO STOP OR READ 32 BITS YES 2493 F01 Figure 1. State Transition Table The LTC2493 automatically performs offset and full-scale calibration every conversion cycle independent of the input channel selected. This calibration is transparent to the user and has no effect on the operation cycle described above. The advantage of continuous calibration is extreme stability of offset and full-scale readings with respect to time, supply voltage variation, input channel, and temperature drift. Easy Drive Input Current Cancellation The LTC2493 combines a high precision, delta-sigma ADC with an automatic, differential, input current cancellation front end. A proprietary front end passive sampling network transparently removes the differential input current. This enables external RC networks and high impedance sensors to directly interface to the LTC2493 without external 2493fa 13 LTC2493 Applications Information amplifiers. The remaining common mode input current is eliminated by either balancing the differential input impedances or setting the common mode input equal to the common mode reference (see the Automatic Differential Input Current Cancellation section). This unique architecture does not require on-chip buffers, thereby enabling signals to swing beyond ground and VCC. Moreover, the cancellation does not interfere with the transparent offset and full-scale auto-calibration and the absolute accuracy (full scale + offset + linearity + drift) is maintained even with external RC networks. Power-Up Sequence The LTC2493 automatically enters an internal reset state when the power supply voltage, VCC, drops below approximately 2.0V. This feature guarantees the integrity of the conversion result and input channel selection. When VCC rises above this threshold, the converter creates an internal power-on-reset (POR) signal with a duration of approximately 4ms. The POR signal clears all internal registers. The conversion immediately following a POR cycle is performed on the input channel IN+ = CH0, IN – = CH1 with simultaneous 50Hz/60Hz rejection and 1x output rate. The first conversion following a POR cycle is accurate within the specification of the device if the power supply voltage is restored to (2.7V to 5.5V) before the end of the POR interval. A new input channel, rejection mode, speed mode, or temperature selection can be programmed into the device during this first data input/output cycle. Reference Voltage Range This converter accepts a truly differential external reference voltage. The absolute/common mode voltage range for the REF+ and REF – pins covers the entire operating range of the device (GND to VCC). For correct converter operation, VREF must be positive (REF+ > REF –). The LTC2493 differential reference input range is 0.1V to VCC. For the simplest operation, REF+ can be shorted to VCC and REF – can be shorted to GND. The converter output noise is determined by the thermal noise of the front end circuits and, as such, its value in nanovolts is nearly constant with reference voltage. A decrease in reference voltage will not significantly improve the converter’s effective resolution. On the other hand, a decreased reference will improve the converter’s overall INL performance. Input Voltage Range The analog inputs are truly differential with an absolute, common mode range for the CH0-CH3 and COM input pins extending from GND – 0.3V to VCC + 0.3V. Outside these limits, the ESD protection devices begin to turn on and the errors due to input leakage current increase rapidly. Within these limits, the LTC2493 converts the bipolar differential input signal VIN = IN+ – IN– (where IN+ and IN – are the selected input channels), from –FS = –0.5 • VREF to +FS = 0.5 • VREF where VREF = REF+ - REF–. Outside this range, the converter indicates the overrange or the underrange condition using distinct output codes (see Table 1). In order to limit any fault current, resistors of up to 5kW may be added in series with the input. The effect of series resistance on the converter accuracy can be evaluated from the curves presented in the Input Current/Reference Current sections. In addition, series resistors will introduce a temperature dependent error due to input leakage current. A 1nA input leakage current will develop a 1ppm offset error on a 5k resistor if VREF = 5V. This error has a very strong temperature dependency. I2C INTERFACE The LTC2493 communicates through an I2C interface. The I2C interface is a 2-wire open-drain interface supporting multiple devices and multiple masters on a single bus. The connected devices can only pull the data line (SDA) low and can never drive it high. SDA is required to be externally 2493fa 14 LTC2493 Applications Information connected to the supply through a pull-up resistor. When the data line is not being driven, it is high. Data on the I2C bus can be transferred at rates up to 100kbits/s in the standard mode and up to 400kbits/s in the fast mode. Each device on the I2C bus is recognized by a unique address stored in that device and can operate either as a transmitter or receiver, depending on the function of the device. In addition to transmitters and receivers, devices can also be considered as masters or slaves when performing data transfers. A master is the device which initiates a data transfer on the bus and generates the clock signals to permit that transfer. Devices addressed by the master are considered a slave. The LTC2493 can only be addressed as a slave. Once addressed, it can receive configuration bits (channel selection, rejection mode, speed mode) or transmit the last conversion result. The serial clock line, SCL, is always an input to the LTC2493 and the serial data line SDA is bidirectional. The device supports the standard mode and the fast mode for data transfer speeds up to 400kbits/s. Figure 2 shows the definition of the I2C timing. The Start and Stop Conditions A Start (S) condition is generated by transitioning SDA from high to low while SCL is high. The bus is considered to be busy after the Start condition. When the data transfer is finished, a Stop (P) condition is generated by transitioning SDA from low to high while SCL is high. The bus is free after a Stop is generated. Start and Stop conditions are always generated by the master. When the bus is in use, it stays busy if a Repeated Start (Sr) is generated instead of a Stop condition. The repeated Start timing is functionally identical to the Start and is used for writing and reading from the device before the initiation of a new conversion. Data Transferring After the Start condition, the I2C bus is busy and data transfer can begin between the master and the addressed slave. Data is transferred over the bus in groups of nine bits, one byte followed by one acknowledge (ACK) bit. The master releases the SDA line during the ninth SCL clock cycle. The slave device can issue an ACK by pulling SDA low or issue a Not Acknowledge (NAK) by leaving the SDA line high impedance (the external pull-up resistor will hold the line high). Change of data only occurs while the clock line (SCL) is low. DATA FORMAT After a Start condition, the master sends a 7-bit address followed by a read/write (R/W) bit. The R/W bit is 1 for a read request and 0 for a write request. If the 7-bit address matches the hard wired LTC2493’s address (one of 9 pin-selectable addresses) the device is selected. When the device is addressed during the conversion state, it will not acknowledge R/W requests and will issue a NAK by leaving the SDA line high. If the conversion is complete, the LTC2493 issues an ACK by pulling the SDA line low. The LTC2493 has two registers. The output register (32 bits long) contains the last conversion result. The input register (16 bits long) sets the input channel, selects the temperature sensor, rejection mode, and speed mode. SDA tLOW tf tSU(DAT) tr tHD(SDA) tf tSP tBUF tr SCL S tHD(SDA) tHD(DAT) tHIGH tSU(STA) Sr tSU(STO) P S 2493 F02 Figure 2. Definition of Timing for Fast/Standard Mode Devices on the I2C Bus 2493fa 15 LTC2493 Applications Information DATA OUTPUT FORMAT binary two’s, complement format. The remaining six bits are sub LSBs below the 24-bit level. The output register contains the last conversion result. After each conversion is completed, the device automatically enters the sleep state where the supply current is reduced to 1µA. When the LTC2493 is addressed for a read operation, it acknowledges (by pulling SDA low) and acts as a transmitter. The master/receiver can read up to four bytes from the LTC2493. After a complete read operation (4 bytes), a new conversion is initiated. The device will NAK subsequent read operations while a conversion is being performed. As long as the voltage on the selected input channels (IN+ and IN–) remains between –0.3V and VCC + 0.3V (absolute maximum operating range) a conversion result is generated for any differential input voltage VIN from –FS = –0.5 • VREF to +FS = 0.5 • VREF . For differential input voltages greater than +FS, the conversion result is clamped to the value corresponding to +FS. For differential input voltages below –FS, the conversion result is clamped to the value –FS – 1LSB. The data output stream is 32 bits long and is shifted out on the falling edges of SCL (see Figure 3a). The first bit is the conversion result sign bit (SIG) (see Tables 1 and 2). This bit is high if VIN ≥ 0 and low if VIN < 0 (where VIN corresponds to the selected input signal IN+ – IN–). The second bit is the most significant bit (MSB) of the result. The first two bits (SIG and MSB) can be used to indicate over and under range conditions (see Table 2). If both bits are HIGH, the differential input voltage is equal to or above +FS. If both bits are set low, the input voltage is below –FS. The function of these bits is summarized in Table 2. The 24 bits following the MSB bit are the conversion result in Table 2. LTC2493 Status Bits Bit 31 SIG Bit 30 MSB VIN ≥ FS 1 1 0V ≤ VIN < FS 1 0 –FS ≤ VIN < 0V 0 1 VIN < –FS 0 0 Input Range INPUT DATA FORMAT The serial input word to the LTC2493 is 13 bits long and is written into the device input register in two 8-bit words. Table 1. Output Data Format Differential Input Voltage VIN* Bit 31 SIG Bit 30 MSB Bit 29 Bit 28 Bit 27 … Bit 6 LSB Bits 5-0 Sub LSBs*** VIN* ≥ FS** 1 1 0 0 0 … 0 00000 FS** – 1LSB 1 0 1 1 1 … 1 XXXXX 0.5 • FS** 1 0 1 0 0 … 0 XXXXX 0.5 • FS** – 1LSB 1 0 0 1 1 … 1 XXXXX 0 1 0 0 0 0 … 0 XXXXX –1LSB 0 1 1 1 1 … 1 XXXXX –0.5 • FS** 0 1 1 0 0 … 0 XXXXX –0.5 • FS** – 1LSB 0 1 0 1 1 … 1 XXXXX –FS** 0 1 0 0 0 … 0 XXXXX VIN* < –FS** 0 0 1 1 1 … 1 11111 *The differential input voltage VIN = IN+ – IN–. **The full-scale voltage FS = 0.5 • VREF . *** Sub LSBs are below the 24-bit level. They may be included in averaging, or discarded without loss of resolution. 2493fa 16 LTC2493 Applications Information The first word (SGL, ODD, A2, A1, A0) is used to select the input channel. The second word of data (IM, FA, FB, SPD) is used to select the frequency rejection, speed mode (1x, 2x), and temperature measurement. If the first three bits shifted into the device are 101, then the next five bits select the input channel for the next conversion cycle (see Table 3). The first input bit (SGL) following the 101 sequence determines if the input selection is differential (SGL = 0) or single-ended (SGL = 1). For SGL = 0, two adjacent channels can be selected to form a differential input. For SGL = 1, one of 4 channels is selected as the positive input. The negative input is COM for all single-ended operations. The remaining four bits (ODD, A2, A1, A0) determine which channel(s) is/are selected and the polarity (for a differential input). After power-up, the device initiates an internal reset cycle which sets the input channel to CH0-CH1 (IN+ = CH0, IN– = CH1), the frequency rejection to simultaneous 50Hz/60Hz, and 1x output rate (auto-calibration enabled). The first conversion automatically begins at power-up using this default configuration. Once the conversion is complete, up to two words may be written into the device. The first three bits of the first input word consist of two preamble bits and one enable bit. Valid settings for these three bits are 000, 100, and 101. Other combinations should be avoided. Once the first word is written into the device, a second word may be input in order to select a configuration mode. The first bit of the second word is the enable bit for the conversion configuration (EN2). If this bit is set to 0, then the next conversion is performed using the previously selected converter configuration. If the first three bits are 000 or 100, the following data is ignored (don’t care) and the previously selected input channel remains valid for the next conversion. 1 SCL … 7 8 9 1 2 … SIG MSB 9 1 2 3 4 5 6 7 8 9 SDA 7-BIT ADDRESS R D31 ACK BY LTC2493 START BY MASTER LSB ACK BY MASTER NAK BY MASTER SUB LSBs SLEEP DATA OUTPUT 2493 F03a Figure 3a. Timing Diagram for Reading from the LTC2493 1 SCL 2 … 7 8 9 1 2 3 4 5 6 7 9 8 1 2 3 4 5 6 7 8 9 SDA 7-BIT ADDRESS 1 W ACK BY LTC2493 START BY MASTER SLEEP 0 EN SGL ODD A2 A1 A0 EN2 ACK BY LTC2493 IM FA FB SPD (OPTIONAL 2ND BYTE) DATA INPUT ACK BY LTC2493 2493 F03b Figure 3b. Timing Diagram for Writing to the LTC2493 2493fa 17 LTC2493 Applications Information set the rejection frequency. The final bit (SPD) is used to select either the 1x output rate if SPD = 0 (auto-calibration is enabled and the offset is continuously calibrated and removed from the final conversion result) or the 2x output rate if SPD = 1 (offset calibration disabled, multiplexing output rates up to 15Hz with no latency). When IM = 1 (temperature measurement) SPD will be ignored and the device will operate in 1x mode. Table 3 Channel Selection MUX ADDRESS SGL ODD/ SIGN A2 A1 CHANNEL SELECTION A0 0 1 IN+ IN– *0 0 0 0 0 0 0 0 0 1 0 1 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 0 0 1 1 1 0 0 0 1 1 0 0 IN– 2 3 IN+ IN– IN– IN+ COM IN+ IN+ IN– IN+ The configuration remains valid until a new input word with EN = 1 (the first three bits are 101 for the first word) and EN2 = 1 (for the second write byte) is shifted into the device. IN– IN+ IN– IN+ 1 IN– *Default at power up Rejection Mode (FA, FB) The second set of configuration data can be loaded into the device by setting EN2 = 1 (see Table 4). The first bit (IM) is used to select the internal temperature sensor. If IM = 1, the following conversion will be performed on the internal temperature sensor rather than the selected input channel. The next two bits (FA and FB) are used to The LTC2493 includes a high accuracy on-chip oscillator with no required external components. Coupled with an integrated fourth order digital low pass filter, the LTC2493 rejects line frequency noise. In the default mode, the LTC2493 simultaneously rejects 50Hz and 60Hz by at least 87dB. If more rejection is required, the LTC2493 can be configured to reject 50Hz or 60Hz to better than 110dB. Table 4. Converter Configuration 1 0 EN SGL ODD A2 A1 A0 EN2 IM FA FB SPD CONVERTER CONFIGURATION 1 0 0 X X X X X X X X X X Keep Previous 1 0 1 X X X X X 0 X X X X Keep Previous 0 0 1 X X X X X X X X X X Keep Previous 1 0 1 X X X X X 1 0 0 0 0 External Input (See Table 3) 50Hz/60Hz Rejection, 1x 1 0 1 X X X X X 1 0 0 1 0 External Input (See Table 3) 50Hz Rejection, 1x 1 0 1 X X X X X 1 0 1 0 0 External Input (See Table 3) 60Hz Rejection, 1x 1 0 1 X X X X X 1 0 0 0 1 External Input (See Table 3) 50Hz/60Hz Rejection, 2x 1 0 1 X X X X X 1 0 0 1 1 External Input (See Table 3) 50Hz Rejection, 2x 1 0 1 X X X X X 1 0 1 0 1 External Input (See Table 3) 60Hz Rejection, 2x 1 0 1 X X X X X 1 1 0 0 X Measure Temperature 50Hz/60Hz Rejection, 1x 1 0 1 X X X X X 1 1 0 1 X Measure Temperature 50Hz Rejection, 1x 1 0 1 X X X X X 1 1 1 0 X Measure Temperature 60Hz Rejection, 1x 1 0 1 X X X X X 1 X 1 1 X Reserved, Do Not Use 2493fa 18 LTC2493 Applications Information Speed Mode (SPD) Every conversion cycle, two conversions are combined to remove the offset (default mode). This result is free from offset and drift. In applications where the offset is not critical, the auto-calibration feature can be disabled with the benefit of twice the output rate. While operating in the 2x mode (SPD = 1), the linearity and full-scale errors are unchanged from the 1x mode performance. In both the 1x and 2x mode there is no latency. This enables input steps or multiplexer changes to settle in a single conversion cycle, easing system overhead and increasing the effective conversion rate. During temperature measurements, the 1x mode is always used independent of the value of SPD. Temperature Sensor The LTC2493 includes an integrated temperature sensor. The temperature sensor is selected by setting IM = 1. The ADC internally connects to the temperature sensor and performs a conversion. The digital output is proportional to the absolute temperature of the device. This feature allows the converter to perform cold junction compensation for external thermocouples or continuously remove the temperature effects of external sensors. The internal temperature sensor output is 28mV at 27ºC (300ºK), with a slope of 93.5µV/ºC independent of VREF (see Figures 4 and 5). Slope calibration is not required if the reference voltage (VREF) is known. A 5V reference has a slope of 314 LSBs24/ºC. The temperature is calculated from the output code (where DATAOUT24 is the decimal representation of the 24-bit result) for a 5V reference using the following formula: TK = DATAOUT24 in Kelvin 314 If a different value of VREF is used, the temperature output is: DATAOUT24 • VREF TK = in Kelvin 1570 If the value of VREF is not known, the slope is determined by measuring the temperature sensor at a known temperature TN (in K) and using the following formula: SLOPE = This value of slope can be used to calculate further temperature readings using: TK = DATAOUT24 SLOPE All Kelvin temperature readings can be converted to TC (ºC) using the fundamental equation: TC = TK – 273 140000 5 VCC = 5V VREF = 5V 120000 SLOPE = 314 LSB /K 24 4 ABSOLUTE ERROR (°C) 3 DATAOUT24 100000 80000 60000 40000 2 1 0 –1 –2 –3 20000 0 DATAOUT24 TN –4 0 100 200 300 TEMPERATURE (K) 400 2493 F04 Figure 4. Internal PTAT Digital Output vs Temperature –5 –55 –30 –5 20 45 70 TEMPERATURE (°C) 95 120 2493 F05 Figure 5. Absolute Temperature Error 2493fa 19 LTC2493 Applications Information Initiating a New Conversion Table 5. Address Assignment When the LTC2493 finishes a conversion, it automatically enters the sleep state. Once in the sleep state, the device is ready for a read operation. After the device acknowledges a read request, the device exits the sleep state and enters the data output state. The data output state concludes and the LTC2493 starts a new conversion once a Stop condition is issued by the master or all 32 bits of data are read out of the device. During the data read cycle, a Stop command may be issued by the master controller in order to start a new conversion and abort the data transfer. This Stop command must be issued during the ninth clock cycle of a byte read when the bus is free (the ACK/NAK cycle). LTC2493 Address The LTC2493 has two address pins (CA0, CA1). Each may be tied high, low, or left floating enabling one of 9 possible addresses (see Table 5). In addition to the configurable addresses listed in Table 5, the LTC2493 also contains a global address (1110111) which may be used for synchronizing multiple LTC2493s or other LTC24XX delta-sigma I2C devices (see Synchronizing Multiple LTC2493s with a Global Address Call section). Operation Sequence The LTC2493 acts as a transmitter or receiver, as shown in Figure 6. The device may be programmed to perform several functions. These include input channel selection, S R/W 7-BIT ADDRESS CONVERSION SLEEP ACK DATA CA1 CA0 ADDRESS LOW LOW 0010100 LOW HIGH 0010110 LOW FLOAT 0010101 HIGH LOW 0100110 HIGH HIGH 0110100 HIGH FLOAT 0100111 FLOAT LOW 0010111 FLOAT HIGH 0100101 FLOAT FLOAT 0100100 measure the internal temperature, selecting the line frequency rejection (50Hz, 60Hz, or simultaneous 50Hz and 60Hz) and a 2x speed mode. Continuous Read In applications where the input channel/configuration does not need to change for each cycle, the conversion can be continuously performed and read without a write cycle (see Figure 7). The configuration/input channel remains unchanged from the last value written into the device. If the device has not been written to since power up, the configuration is set to the default value. At the end of a read operation, a new conversion automatically begins. At the conclusion of the conversion cycle, the next result may be read using the method described above. If the conversion cycle is not concluded and a valid address selects the device, the LTC2493 generates a NAK signal indicating the conversion cycle is in progress. Sr DATA TRANSFERRING DATA INPUT/OUTPUT P CONVERSION 2493 F05 Figure 6. Conversion Sequence 2493fa 20 LTC2493 Applications Information Continuous Read/Write and 60Hz) is used but the channel is changed, a Stop or Repeated Start may be issued after the first byte (channel selection data) is written into the device. Once the conversion cycle is concluded, the LTC2493 can be written to and then read from using the Repeated Start (Sr) command. Discarding a Conversion Result and Initiating a New Conversion with Optional Write Figure 8 shows a cycle which begins with a data Write, a repeated Start, followed by a Read and concluded with a Stop command. The following conversion begins after all 32 bits are read out of the device or after a Stop command. The following conversion will be performed using the newly programmed data. In cases where the same speed (1x/2x mode) and rejection frequency (50Hz, 60Hz, 50Hz S 7-BIT ADDRESS CONVERSION R ACK SLEEP READ At the conclusion of a conversion cycle, a write cycle can be initiated. Once the write cycle is acknowledged, a Stop command will start a new conversion. If a new input channel or conversion configuration is required, this data can be written into the device and a Stop command will initiate the next conversion (see Figure 9). P S DATA INPUT 7-BIT ADDRESS CONVERSION R ACK SLEEP READ P DATA OUTPUT CONVERSION 2493 F07 Figure 7. Consecutive Reading with the Same Input/Configuration S 7-BIT ADDRESS CONVERSION W ACK SLEEP WRITE Sr DATA INPUT 7-BIT ADDRESS R ACK ADDRESS READ DATA OUTPUT P CONVERSION 2493 F08 Figure 8. Write, Read, Start Conversion S 7-BIT ADDRESS CONVERSION W ACK SLEEP WRITE (OPTIONAL) DATA INPUT P CONVERSION 2493 F09 Figure 9. Start a New Conversion Without Reading Old Conversion Result 2493fa 21 LTC2493 Applications Information Synchronizing Multiple LTC2493s with a Global Address Call to synchronize multiple converters without changing the channel or configuration, a Stop may be issued after acknowledgement of the global write command. Global read commands are not allowed and the converters will NAK a global read request. In applications where several LTC2493s (or other I2C delta-sigma ADCs from Linear Technology Corporation) are used on the same I2C bus, all converters can be synchronized through the use of a global address call. Prior to issuing the global address call, all converters must have completed a conversion cycle. The master then issues a Start, followed by the global address 1110111, and a write request. All converters will be selected and acknowledge the request. The master then sends a write byte (optional) followed by the Stop command. This will update the channel selection (optional) converter configuration (optional) and simultaneously initiate a start of conversion for all delta-sigma ADCs on the bus (see Figure 10). In order Driving the Input and Reference The input and reference pins of the LTC2493 are connected directly to a switched capacitor network. Depending on the relationship between the differential input voltage and the differential reference voltage, these capacitors are switched between these four pins. Each time a capacitor is switched between two of these pins, a small amount of charge is transferred. A simplified equivalent circuit is shown in Figure 11. SCL SDA LTC2493 S LTC2493 GLOBAL ADDRESS W ACK ALL LTC2493s IN SLEEP … LTC2493 WRITE (OPTIONAL) P CONVERSION OF ALL LTC2493s DATA INPUT 2493 F10 Figure 10. Synchronize Multiple LTC2493s with a Global Address Call IIN + IN+ INPUT MULTIPLEXER 100Ω INTERNAL SWITCH NETWORK ( ) I IN+ 10kΩ ( I REF + IIN– IN– AVG 100Ω SWITCHING FREQUENCY fSW = 123kHz INTERNAL OSCILLATOR fSW = 0.4 • fEOSC EXTERNAL OSCILLATOR CEQ 12µF 0.5 • REQ ( 1.5VREF + VREF(CM) – VIN(CM) 0.5 • REQ 10kΩ REF + – REF − VREF(CM) = 2 )– VIN2 VREF • REQ VIN = IN+ − IN− , WHERE IN+ AND IN− ARE THE SELECTED INPUT CHANNELS IN+ – IN− VIN(CM) = 2 REQ = 2.71MW INTERNAL OSCILLATOR 60Hz MODE 10kΩ 2493 F11a REQ = 2.98MW INTERNAL OSCILLATOR 50Hz/60Hz MODE ( ) REQ = 0.833 • 1012 /fEOSC EXTERNAL OSCILLATOR Figure 11. Equivalent Analog Input Circuit 22 VIN(CM) − VREF(CM) = VREF = REF + − REF − IREF– REF– AVG ≈ AVG where : 10kΩ IREF+ REF+ ) ( ) = I IN– 2493fa LTC2493 Applications Information When using the LTC2493’s internal oscillator, the input capacitor array is switched at 123kHz. The effect of the charge transfer depends on the circuitry driving the input/reference pins. If the total external RC time constant is less than 580ns the errors introduced by the sampling process are negligible since complete settling occurs. Typically, the reference inputs are driven from a low impedance source. In this case, complete settling occurs even with large external bypass capacitors. The inputs (CH0-CH3, COM), on the other hand, are typically driven from larger source resistances. Source resistances up to 10k may interface directly to the LTC2493 and settle completely; however, the addition of external capacitors at the input terminals in order to filter unwanted noise (antialiasing) results in incomplete settling. Automatic Differential Input Current Cancellation In applications where the sensor output impedance is low (up to 10kW with no external bypass capacitor or up to 500W with 0.001µF bypass), complete settling of the input occurs. In this case, no errors are introduced and direct digitization is possible. For many applications, the sensor output impedance combined with external input bypass capacitors produces RC time constants much greater than the 580ns required for 1ppm accuracy. For example, a 10kW bridge driving a 0.1µF capacitor has a time constant an order of magnitude greater than the required maximum. The LTC2493 uses a proprietary switching algorithm that forces the average differential input current to zero independent of external settling errors. This allows direct digitization of high impedance sensors without the need for buffers. The switching algorithm forces the average input current on the positive input (IIN+) to be equal to the average input current on the negative input (IIN–). Over the complete conversion cycle, the average differential input current (IIN+ – IIN–) is zero. While the differential input current is zero, the common mode input current (IIN+ + IIN–)/2 is proportional to the difference between the common mode input voltage (VIN(CM)) and the common mode reference voltage (VREF(CM)). In applications where the input common mode voltage is equal to the reference common mode voltage, as in the case of a balanced bridge, both the differential and common mode input current are zero. The accuracy of the converter is not compromised by settling errors. In applications where the input common mode voltage is constant but different from the reference common mode voltage, the differential input current remains zero while the common mode input current is proportional to the difference between VIN(CM) and VREF(CM). For a reference common mode voltage of 2.5V and an input common mode of 1.5V, the common mode input current is approximately 0.74µA (in simultaneous 50Hz/60Hz rejection mode). This common mode input current does not degrade the accuracy if the source impedances tied to IN+ and IN– are matched. Mismatches in source impedance lead to a fixed offset error but do not effect the linearity or full-scale reading. A 1% mismatch in a 1kW source resistance leads to a 74µV shift in offset voltage. In applications where the common mode input voltage varies as a function of the input signal level (single-ended type sensors), the common mode input current varies proportionally with input voltage. For the case of balanced input impedances, the common mode input current effects are rejected by the large CMRR of the LTC2493, leading to little degradation in accuracy. Mismatches in source impedances lead to gain errors proportional to the difference between the common mode input and common mode reference. 1% mismatches in 1k source resistances lead to gain errors on the order of 15ppm. Based on the stability of the internal sampling capacitors and the accuracy of the internal oscillator, a one-time calibration will remove this error. In addition to the input sampling current, the input ESD protection diodes have a temperature dependent leakage current. This current, nominally 1nA (±10nA max), results in a small offset shift. A 1k source resistance will create a 1µV typical and a 10µV maximum offset voltage. 2493fa 23 LTC2493 Applications Information Reference Current Similar to the analog inputs, the LTC2493 samples the differential reference pins (REF+ and REF–) transferring small amounts of charge to and from these pins, thus producing a dynamic reference current. If incomplete settling occurs (as a function the reference source resistance and reference bypass capacitance) linearity and gain errors are introduced. For relatively small values of external reference capacitance (CREF < 1nF), the voltage on the sampling capacitor settles 90 60 50 0 CREF = 0.01µF CREF = 0.001µF CREF = 100pF CREF = 0pF 40 30 20 –20 –30 –40 –50 VCC = 5V –60 VREF = 5V V + = 1.25V –70 VIN– = 3.75V IN –80 FO = GND TA = 25°C –90 10 0 10 0 0 10 1k 100 RSOURCE (Ω) 10k 100k 2493 F12 Figure 12. +FS Error vs RSOURCE at VREF (Small CREF) 500 VCC = 5V VREF = 5V VIN+ = 3.75V VIN– = 1.25V FO = GND TA = 25°C +FS ERROR (ppm) 400 300 2493 F13 –100 CREF = 0.1µF CREF = 0.01µF –200 CREF = 1µF, 10µF –300 VCC = 5V VREF = 5V VIN+ = 1.25V VIN– = 3.75V FO = GND TA = 25°C –400 100 200 100k 0 CREF = 0.01µF 0 10k CREF = 1µF, 10µF 200 0 1k 100 RSOURCE (Ω) Figure 13. –FS Error vs RSOURCE at VREF (Small CREF) –FS ERROR (ppm) –10 CREF = 0.01µF CREF = 0.001µF CREF = 100pF CREF = 0pF –10 –FS ERROR (ppm) +FS ERROR (ppm) 70 In cases where large bypass capacitors are required on the reference inputs (CREF > .01µF), full-scale and linearity errors are proportional to the value of the reference resistance. Every ohm of reference resistance produces a full-scale error of approximately 0.5ppm (while operating in simultaneous 50Hz/60Hz mode (see Figures 14 and 15)). If the input common mode voltage is equal to 10 VCC = 5V VREF = 5V VIN+ = 3.75V VIN– = 1.25V FO = GND TA = 25°C 80 for reference impedances of many kW (if CREF = 100pF up to 10kW will not degrade the performance (see Figures 12 and 13)). 600 400 RSOURCE (Ω) 800 1000 2493 F14 Figure 14. +FS Error vs RSOURCE at VREF (Large CREF) –500 0 200 CREF = 0.1µF 600 400 RSOURCE (Ω) 800 1000 2493 F15 Figure 15. –FS Error vs RSOURCE at VREF (Large CREF) 2493fa 24 LTC2493 Applications Information In addition to the reference sampling charge, the reference ESD protection diodes have a temperature dependent leakage current. This leakage current, nominally 1nA (±10nA max) results in a small gain error. A 100W reference resistance will create a 0.5µV full-scale error. Normal Mode Rejection and Antialiasing One of the advantages delta-sigma ADCs offer over conventional ADCs is on-chip digital filtering. Combined with a large oversample ratio, the LTC2493 significantly simplifies antialiasing filter requirements. Additionally, the input current cancellation feature allows external low pass filtering without degrading the DC performance of the device. The SINC4 digital filter provides excellent normal mode rejection at all frequencies except DC and integer multiples of the modulator sampling frequency (fS) (see Figures 17 and 18). The modulator sampling frequency is fS = 15,360Hz while operating with its internal oscillator and fS = fEOSC/20 when operating with an external oscillator of frequency fEOSC . 0 INPUT NORMAL MODE REJECTION (dB) the reference common mode voltage, a linearity error of approximately 0.67ppm per 100W of reference resistance results (see Figure 16). In applications where the input and reference common mode voltages are different, the errors increase. A 1V difference in between common mode input and common mode reference results in a 6.7ppm INL error for every 100W of reference resistance. –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 –120 0 fS 2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS 10fS11fS12fS DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz) 2493 F17 Figure 17. Input Normal Mode Rejection, Internal Oscillator and 50Hz Rejection Mode VCC = 5V 8 VREF = 5V VIN(CM) = 2.5V 6 T = 25°C A 4 CREF = 10µF 0 R = 1k 2 R = 500Ω 0 R = 100Ω –2 –4 –6 –8 –10 – 0.5 – 0.3 0.1 – 0.1 VIN/VREF 0.3 0.5 2493 F16 Figure 16. INL vs Differential Input Voltage and Reference Source Resistance for CREF > 1µF INPUT NORMAL MODE REJECTION (dB) INL (ppm OF VREF) 10 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 –120 0 fS 2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS 10fS DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz) 2493 F18 Figure 18. Input Normal Mode Rejection, Internal Oscillator and 60Hz Rejection Mode 2493fa 25 LTC2493 Applications Information When using the internal oscillator, the LTC2493 is designed to reject line frequencies. As shown in Figure 19, rejection nulls occur at multiples of frequency fN, where fN is determined by the input control bits FA and FB (fN = 50Hz or 60Hz or 55Hz for simultaneous rejection). Multiples of the modulator sampling rate (fS = fN • 256) only reject noise to 15dB (see Figure 20); if noise sources are present at these frequencies antialiasing will reduce their effects. The user can expect to achieve this level of performance using the internal oscillator, as shown in Figures 21, 22, and 23. Measured values of normal mode rejection are shown superimposed over the theoretical values in all three rejection modes. INPUT NORMAL MODE REJECTION (dB) 0 Traditional high order delta-sigma modulators suffer from potential instabilities at large input signal levels. The proprietary architecture used for the LTC2493 third order modulator resolves this problem and guarantees stability with input signals 150% of full scale. In many industrial applications, it is not uncommon to have microvolt level signals superimposed over unwanted error sources with several volts if peak-to-peak noise. Figures 24 and 25 show measurement results for the rejection of a 7.5V peak-to-peak noise source (150% of full scale) applied to the LTC2493. These curves show that the rejection performance is maintained even in extremely noisy environments. fN = fEOSC/5120 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 –120 0 fN 2fN 3fN 4fN 5fN 6fN 7fN INPUT SIGNAL FREQUENCY (Hz) 8fN 2493 F19 Figure 19. Input Normal Mode Rejection at DC INPUT NORMAL MODE REJECTION (dB) 0 –10 fN = fEOSC/5120 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 –120 250fN 252fN 254fN 256fN 258fN 260fN 262fN INPUT SIGNAL FREQUENCY (Hz) 2493 F20 Figure 20. Input Normal Mode Rejection at fS = 256 • fN 2493fa 26 LTC2493 Applications Information MEASURED DATA CALCULATED DATA –20 –40 VCC = 5V VREF = 5V VIN(CM) = 2.5V VIN(P-P) = 5V TA = 25°C – 60 –80 –100 –120 0 15 30 45 60 75 0 NORMAL MODE REJECTION (dB) NORMAL MODE REJECTION (dB) 0 –40 VCC = 5V VREF = 5V VIN(CM) = 2.5V VIN(P-P) = 5V TA = 25°C – 60 –80 –100 –120 90 105 120 135 150 165 180 195 210 225 240 INPUT FREQUENCY (Hz) MEASURED DATA CALCULATED DATA –20 0 12.5 25 37.5 50 62.5 75 87.5 100 112.5 125 137.5 150 162.5 175 187.5 200 INPUT FREQUENCY (Hz) 2493 F21 NORMAL MODE REJECTION (dB) 0 MEASURED DATA CALCULATED DATA –20 –40 VCC = 5V VREF = 5V VIN(CM) = 2.5V VIN(P-P) = 5V TA = 25°C – 60 –80 –100 –120 0 12.5 25 37.5 50 62.5 75 87.5 100 112.5 125 137.5 150 162.5 175 187.5 200 INPUT FREQUENCY (Hz) Figure 22. Input Normal Mode Rejection vs Input Frequency with Input Perturbation of 100% (50Hz Notch) 0 NORMAL MODE REJECTION (dB) Figure 21. Input Normal Mode Rejection vs Input Frequency with Input Perturbation of 100% (60Hz Notch) 2493 F22 VIN(P-P) = 5V VIN(P-P) = 7.5V (150% OF FULL SCALE) –20 VCC = 5V VREF = 5V VIN(CM) = 2.5V TA = 25°C –40 – 60 –80 –100 –120 0 15 30 45 60 75 90 105 120 135 150 165 180 195 210 225 240 INPUT FREQUENCY (Hz) 2493 F24 2493 F23 Figure 23. Input Normal Mode Rejection vs Input Frequency with Input Perturbation of 100% (50Hz/60Hz Notch) Figure 24. Measure Input Normal Mode Rejection vs Input Frequency with Input Perturbation of 150% (60Hz Notch) NORMAL MODE REJECTION (dB) 0 VIN(P-P) = 5V VIN(P-P) = 7.5V (150% OF FULL SCALE) –20 VCC = 5V VREF = 5V VIN(CM) = 2.5V TA = 25°C –40 – 60 –80 –100 –120 0 12.5 25 37.5 50 62.5 75 87.5 100 112.5 125 137.5 150 162.5 175 187.5 200 INPUT FREQUENCY (Hz) 2493 F25 Figure 25. Measure Input Normal Mode Rejection vs Input Frequency with Input Perturbation of 150% (50Hz Notch) 2493fa 27 LTC2493 Applications Information Using the 2X speed mode of the LTC2493 alters the rejection characteristics around DC and multiples of fS. The device bypasses the offset calibration in order to increase the output rate. The resulting rejection plots are shown in Figures 26 and 27. 1x type frequency rejection can be achieved using the 2x mode by performing a running average of the previoius two conversion results (see Figure 28). Output Data Rate When using its internal oscillator, the LTC2493 produces up to 15 samples per second (sps) with a notch frequency of 60Hz. The actual output data rate depends upon the length of the sleep and data output cycles which are controlled by the user and can be made insignificantly short. When operating with an external conversion clock (fO connected to an external oscillator), the LTC2493 output data rate can be increased. The duration of the conversion cycle is 41036/fEOSC. If fEOSC = 307.2kHz, the converter behaves as if the internal oscillator is used. A change in fEOSC results in a proportional change in the internal notch position. This leads to reduced differential mode rejection of line frequencies. The common mode rejection of line frequencies remains unchanged, thus fully differential input signals with a high degree of symmetry on both the IN+ and IN – pins will continue to reject line frequency noise. An increase in fEOSC also increases the effective dynamic input and reference current. External RC networks will continue to have zero differential input current, but the time required for complete settling (580ns for fEOSC = 307.2kHz) is reduced, proportionally. Once the external oscillator frequency is increased above 1MHz (a more than 3x increase in output rate) the effectiveness of internal auto calibration circuits begins to degrade. This results in larger offset errors, full-scale errors, and decreased resolution, as seen in Figures 29-36. 0 0 –20 –20 INPUT NORMAL REJECTION (dB) INPUT NORMAL REJECTION (dB) An increase in fEOSC over the nominal 307.2kHz will translate into a proportional increase in the maximum output data rate (up to a maximum of 100sps). The increase in output rate leads to degradation in offset, full-scale error, and effective resolution as well as a shift in frequency rejection. When using the integrated temperature sensor, the internal oscillator should be used or an external oscillator fEOSC = 307.2kHz maximum. –40 –60 –80 –100 –120 0 fN 2fN 3fN 4fN 5fN 6fN 7fN INPUT SIGNAL FREQUENCY (fN) 8fN 2493 F26 Figure 26. Input Normal Mode Rejection 2x Speed Mode –40 –60 –80 –100 –120 248 250 252 254 256 258 260 262 264 INPUT SIGNAL FREQUENCY (fN) 2493 F27 Figure 27. Input Normal Mode Rejection 2x Speed Mode 2493fa 28 LTC2493 Applications Information 50 OFFSET ERROR (ppm OF VREF) NO AVERAGE –90 WITH RUNNING AVERAGE –100 –110 –120 –130 30 TA = 85°C 20 10 0 –FS ERROR (ppm OF VREF) RESOLUTION (BITS) –2000 –2500 V = VREF(CM) –3000 IN(CM) VCC = VREF = 5V FO = EXT CLOCK –3500 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) 20 20 TA = 85°C 16 14 12 10 VIN(CM) = VREF(CM) VCC = VREF = 5V VIN = 0V FO = EXT CLOCK RES = LOG 2 (VREF/NOISERMS) 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) RESOLUTION (BITS) VCC = 5V, VREF = 2.5V 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) 2493 F34 Figure 34. Offset Error vs Output Data Rate and Temperature TA = 25°C 14 2493 F33 20 Figure 33. Resolution (INLMAX ≤ 1LSB) vs Output Data Rate and Temperature 22 VCC = VREF = 5V 22 0 TA = 85°C VIN(CM) = VREF(CM) 12 VCC = VREF = 5V FO = EXT CLOCK RES = LOG 2 (VREF/INLMAX) 10 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) 24 VIN(CM) = VREF(CM) VIN = 0V 15 FO = EXT CLOCK TA = 25°C VCC = VREF = 5V 16 Figure 32. Resolution (NoiseRMS ≤ 1LSB) vs Output Data Rate and Temperature 20 10 18 2493 F32 2493 F31 OFFSET ERROR (ppm OF VREF) 22 18 Figure 31.–FS Error vs Output Data Rate and Temperature –10 2493 F30 TA = 25°C 22 TA = 85°C 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) Figure 30. +FS Error vs Output Data Rate and Temperature 24 TA = 25°C –1500 0 2493 F29 –500 –5 500 Figure 29. Offset Error vs Output Data Rate and Temperature 0 –1000 TA = 25°C 1000 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) 2493 F28 Figure 28. Input Normal Mode Rejection 2x Speed Mode with and Without Running Averaging 5 1500 0 –10 TA = 85°C 2000 TA = 25°C 60 62 48 50 54 56 58 52 DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz) VIN(CM) = VREF(CM) VCC = VREF = 5V FO = EXT CLOCK 2500 RESOLUTION (BITS) –140 40 3000 20 VCC = 5V, VREF = 2.5V 18 16 14 VIN(CM) = VREF(CM) VIN = 0V FO = EXT CLOCK 12 T = 25°C A RES = LOG 2 (VREF/NOISERMS) 10 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) RESOLUTION (BITS) NORMAL MODE REJECTION (dB) –80 3500 VIN(CM) = VREF(CM) VCC = VREF = 5V VIN = 0V FO = EXT CLOCK +FS ERROR (ppm OF VREF) –70 2493 F35 Figure 35. Resolution (NoiseRMS ≤ 1LSB) vs Output Data Rate and Temperature 18 VCC = VREF = 5V 16 VCC = 5V, VREF = 2.5V VIN(CM) = VREF(CM) 14 VIN = 0V REF– = GND 12 FO = EXT CLOCK TA = 25°C RES = LOG 2 (VREF/INLMAX) 10 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) 2493 F36 Figure 36. Resolution (INLMAX ≤ 1LSB) vs Output Data Rate and Temperature 2493fa 29 LTC2493 Applications Information Easy Drive ADCs Simplify Measurement of High Impedance Sensors Delta-Sigma ADCs, with their high accuracy and high noise immunity, are ideal for directly measuring many types of sensors. Nevertheless, input sampling currents can overwhelm high source impedances or low-bandwidth, micropower signal conditioning circuits. The LTC2493 solves this problem by balancing the input currents, thus simplifying or eliminating the need for signal conditioning circuits. A common application for a delta-sigma ADC is thermistor measurement. Figure 37 shows two examples of thermistor digitization benefiting from the Easy Drive technology. The first circuit (applied to input channels CH0 and CH1) uses balanced reference resistors in order to balance the common mode input/reference voltage and balance the differential input source resistance. If reference resistors R1 and R4 are exactly equal, the input current is zero and no errors result. If these resistors have a 1% tolerance, the maximum error in measured resistance is 1.6Ω due to a shift in common mode voltage; far less than the 1% error of the reference resistors themselves. No amplifier is required, making this an ideal solution in micropower applications. Easy Drive also enables very low power, low bandwidth amplifiers to drive the input to the LTC2493. As shown in Figure 7, CH2 is driven by the LT1494. The LT1494 has excellent DC specs for an amplifier with 1.5µA supply current (the maximum offset voltage is 150µV and the open loop gain is 100,000). Its 2kHz bandwidth makes it unsuitable for driving conventional delta sigma ADCs. Adding a 1kΩ, 0.1µF filter solves this problem by providing a charge reservoir that supplies the LTC2493 instantaneous current, while the 1k resistor isolates the capacitive load from the LT1494. Conventional delta sigma ADCs input sampling current lead to DC errors as a result of incomplete settling in the external RC network. The Easy Drive technology cancels the differential input current. By balancing the negative input (CH3) with a 1kΩ, 0.1µF network errors due to the common mode input current are cancelled. 2493fa 30 LTC2493 Package Description DE Package 14-Lead Plastic DFN (4mm × 3mm) (Reference LTC DWG # 05-08-1708 Rev B) 0.70 ±0.05 3.30 ±0.05 3.60 ±0.05 2.20 ±0.05 1.70 ± 0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 3.00 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ±0.10 (2 SIDES) R = 0.05 TYP 3.00 ±0.10 (2 SIDES) R = 0.115 TYP 8 0.40 ± 0.10 14 3.30 ±0.10 1.70 ± 0.10 PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER PIN 1 TOP MARK (SEE NOTE 6) (DE14) DFN 0806 REV B 7 0.200 REF 1 0.25 ± 0.05 0.50 BSC 0.75 ±0.05 3.00 REF 0.00 – 0.05 BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING PROPOSED TO BE MADE VARIATION OF VERSION (WGED-3) IN JEDEC PACKAGE OUTLINE MO-229 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 2493fa 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. 31 LTC2493 Typical Application 5V 5V R1 51.1k C4 0.1µF 12 10µF IIN+ = 0 R3 10k TO 100k R4 IIN– = 0 51.1k 14 8 5V 9 10 5V 102k 11 + 0.1µF 10k TO 100k 7 1k LT1494 – = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR 1 FO LTC2493 13 0.1µF C3 0.1µF VCC 1.7k REF + REF– 5 SDA 4 SCL 2-WIRE I2C INTERFACE CH0 CH1 CA0 CH2 CA1 CH3 COM GND 2 3 9-PIN SELECTABLE ADDRESSES 6 2493 F37 0.1µF 1k 0.1µF Figure 37. Easy Drive ADCs Simplify Measurement of High Impedance Sensors Related Parts PART NUMBER DESCRIPTION COMMENTS LT1236A-5 Precision Bandgap Reference, 5V 0.05% Max Initial Accuracy, 5ppm/°C Drift LT1460 Micropower Series Reference 0.075% Max Initial Accuracy, 10ppm/°C Max Drift LT1790 Micropower SOT-23 Low Dropout Reference Family 0.05% Max Initial Accuracy, 10ppm/°C Max Drift LTC2400 24-Bit, No Latency ΔΣ ADC in SO-8 0.3ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200µA LTC2410 24-Bit, No Latency ΔΣ ADC with Differential Inputs 0.8µVRMS Noise, 2ppm INL LTC2440 24-Bit, High Speed, Low Noise ΔΣ ADC 3.5kHz Output Rate, 200nV Noise, 24.6 ENOBs LTC2442 24-Bit, High Speed, 2-/4-Channel ΔΣ ADC with Integrated Amplifier 8kHz Output Rate, 200nV Noise, Simultaneous 50Hz/60Hz Rejection LTC2449 24-Bit, High Speed, 8-/16-Channel ΔΣ ADC 8kHz Output Rate, 200nV Noise, Simultaneous 50Hz/60Hz Rejection LTC2480/LTC2482/ LTC2484 16-Bit/24-Bit ΔΣ ADCs with Easy Drive Inputs, 600nV Noise, Programmable Gain, and Temperature Sensor Pin Compatible 16-Bit and 24-Bit Versions LTC2481/LTC2483/ LTC2485 16-Bit/24-Bit ΔΣ ADCs with Easy Drive Inputs, 600nV Noise, I2C Interface, Programmable Gain, and Temperature Sensor Pin Compatible 16-Bit and 24-Bit Versions LTC2486/LTC2488/ LTC2492 16-Bit/24-Bit 2-/4-Channel ΔΣ ADC with Easy Drive Inputs, SPI Interface, Programmable Gain, and Temperature Sensor Pin-Compatible 16-Bit and 24-Bit Versions LTC2487 16-Bit 2-/4-Channel ΔΣ ADC with Easy Drive Inputs and I2C Interface, Temperature Sensor Pin-Compatible with LTC2493/LTC2489 LTC2489 16-Bit 2-/4-Channel ΔΣ ADC with Easy Drive Inputs Pin-Compatible with LTC2487/LTC2493 LTC2495/LTC2497/ LTC2499 16-Bit/24-Bit 8-/16-Channel ΔΣ ADC with Easy Drive Inputs and I2C Interface, Programmable Gain, and Temperature Sensor Pin-Compatible 16-Bit and 24-Bit Versions LTC2496/LTC2498 16-Bit 8-/16-Channel ΔΣ ADC with Easy Drive Inputs and SPI Interface Pin-Compatible with LTC2498/LTC2449 2493fa 32 Linear Technology Corporation LT/CGRAFX 0407 REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2007