Application Note 78 August 1999 A Collection of Differential to Single-Ended Signal Conditioning Circuits for Use with the LTC2400, a 24-Bit No Latency ∆Σ ADC in an SO-8 By Kevin R. Hoskins and Derek V. Redmayne INTRODUCTION The LTC®2400 is the industry’s first No Latency ∆ΣTM ADC that combines automatic offset and full-scale calibration, an internal oscillator, a sinc4 digital filter, and serial I/O to yield a 24-bit ADC with 1.5µVRMS input noise and singleshot conversion time architecture. It is the ideal A/D converter for temperature measurement and high effective resolution instrumentation applications, such as digital multimeters. This application note contains six circuits that extend the LTC2400’s capabilities using a number of low power differential-to-single-ended signal conditioning circuits. These circuits offer the customer a number of choices for conditioning differential input signals as low as 5mV to as high as ±2.5V, as well as operation on a single 5V or ±5V supplies. In each case, careful circuit design and implementation techniques were used to maintain or preserve the LTC2400’s inherently high effective resolution. In some cases, circuit accuracies (uncalibrated) exceed 17 bits. , LTC and LT are registered trademarks of Linear Technology Corporation. No Latency ∆Σ is a trademark of Linear Technology Corporation. TABLE OF CONTENTS Circuit 1. LTC2400 High Accuracy Differential to Single-Ended Converter for 5V Supplies.................... AN78-2 Differential to Single-Ended Converter Has Very High Uncalibrated Accuracy and Low Offset and Drift Circuit 2. Simple Differential Front-End for the LTC2400 ........................................................................AN78-4 Simple Rail-to-Rail Circuit Converts Differential Signals to Single-Ended Signals and Operates on Single or Dual Supplies Where Resolution Is More Important Than Accuracy Circuit 3. Bipolar Input 24-Bit A/D Converter Accepts ± 2.5V Inputs ...................................................... AN78-6 Differential Input 24-Bit A/D Converter Provides Half-Scale Zero for Bipolar Input Signals Circuit 4. High Accuracy, Differential to Single-Ended Conversion for Wide Range Bipolar Input Signals .......................................................................................................................... AN78-8 Bipolar Differential to Single-Ended Converter Drives the LTC2400’s Input Rail-to-Rail Circuit 5. Low Level, High Accuracy, Bipolar Input Differential to Single-Ended Signal Conversion for 24-Bit A/D ..................................................................................................... AN78-10 Single Supply Differential to Single-Ended Conversion Circuit Amplifies Low Level Bipolar Signals and Maintains the LTC2400’s High Accuracy Circuit 6. LTC2400 High Accuracy Differential to Single-Ended Converter for Single 5V Supply ......... AN78-12 This Converter Has High Accuracy, Very Low Offset and Offset Drift, Rail-to-Rail Input Common Mode Range and is “Live at Zero” LTC2400 Bonus Circuits #1: An Extremely High Resolution LTC2400-Pt RTD Temperature Digitizer ............................................ AN78-14 #2: A High Resolution LTC2400-Based Type S Thermocouple Temperature Digitizer with Improved Cold Junction Compensation .............................................................................. AN78-15 LTC2400 Key Specifications Summary ........................................................................................... AN78-16 an78fs AN78-1 Application Note 78 Circuit 1 LTC2400 High Accuracy Differential to Single-Ended Converter for ± 5V Supplies Differential to Single-Ended Converter Has Very High Uncalibrated Accuracy and Low Offset and Drift SPECIFICATIONS ® VCC = VREF = LT 1236-5; VFS = 40mV; RSOURCE = 175Ω (Balanced) PARAMETER Input Voltage Range CIRCUIT TOTAL (MEASURED) LTC2400 (UNITS) – 3 to 40 Zero Error 12.7 mV 1.5 µV Input Current See Text Nonlinearity ±1 4 ppm Input-Referred Noise (without averaging) 0.3* 1.5 µVRMS Input-Referred Noise (averaged 64 readings) 0.05* µVRMS Resolution (with averaged readings) 19.6 Bits Overall Accuracy (uncalibrated**) 18.1 Bits Supply Voltage ±5 5 V Supply Current 1.6 0.2 mA CMRR 120 dB Common Mode Range ±5 V *Input-referred noise with a gain of 101. **Does not include gain setting resistors. OPERATION The circuit in Figure 1 is ideal for low level differential signals in applications that have a ±5V supply and need high accuracy without calibration. The circuit combines an LTC 1043 and LTC1050 as a differential to single-ended amplifier that has an input common mode range that includes the power supplies. It uses the LTC1043 to sample a differential input voltage, holds it on CS and transfers it to a ground-referred capacitor, CH. The voltage on CH is applied to the LTC1050’s noninverting input and amplified by the gain set by resistors R1 and R2 (101 for the values shown). The amplifier’s output is then converted to a digital value by the LTC2400. The LTC1043 achieves its best differential to single-ended conversion when its internal switching frequency operates at a nominal 300Hz, as set by the 0.01µF capacitor C1 and when 1µF capacitors are used for CS and CH. CS and CH should be a film type such as mylar or polypropylene. Conversion accuracy is enhanced by placing a guard shield around CS and connecting the shield to Pin 10 of the LTC1043. This minimizes nonlinearity that results from stray capacitance transfer errors associated with CS. To minimize the possibility of PCB leakage currents introducing an error source into CH, an optional guard circuit could be added as shown. The common point of these two resistors produces the potential for the guard ring. Consult the LTC1043 data sheet for more information. As is good practice in all high precision circuits, keep all lead lengths as short as possible to minimize stray capacitance and noise pickup. The LTC1050’s closed-loop gain accuracy is affected by the tolerance of the ratio of the gain-setting resistors. If cost considerations preclude using low tolerance resistors (0.02% or better), the processor to which the LTC2400 is connected can be used to perform software correction. Operated as a follower, the LTC1050’s gain and linearity error is less than 0.001%. As stated above, the LTC1043 has the highest transfer accuracy when using 1.0µF capacitors. For example, 0.1µF will typically increase the circuit’s overall nonlinearity tenfold. Another source of errors is thermocouple effects that occur in soldered connections. Their effects are most pronounced in the circuit’s low level portion, before the LTC1050’s output. Any temperature changes in any of the low level circuitry’s connections will cause linearity perturbations in the final conversion result. Their effects can be minimized by balancing the thermocouple connections an78fs AN78-2 Application Note 78 with reversed redundant connections and by sealing the circuit against moving air. A subtle source of error arises from ground lead impedance differences between the LTC1043 circuit, the LTC1050 preamplifier and the LTC2400. This error can be avoided by connecting Pin 14 of the LTC1043, the bottom end of R2 and Pin 4 of the LTC2400 to a single-point “star” ground. The circuit’s input current is dependent on the input signal’s common mode voltage. The input current is approximately – 100nA at VIN(CM) = – 5V, 100nA at VIN(CM) = 5V and 0µA at VIN(CM) = 0V. The values may vary from 5V 0.1µF part to part. Figure 1’s input is analogous to a 2µF capacitor in parallel with a 25MΩ connected to ground. The LTC1043’s nominal 800Ω switch resistance is between the source and the 2µF capacitance. The circuit schematic shows an optional resistor, RS. This resistor can be placed in series with the LTC2400’s input to limit current if the input goes below – 300mV. The resistor does not degrade the converter’s performance as long as any capacitiance, stray or otherwise, connected between the LTC2400’s input and ground is less than 100pF. Higher capacitance will increase offset and fullscale errors. OPTIONAL GUARD CIRCUIT FOR CH R4 R3 90.9Ω 9.09k 5V BRIDGE— TYPICAL INPUT 0.1µF 8 2 350Ω DIFFERENTIAL INPUT 11 10 + CS 1µF (EXT) + 7 – 6 R S* 5.1k CH 1µF 3 VIN LTC2400 GND 0.1µF R1 9.09k R2 90.9Ω 12 CS VREF 4 5 6 7 CHIP SELECT SERIAL DATA OUT SERIAL CLOCK FO 8 *OPTIONAL—LIMITS INPUT CURRENT IF THE INPUT VOLTAGE GOES BELOW –300mV R1, R2 = 0.02% INITIAL TOLERANCE OR BETTER R3, R4 = 1% 14 13 SDO SCK 4 –5V 350Ω AGND OR –VEXT 1 VCC 2 3 LTC1050 350Ω 0.1µF 4 7 350Ω VREFIN 5V DSOL1 F01 16 C1 0.01µF 17 1/2 LTC1043 SINGLE-POINT OR “STAR” GROUND 0.1µF –5V Figure 1. Differential to Single-Ended Converter for Low Level Inputs, Such as Bridges, Maintains the LTC2400’s High Accuracy an78fs AN78-3 Application Note 78 Circuit 2 Simple Differential Front-End for the LTC2400 Simple Rail-to-Rail Circuit Converts Differential Signals to Single-Ended Signals and Operates on Single or Dual Supplies Where Resolution Is More Important Than Accuracy SPECIFICATIONS VCC = VREF = LT®1236-5; VFS = 5V; RSOURCE = 175Ω (Balanced) PARAMETER Input Voltage Range Zero Error CIRCUIT TOTAL (MEASURED) LTC2400 (UNITS) – 0.3 to 5.3 V 2.75 mV Input Current See Text Nonlinearity ±35 4 ppm Input-Referred Noise (without averaging) 10 1.5 µVRMS Input-Referred Noise (averaged 64 readings) 1.5 µVRMS Resolution (with averaged readings) 21.7 Bits Supply Voltage 5 5 V Supply Current 0.45 0.2 mA CMRR 118 dB –5 to 5 V Common Mode Range* *0V to 5V for single 5V supply OPERATION The circuit in Figure 2 is ideal for wide dynamic range differential signals in applications that have a 5V or ±5V supply where absolute accuracy is secondary to high resolution. The circuit uses one-half of an LTC®1043 to perform a differential to single-ended conversion over an input common mode range that includes the power supplies. It uses the LTC1043 to sample a differential input voltage, holds it on CS and transfers it to a ground-referred capacitor CH. The voltage on CH is applied to the LTC2400’s input and converted to a digital value. The LTC1043 achieves its best differential to single-ended conversion when its internal switching frequency operates at a nominal 300Hz, as set by the 0.01µF capacitor C1 and when 1µF capacitors are used for CS and CH. CS and CH should be a film type such as mylar or polypropylene. Conversion accuracy is enhanced by placing a guard shield around CS and connecting the shield to Pin 10 of the LTC1043. This minimizes nonlinearity that results from stray capacitance transfer errors associated with CS. Consult the LTC1043 data sheet for more information. As is good practice in all high precision circuits, keep all lead lengths as short as possible to minimize stray capacitance and noise pickup. Like all delta-sigma converters, the LTC2400’s input circuitry causes small current spikes on the input signal. These current spikes perturb the voltage on the LTC1043’s CH, which results in an effective increase in offset voltage and gain error. These errors remain constant and can be removed through software. Without this end-point correction that reduces the effects of zero and full-scale error, the overall accuracy is degraded. The input dynamic range, however, is not compromised and the overall linearity remains at ±35ppm, or 14.5bits. For inputs with common mode voltages that swing above and below ground, connect Pin 17 to a negative supply, as shown in Figure 2. When applying differential voltages with common mode voltages between ground and the LTC1043’s positive supply, connect Pin 17 (V –) to ground for single supply operation. As stated above, the LTC1043 has the highest transfer accuracy when using 1µF capacitors. Using any other value will compromise the accuracy. For example, 0.1µF will typically increase the circuit’s overall nonlinearity and decrease the CMRR by a factor of 10. The LTC1043’s internal oscillator’s frequency will vary with changes in supply voltage. This variation shows up as increased noise and/or gain error. For example, a 100mV change in the LTC1043’s supply voltage causes 14ppm gain error in the LTC2400. If this variation is short term, an78fs AN78-4 Application Note 78 this error appears as noise. The LTC1043 shows the largest gain error at a nominal common mode input of 3V. These errors can be reduced by using an external clock. As the LTC1043’s VCC increases from a nominal 5V, gain errors are most significant and below 5V, linearity errors become more significant. The circuit’s input current is dependent on the input signal’s magnitude and the reference voltage. For a 5V reference, the input current is approximately –1µA at zero scale, 1µA at full scale and 0µA at midscale. The values may vary from part to part. Figure 2’s input is analogous to a 2µF capacitor in parallel with a 2.5MΩ connected to VREF/2. The LTC1043’s nominal 800Ω switch resistance is between the source and the 2µF capacitance. This description applies to cases where a capacitor is connected in parallel to the LTC2400’s input. This circuit is best suited to applications with large signal swings, and source impedances under 500Ω. VREFIN 5V 0.1µF 5V 0.1µF 1 VCC 2 4 3 8 7 CS VREF VIN LTC2400 SDO SCK GND 11 LARGE MAGNITUDE DIFFERENTIAL INPUT 10 + CS 1µF (EXT) 6 7 CHIP SELECT SERIAL DATA OUT SERIAL CLOCK FO 8 CH 1µF KEEP LEAD LENGTH SHORT 12 13 4 5 14 16 C1 0.01µF SINGLE-POINT OR “STAR” GROUND 17 1/2 LTC1043 0.1µF –5V DSOL2 F01 Figure 2. Simple Rail-to-Rail Circuit Converts Differential Signals to Single-Ended Signals an78fs AN78-5 Application Note 78 Circuit 3 Bipolar Input 24-Bit A/D Converter Accepts ±2.5V Inputs Differential Input 24-Bit A/D Converter Provides Half-Scale Zero for Bipolar Input Signals SPECIFICATIONS VCC = VREF = LT1236-5; VFS = ±2.5V; RSOURCE = 175Ω (Balanced) PARAMETER Input Voltage Range Zero Error CIRCUIT TOTAL (MEASURED) LTC2400 (UNITS) V ±2.8 70 1.5 µV Input Current See Text Nonlinearity ±35 4 ppm Input-Referred Noise (without averaging) 10 1.5 µVRMS Input Referred Noise (averaged 64 readings) 1.5 Resolution (with averaged readings) 21.7 µVRMS Bits Supply Voltage 5 5 V Supply Current 0.5 0.2 mA CMRR Common Mode Range 118 dB 0 to 5 V OPERATION The circuit in Figure 3 is ideal for wide dynamic range differential signals in applications that have a 5V supply. The circuit uses one-half of an LTC1043 to perform a differential to single-ended conversion over an input common mode range that includes the power supplies. This half of the LTC1043 samples a differential input voltage, holds it on CS1 and transfers it to capacitor CH1. The voltage on CH1 is applied to the LTC2400’s input and converted to a digital value. A reference voltage is applied to the LTC2400’s VREF pin and the LTC1043’s Pin 6. The remaining half of the LTC1043 divides the reference voltage by two with a high degree of accuracy. This VREF/2 voltage is applied to the bottom of CH1, centering the LTC1043’s output voltage at midscale (2.5V). This allows the converter to accept bipolar input voltages that swing about a VREF/2 point when operating on a single supply. The LTC1043 achieves its best differential to single-ended conversion when its internal switching frequency operates at a nominal 300Hz, as set by the 0.01µF capacitor C1 and when 1µF capacitors are used for CS1, CS2, CH1 and CH2. Each of the four capacitors should be a film type such as mylar or polypropylene. Conversion accuracy is enhanced by placing a guard shield around CS1 and connecting the shield to Pin 10 of the LTC1043. This minimizes nonlinearity that results from stray capacitance transfer errors associated with CS1. Consult the LTC1043 data sheet for more information. As is good practice in all high precision circuits, keep all lead lengths as short as possible to minimize stray capacitance and noise pickup. Like all delta-sigma converters, the LTC2400’s input circuitry causes small current spikes on the input signal. These current spikes perturb the voltage on the LTC1043’s CH1, which results in an effective increase in offset voltage and gain error. These errors remain constant over a short time interval and can be removed through software. Without this end-point correction that reduces the effects of zero and full-scale error, the overall accuracy is degraded. The input dynamic range, however, is not compromised and the overall linearity remains at ±35ppm, or 14.5bits. As stated above, the LTC1043 has the highest transfer accuracy when using 1µF capacitors. Using any other value will compromise the accuracy. For example, 0.1µF will typically increase the circuit’s overall nonlinearity by a factor of 10. The LTC1043’s internal oscillator’s frequency will vary with changes in supply voltage. This variation shows up as increased noise and/or gain error. For example, a 100mV change in the LTC1043’s supply voltage causes 14ppm gain error in the LTC2400. If this variation is short term, this error appears as noise. The LTC1043 shows the largest gain error at a nominal 3V input. These errors can be reduced by using an external clock. As the LTC1043’s an78fs AN78-6 Application Note 78 VCC increases from a nominal 5V, gain errors are most significant and below 5V, linearity errors become more significant. is between the source and the 2µF capacitance. This description applies to cases where a capacitor is connected in parallel to the LTC2400’s input. The circuit’s input current is dependent on the input signal’s magnitude and the reference voltage. For a 5V reference, the input current is approximately –1µA at – 2.5V, 1µA at 2.5V and 0µA at midscale (0V). The values may vary from part to part. Figure 3’s input is analogous to a 2µF capacitor in parallel with a 2.5MΩ connected to ground. The LTC1043’s nominal 800Ω switch resistance This topology is better suited to lower level signals and higher source impedances than a similar topology without the 1/2 reference point. Operation about the 1/2 reference point minimizes the input current passed from the LTC2400 and reduces the effect of the gain error variation that results from internal oscillator frequency change in the LTC1043. 5V 0.1µF 4 8 7 11 LARGE MAGNITUDE DIFFERENTIAL INPUT + CS1 1µF ( EXT) 10 CH1 1µF 12 MAKE LEAD LENGTH SHORT 14 13 VREFIN 5V 0.1µF VREFIN 5 6 1 VCC 2 2 + CS2 1µF (EXT) CH2 1µF 3 CS VREF VIN LTC2400 SDO 3 KEEP SHORT 4 FO 6 7 CHIP SELECT SERIAL DATA OUT SERIAL CLOCK 8 15 18 C1 0.01µF GND SCK 5 16 LTC1043 SINGLE-POINT OR “STAR” GROUND 17 DSOL3 F01 Figure 3. Differential Input 24-Bit A/D Converter with Half-Scale Zero for Bipolar Input Signals an78fs AN78-7 Application Note 78 Circuit 4 High Accuracy, Differential to Single-Ended Conversion for Wide Range Bipolar Input Signals Bipolar Differential to Single-Ended Converter Drives the LTC2400’s Input Rail-to-Rail midscale. This allows the converter to accept bipolar input voltages that swing about a VREF/2 point when operating on a single supply. SPECIFICATIONS VCC = VREF = LT1236-5; VFS = ±2.45V; RSOURCE = 175Ω (Balanced) PARAMETER Input Voltage Range Zero Error CIRCUIT TOTAL (MEASURED) LTC2400 (UNITS) V ±2.45 22 1.5 µV Input Current See Text Nonlinearity ±2.5 4 ppm Input-Referred Noise (without averaging) 6.5 1.5 µVRMS Input-Referred Noise (averaged 64 readings) 1 µVRMS Resolution (with averaged readings) 22.2 Bits Overall Accuracy (uncalibrated) 17.1 Bits Supply Voltage 5 5 V Supply Current 2.1 0.2 mA CMRR 118 dB 0 to 5 V Common Mode Range OPERATION The circuit in Figure 4 is ideal for wide dynamic range differential signals in applications that have a 5V supply. The circuit uses one-half of an LTC1043 to perform a differential to single-ended conversion over an input common mode range that includes the power supplies. This half of the LTC1043 samples a differential input voltage, holds it on CS1 and transfers it to capacitor CH1. The voltage on CH1 is buffered, applied to the LTC2400’s input and converted to a digital value. A reference voltage is applied to the LTC2400’s VREF pin and the LTC1043’s Pin 6. The remaining half of the LTC1043 divides the reference voltage by two with a high degree of accuracy. This VREF/2 voltage is applied to the bottom of CH1, centering the LTC1043’s output voltage at The LTC1043 achieves its best differential to single-ended conversion when its internal switching frequency operates at a nominal 300Hz, as set by the 0.01µF capacitor C1 and when 1µF capacitors are used for CS1, CS2, CH1 and CH2. Each of these four capacitors should be a film type such as mylar or polypropylene. Conversion accuracy is enhanced by placing a guard shield around CS1 and connecting the shield to Pin 10 of the LTC1043. This minimizes nonlinearity that results from stray capacitance transfer errors associated with CS1. Consult the LTC1043 data sheet for more information. As is good practice in all high precision circuits, keep all lead lengths as short as possible to minimize stray capacitance and noise pickup. The circuit in Figure 4 improves on unbuffered LTC1043 circuits, providing an order of magnitude improvement in linearity (±2.5ppm) by buffering the voltage on CH1. The circuit also improves linearity by buffering the voltage on CH1 with an LTC1152 operating at unity gain. If a 10V supply is available, the LTC1050 can be used instead of the LTC1152. As stated above, the LTC1043 has the highest transfer accuracy when using 1µF capacitors. Using any other value will compromise the accuracy. For example, 0.1µF will typically increase the circuit’s overall nonlinearity, and degrade CMRR by an order of magnitude. Another source of error is thermocouple effects that occur in soldered connections starting with the LTC1043’s input traces and ending with the connections to the LTC2400’s input and ground pins. Any temperature changes in any of the low level circuitry’s connections will cause perturbations in the final conversion result. Their effects can be an78fs AN78-8 Application Note 78 minimized by controlling thermal gradients between pairs of connection by judicious placement of heat sources, components and copper heat spreaders under the pads and by insulating the circuit against moving air. approximately – 100nA at VIN(CM) = – 2.5V, 100nA at VIN(CM) = 2.5V and 0µA at VIN(CM) = 0V. The values may vary from part to part. Figure 4’s input is analogous to a 2µF capacitor in parallel with a 25MΩ connected to ground. The LTC1043’s nominal 800Ω switch resistance is between the source and the 2µF capacitance. The circuit’s input current is dependent on the input signal’s common mode voltage. The input current is 5V 0.1µF 4 8 7 11 LARGE MAGNITUDE DIFFERENTIAL INPUT + CS1 1µF ( EXT) 10 CH1 1µF MAKE LEAD LENGTH SHORT 12 14 13 VREFIN 5V 0.1µF VREFIN 5 6 5V 0.1µF 1 VCC 2 3 2 + CS2 1µF (EXT) 3 CH2 1µF + 7 LTC1152* 2 – 4 6 3 CS VREF VIN LTC2400 SCK GND 4 16 LTC1043 17 6 7 CHIP SELECT SERIAL DATA OUT SERIAL CLOCK FO 8 DSOL 4 F01 15 18 C1 0.01µF SDO 5 *THE LTC1050 CAN ALSO BE USED IF A 10V SUPPLY IS AVAILABLE SINGLE-POINT OR “STAR” GROUND Figure 4. High Accuracy, Bipolar Differential to Single-Ended Converter Drives the LTC2400’s Input Rail-to-Rail an78fs AN78-9 Application Note 78 Circuit 5 Low Level, High Accuracy, Bipolar Input Differential to Single-Ended Signal Conversion for 24-Bit A/D Single Supply Differential to Single-Ended Conversion Circuit Amplifies Low Level Bipolar Signals and Maintains the LTC2400’s High Accuracy SPECIFICATIONS VCC = VREF = LT1236-5; VFS = ±125mV; RSOURCE = 175Ω (Balanced) PARAMETER Input Voltage Range CIRCUIT LTC2400 TOTAL (MEASURED) ONLY (UNITS) mV ±125 Zero Error 25 1.5 µV Input Current See Text Nonlinearity ±2.5 4 ppm 1* 1.5 µV Input-Referred Noise (without averaging) Input-Referred Noise (averaged 64 readings) 0.12* µV Resolution (with averaged readings) 21.0 Bits Overall Accuracy (uncalibrated)** 17.0 Bits Supply Voltage 5 5 V Supply Current 1.2 0.2 mA CMRR 118 dB 0 to 5 V Common Mode Range * Input-referred noise with a gain of 20. ** Does not include gain setting resistors. OPERATION The circuit in Figure 5 is ideal for wide dynamic range differential bridge outputs in applications that have a 5V supply. The circuit uses one-half of an LTC1043 to perform a differential to single-ended conversion over an input common mode range that includes the power supplies. This half of the LTC1043 samples a differential input voltage, holds it on CS1 and transfers it to capacitor CH1. The voltage on CH1 is buffered, applied to the LTC2400’s input and converted to a digital value. A reference voltage is applied to the LTC2400’s VREF pin and the LTC1043’s Pin 6. The remaining half of the LTC1043 divides the reference voltage by two with a high degree of accuracy. This VREF/2 voltage is applied to the bottom of CH1, centering the LTC1043’s output voltage at midscale. This allows the converter to accept bipolar input voltages that swing about a VREF/2 point when operating on a single supply. The LTC1043 achieves its best differential to single-ended conversion when its internal switching frequency operates at a nominal 300Hz, as set by the 0.01µF capacitor C1 and when 1µF capacitors are used for CS1, CS2, CH1 and CH2. Each of these four capacitors should be a film type such as mylar or polypropylene. Conversion accuracy is enhanced by placing a guard shield around CS1 and connecting the shield to Pin 10 of the LTC1043. This minimizes nonlinearity that results from stray capacitance transfer errors associated with CS1. Consult the LTC1043 data sheet for more information. As is good practice in all high precision circuits, keep all lead lengths as short as possible to minimize stray capacitance and noise pickup. The circuit in Figure 5 improves on unbuffered LTC1043 circuits, providing an order of magnitude improvement in linearity (±2.5ppm) by buffering the voltage on CH1. Onehalf of an LTC1051 is used, with its gain set by R1 and R2 (AV = 1 + R1/R2). The remaining half of the LTC1051 buffers the voltage on the bottom of CH1 before it is applied to R2. As stated above, the LTC1043 has the highest transfer accuracy when using 1µF capacitors. Using any other value will compromise the accuracy. For example, 0.1µF will typically increase the circuit’s overall nonlinearity by an order of magnitude. Another source of error is thermocouple effects that occur in soldered connections starting with the LTC1043’s input traces and ending with the connection at the LTC2400’s input and ground pins. Any temperature changes in any of the low level circuitry’s connections will cause perturbations in the final conversion result. Their effects can be an78fs AN78-10 Application Note 78 voltage is increased. At a gain of less than five, the common mode input range of the LTC1051 becomes a constraint, which prevents using the A/D’s full dynamic range. minimized by ensuring uniform temperature at pairs of junctions between dissimilar metals and by sealing the circuit against moving air. The circuit’s input current is dependent on the input signal’s common mode voltage. The input current is approximately –100nA at – 2.5V, 100nA at 2.5V and 0µA at midscale (0V). The values may vary from part to part. Figure 5’s input is analogous to a 2µF capacitor in parallel with a 25MΩ connected to ground. The LTC1043’s nominal 800Ω switch resistance is between the source and the 2µF capacitance. The use of a higher supply voltage for the LTC1051 allows a higher common mode input voltage and, therefore, is suitable for lower gain and greater differential input voltage. The higher supply voltage is not necessary if the railto-rail LTC1152 is used. Lower reference voltage provides an additional benefit of increased LTC2400 linearity. Whereas the linearity error is ±4ppm with a 5V reference, it drops to ±2ppm with a 2.5V reference. This translates to a lower offset when using midscale as the zero point. Resistors R1 and R2 set the gain of the op amp that drives the LTC2400. The practical gain range with this topology as shown is from 5 to 100 unless the op amp supply 5V 0.1µF 5V 0.1µF 4 5 8 7 11 6 CS1 1µF (EXT) DIFFERENTIAL INPUT CH1 1µF + 8 1/2 LTC1051 7 – MAKE LEAD LENGTH SHORT R1* 20k 12 3 14 13 2 VREFIN + 1/2 LTC1051 – VREFIN R2* 1.05k 0.1µF 1 2 4 5 6 3 2 CS2 1µF (EXT) 1 VCC CS VREF VIN LTC2400 SDO SCK CH2 1µF GND 5 6 7 SERIAL DATA OUT SERIAL CLOCK 8 KEEP LEAD LENGTH SHORT 15 CHIP SELECT FO 4 3 18 5V DSOL5 F01 16 C1 0.01µF LTC1043 SINGLE-POINT OR “STAR” GROUND 17 *SEE TEXT Figure 5. This Single Supply Differential to Single-Ended Conversion Circuit Amplifies Low Level Bipolar Signals and Maintains the LTC2400’s High Accuracy an78fs AN78-11 Application Note 78 Circuit 6 LTC2400 Differential to Single-Ended Converter for Single 5V Supply This Converter Has High Accuracy, Very Low Offset and Offset Drift, Rail-to-Rail Input Common Mode Range and is “Live at Zero” SPECIFICATIONS VCC = VREF = LT1019-2.5; RSOURCE = 175Ω (Balanced) PARAMETER Input Voltage Range Zero Error CIRCUIT TOTAL (MEASURED) LTC2400 (UNITS) – 0.5 to 5 2 mV 1.5 µV Input Current See Text Nonlinearity ±5 4 ppm Noise (without averaging) 0.21* 1.5 µVRMS Noise (averaged 64 readings) 0.026* µVRMS Resolution (with averaged readings) 17.6 Bits Overall Accuracy (uncalibrated**) 17.6 Bits Supply Voltage 5 5 V Supply Current 2.6 0.2 mA CMRR 120 dB 0 to 5 V Common Mode Range *Input referred noise with a gain of 101 **Does not include gain setting resistors, offset and gain error removed OPERATION The circuit in Figure 6 is ideal for low level differential signals, typically 2mV/ V, in single supply applications and features a “live at zero” operation. The circuit combines an LTC1043 and LTC1050 as a differential to single-ended amplifier that has an input common mode range that includes the power supplies. It uses the LTC1043 to sample a differential input voltage, holds it on CS and transfers it to a ground-referred capacitor CH, completing the conversion to single-ended. The voltage on CH is applied to the LTC1050’s noninverting input and amplified by the gain set by resistors R1 and R2 (101X for the values shown). The amplifier’s output is then converted to a digital value by the LTC2400. The circuit uses a simple voltage reference (the Schottky diode and NPN transistor) to bias the single-ended signal approximately 270mV above ground. For single supply applications, this bias voltage and the circuit’s “live at zero” operation allows the LTC1050 and the LTC2400 to amplify and convert signals that include inputs below ground. The LTC1043 achieves its best differential to single-ended conversion when its internal switching frequency operates at a nominal 300Hz, as set by the 0.01µF capacitor C1, and when 1µF capacitors are used for CS and CH. CS and CH should be a film type such as mylar or polypropylene. Conversion accuracy is enhanced by placing a guard shield around CS and connecting the shield to Pin 10 of the LTC1043. This minimizes nonlinearity that results from stray capacitance transfer errors associated with CS. Consult the LTC1043 data sheet for more information. As is good practice in all high precision circuits, keep all lead lengths as short as possible to minimize stray capacitance and noise pickup. As stated above, the LTC1043 has the highest transfer accuracy when using 1µF capacitors. Using any other value will compromise the accuracy. For example, 0.1µF will typically increase the circuit’s overall nonlinearity tenfold. The LTC1050’s closed-loop gain accuracy is affected by the tolerance of the ratio of the gain-setting resistors. If cost considerations preclude using low tolerance resistors (0.02% or better), the processor to which the LTC2400 is connected can be used to perform software correction. Operated as a follower, the LTC1050’s gain and linearity error is less than 0.001%. an78fs AN78-12 Application Note 78 The circuit uses 2.5V to excite the 2mV/V bridge, producing a low level output. Best performance is achieved using bandwidth limiting as shown and the attenuator at the LTC2400’s input to reduce the input-referred noise. The LTC1050’s noise gain of 100 allows adequate headroom for the expected signal magnitude. This is followed by an attenuator that reduces the signal for an overall gain of 16.8. This gain is the typical point where the input-referred noise is minimized. circuitry’s connections will cause linearity perturbations in the final conversion result. There effects can be minimized by balancing the thermocouple connections with reversed redundant connections and by sealing the circuit against moving air. The circuit’s input current is dependent on the input signal’s common mode voltage. The input current is approximately 100nA at VIN(CM) = 5V, dropping to zero at VIN(CM) = 0V. The values may vary from part to part. Figure 6’s input is analogous to a 2µF capacitor in parallel with a 25MΩ connected to ground. The LTC1043’s nominal 800Ω switch resistance is between the source and the 2µF capacitance. A source of errors is thermocouple effects that occur in soldered connections. Their effects are most pronounced in the circuit’s low level portion, before the LTC1050’s output. Any temperature changes in any of the low level 5V CH 0.1µF 5V 0.1µF 0.1µF 5V BRIDGE— TYPICAL INPUT KEEP SHORT 4 350Ω 350Ω 2mV/V 350Ω 7 CH 1µF – 4 6 RS 5.1k 3 C1 0.1µF 5.1k 1 VCC CS VIN LTC2400 4 SDO FO 8 14 16 1/2 LTC1043 R2 1k SCK 5 6 SERIAL DATA OUT 7 5.1k SERIAL CLOCK 470Ω 350Ω C1 0.01µF CHIP SELECT 470Ω VREF GND R1 100k 12 13 4.3k 2 CS 1µF (EXT) DIFFERENTIAL INPUT + LTC1050 11 0.1µF 0.1µF 2 3 8 7 5V 5V 5V LT1019-2.5 R3 1k R1, R2: 0.1% OR BETTER, 10ppm/°C 'HC14 OR EQUIVALENT DSOL6 F01 17 1N5711 2N5210 SINGLE POINT “STAR” CONNECTION Figure 6. Single Supply Differential to Single-Ended Converter for Low Level Inputs with “Live at Zero” Operation an78fs AN78-13 Application Note 78 LTC2400 Bonus Circuit #1 An Extremely High Resolution LTC2400-Pt RTD Temperature Digitizer The circuit shown below uses an LTC2400 to digitize the output of a conditioned 100Ω Pt RTD. Using an RTD in combination with the LTC2400, temperatures to 200°C can be measured with a high degree of resolution. The circuit below incorporates a low noise bipolar operational amplifier, the LT®1028, configured for a gain of 92. In using low noise preamplification, the effective noise floor of the LTC2400 is reduced by the same amount. As a result, the circuit offers a potential resolution of 0.001°C. R1, R2, R3 and R4 should be stable, precision resistors, such as Vishay S102 types or their equivalent. Furthermore, these resistors should exhibit very low coefficient of temperature or should be temperature-stabilized by placing the preamplifier circuit in an enclosure. Alternatively, precision resistor networks can be used and are available from Vishay or Caddock. The excitation current generated by the VREF-R1-R2 combination is low enough for most sensors that RTD self-heating effect is near the noise floor of the LTC2400 (1.5µVRMS). Achieving this level of resolution requires careful thermal design and minimizing RTD self-heating effects. Resistors VREF 5V R1* 9.09k F R2* 9.09k S 3 Pt RTD 100Ω 1 5V 2 + LT1028 2 6 – 300Ω – 5V R3* 9.09k R4** 100Ω 0.1µF 1k 3 VCC VREF VIN CS SDO LTC2400 SCK GND 0.1µF 5 6 7 FO 4 8 5V 60Hz 50Hz *VISHAY S102 OR EQUIVALENT SINGLE-POINT OR “STAR” GROUND AN78 BC#1 LTC2400 Bonus Circuit #1: An Extremely High Resolution LTC2400-Pt RTD Temperature Digitizer an78fs AN78-14 Application Note 78 LTC2400 Bonus Circuit #2 A High Resolution LTC2400-Based Type S Thermocouple Digitizer with Improved Cold Junction Compensation The figure shown below illustrates a simple interface circuit that demonstrates the practicality of direct thermocouple connection to the LTC2400 using low output voltage thermocouples (a Type S thermocouple, as shown, produces a full-scale output voltage of 18mV). This circuit uses the LT1025, a micropower thermocouple cold junction compensator, to sense the temperature of the cold junction and introduce an offset voltage. This offset voltage is equal in magnitude, but opposite in polarity, to the voltage generated by the thermocouple cable/PC board termination. (note the polarity of the wires!) and connect to the most appropriate output pin on the LT1025. To minimize any additional error into the measurement, the LT1025 must be mounted at the cold junction and the connections made to the LT1025, the thermocouple, and the LTC2400 must be isothermal. Because of the LTC2400’s noise floor, this circuit is capable of resolving temperatures to within 0.25°C without averaging. Since the LTC2400 does not exhibit any easily discernible quantization effects, averaging multiple readings can significantly extend the resolution for slowvarying processes. This circuit can be easily adapted for use with other thermocouple types—simply replace the thermocouple 5V 0.1µF LT1025 NC 1 2 NC 3 4 E VIN VO GND J K, T R, S R– 8 7 6 1 NC 2 NC – + 3 VCC VREF VIN CS LTC2400 SDO 5 SCK GND TYPE S 4 THERMOCOUPLE TYPE E J K, T R, S 5 6 7 FO SEEBECK COEFFICIENT 60.9µV/°C 51.7µV/°C 40.6µV/°C 6µV/°C 8 AN78 BC#2 5V 60Hz 50Hz SINGLE-POINT OR “STAR” GROUND LTC2400 Bonus Circuit #2: A High Resolution LTC2400-Based Type S Thermocouple Digitizer with Improved Cold Junction Compensation an78fs 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. AN78-15 Application Note 78 LTC2400: A 24-Bit µPower No Latency ∆Σ ADC in SO-8 KEY SPECIFICATIONS PARAMETER PACKAGE PINOUT CONDITIONS TOP VIEW Resolution (No Missing Codes) 0.1V ≤ VREF ≤ VCC 24 Bits (min) Integral Nonlinearity VREF = 2.5V VREF = 5V 2ppm of VREF 4ppm of VREF Offset Error 2.5V ≤ VREF ≤ VCC 0.5ppm of VREF Offset Error Drift 2.5V ≤ VREF ≤ VCC 0.01ppm of VREF/°C Full-Scale Error 2.5V ≤ VREF ≤ VCC 4ppm of VREF Full-Scale Error Drift 2.5V ≤ VREF ≤ VCC 0.02ppm of VREF/°C Total Unadjusted Error VREF = 2.5V VREF = 5V Normal Mode Rejection 50Hz ±2 110dB (min) Input Voltage Range (Live at Zero) –1.25V • VREF to 1.125V • VREF Reference Voltage Range 0.1V ≤ VREF ≤ VCC Supply Voltage 2.7V ≤ VCC ≤ 5.5V Supply Current Conversion Mode Sleep Mode SCK VIN 3 6 SDO GND 4 5 CS 10 VCC = 5V VREF = 5V TA = 25°C FO = LOW 8 LINEARITY ERROR (ppm) 110dB (min) FO 7 Total Unadjusted Error vs Output Code 1.5µVRMS Normal Mode Rejection 60Hz ±2% 8 S8 PACKAGE 8-LEAD PLASTIC SO 5ppm of VREF 1ppm of VREF Output Noise VCC 1 VREF 2 6 4 2 0 –2 –4 –6 –8 CS = 0V CS = VCC 200µA 20µA –10 0 8,338,608 OUTPUT CODE (DECIMAL) 16,777,215 2400 TA02 Noise Histogram NUMBER OF READINGS 1500 Rejection vs Frequency at VIN –60 VCC = 5V VREF = 5V VIN = 0V –70 –80 REJECTION (dB) 1000 500 –90 –100 –110 –120 –130 0 –1.0 – 0.5 0 0.5 1.0 OUTPUT CODE (ppm) 1.5 2400 G14 –140 –12 –8 –4 0 4 8 12 INPUT FREQUENCY DEVIATION FROM NOTCH FREQUENCY (%) 2400 G25 an78fs AN78-16 Linear Technology Corporation LT/TP 0899 4K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com © LINEAR TECHNOLOGY CORPORATION 1999