LTC2400 High Accuracy Differential to Single-Ended Converter for ±5V Supplies – Design Note 207 Differential to Single-Ended Converter Has Very High Uncalibrated Accuracy and Low Offset and Drift Kevin R. Hoskins and Derek V Redmayne Introduction 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 achieves 19.6-bit resolution and 18.1-bit accuracy. These and other specifications are summarized in Table 1. Operation The circuit in Figure 1 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 5V 0.1μF 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 singleended 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 L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 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 RS* 5.1k 3 VIN LTC2400 GND 0.1μF R1 9.09k R2 90.9Ω 12 CS VREF SDO SCK 4 CH 1μF –5V 350Ω AGND OR –VEXT 1 VCC 2 3 LTC1050 350Ω 0.1μF 4 7 350Ω VREFIN 5V 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 07/99/207_conv SERIAL DATA OUT SERIAL CLOCK *OPTIONAL—LIMITS INPUT CURRENT IF THE INPUT VOLTAGE GOES BELOW –300mV R1, R2 = 0.02% INITIAL TOLERANCE OR BETTER R3, R4 = 1% 16 17 7 CHIP SELECT 8 DN207 F01 C1 0.01μF 6 FO 4 14 13 5 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 to 0.001% or 10ppm. 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 circuitry’s connections will cause linearity perturbations in the final conversion result. Their effects can be minimized by balancing the thermocouple connections 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 consists of common mode and differential mode components. The differential mode input current can be as much as ±25nA when VIN(CM) is equal to the + or –5V common mode limit, and VDIFF is 40mV (typ). The ratio of common mode to differential input current is nominally 3:1. Under balanced input conditions (VIN(CM) = 0), the total input Data Sheet Download www.linear.com Linear Technology Corporation current is typically 1nA. The values may vary from 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 capacitance, stray or otherwise, connected between the LTC2400’s input and ground is less than 100pF. Higher capacitance will increase offset and full-scale errors. Other differential to single-ended conditioning circuits for the LTC2400 are available on LTC’s web site. Refer to the URL at the bottom of this page. Table 1. Performance Specifications for Figure 1’s Circuit. VCC = VREF = LT®1236-5; VFS = 40mV, RSOURCE = 175Ω (Balanced) CIRCUIT (MEASURED) LTC2400 PARAMETER Input Voltage Range –3 to 40 Zero Error 12.7 TOTAL (UNITS) 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. For applications help, call (408) 432-1900 dn207f_conv LT/TP 0799 370K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1999