Design Solutions 2 April 1999 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 by Kevin R. Hoskins and Derek V. Redmayne 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 1 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 1. 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, , LTC and LT are registered trademarks of Linear Technology Corporation. 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. 1 Design Solutions 2 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 1’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 1. Simple Rail-to-Rail Circuit Converts Differential Signals to Single-Ended Signals 2 Linear Technology Corporation dsol2 LT/TP 0499 2K • 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