This article presents a simple ADC driver circuit that converts a ±10V single-ended input signal into a fully differential signal capable of driving the LTC2377-20 20-bit SAR ADC with a combined linearity error of only 2ppm. Options for providing higher input impedance and a lower overall supply current are also examined.
The circuit of Figure 1 converts a ±10V single-ended signal into the ±5V fully differential signal required by the LTC2377-20 (U1). The LTC2377-20 is a 20-bit, 500ksps, low power SAR ADC with a typical integral nonlinearity (INL) of ±0.5ppm. The voltage at AIN is buffered by U4, which in turn drives the U5 resistor string, acting as a precision divider. U3 operates in a gain of minus one-half and drives the center of the U5 resistor string to maintain the ADC common mode voltage at VREF/2.
U3 and U4 are LT1468A low offset highly linear op amps. U5 is a LT5400A quad matched resistor network with a guaranteed maximum mismatch of 0.01%. Matched resistor values in U5 are important because any mismatch contributes to both offset and full-scale error in this circuit. For this reason and because of their extremely low voltage coefficient, do not use discrete resistors instead of the LT5400A. R4 provides a quarter-scale shift to the output of U3. R1 and R2 form a divider that biases the noninverting input of U3 at VREF/2.