Buffer Op Amp to Analog-to-Digital Converter Design: Differential Input, Single Supply, Transformer Coupled

Problem:
How do I buffer the input of an Analog to Digital Converter (ADC) when I have a differential input, single power supply and transformer coupling?

Solution:


Double click on schematic to enlarge.

Resistors R1 through R4 set the inverting gain of the op amps to the desired level. Using the common mode output of the ADC to drive the non-inverting inputs of the op amps creates the virtual ground. The virtual ground level is isolated from the input by C1 and C3. Resistor RT2A and RT2B isolate the output of the op amp from the capacitive / inductive load created by coupling capacitors C2 and C4, and the primary of T1. It also acts as a matching resistor for termination resistor RT3, which terminates the secondary of transformer. Resistors R5 and R6, and capacitors C5 and C6 are compensation components specified on the ADC data sheet. Capacitor C7 is a decoupling capacitor selected to reject noise in the system. The common mode point of the ADC is connected to the center tap of the secondary of T1, which sets the common mode point for the signals connected to IN+ and IN-.

The primary use of this circuit is for gains of less than 1, which are adjusted by the turn ratio of transformer T1 - not by attempting a gain of less than 1 in the buffer op amp stages.