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

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

Solution:


Double click on schematic to enlarge.

Resistors R1 and R2 set the inverting gain of the op amp to the desired level. Using the common mode output of the ADC to drive the non-inverting input of the op amp creates the virtual ground for the op amp. The virtual ground level is isolated from the input by C1. Resistor RT2 isolates the output of the op amp from the capacitive / inductive load created by coupling capacitor C2 and the primary of T1. It also acts as a matching resistor for termination resistor RT3, which terminates the secondary of transformer. Resistors R3 and R4, and capacitors C3 and C4 are compensation components specified on the ADC data sheet. Capacitor C5 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-.