Buffer Op Amp to Analog-to-Digital Converter Design: DC Coupled Single-Ended Input, Dual Supply, Single-Ended Op Amps, Single Input to ADC

Problem:
How do I buffer the input of an Analog to Digital Converter (ADC) when I have an DC coupled single-ended input, dual power supply, single-ended op amps, and single input to ADC?

Solution:


Double click on schematic to enlarge.

The ADC IN- input is connected to the common mode output of the ADC, and decoupled with C3. This sets the input range of the ADC as the common mode voltage to +Ref_A. Resistors R1 and R2 set the inverting gain of the op amp. Because it is dc coupled, the designer needs to consider the dc offset of the source, and the dc gain of the stage. Reference B is buffered and used to generate the common mode point of the stage through voltage divider resistors R3 and R4. The non-inverting dc gain of the stage on this common mode voltage must also be considered in the design. Capacitor C1 decouples the common mode voltage of the stage. Resistor R5 and capacitor C2 are ADC compensation components.

The Ref_B output of the ADC probably does not have enough drive capability to drive the resistive voltage divider, and must be buffered. This buffer is shown as a generic amplifier - it can be as simple as a non-inverting op amp buffer or more sophisticated if low drift is needed.

This circuit has a potential dc offset error, because the common mode voltage level at the op amp may not perfectly match that of the ADC.