Electronic – Difference between virtual and ground plane

analogcircuit analysisgroundpcb

Ive created two PCBs, a power board to output a +-5V supply and a midway voltage reference (using the TLE2426) at 0V. This PCB powers a biosignal acquistion board shown in the spice diagram below.

The amplifier is an INA129 and the rest of the op amps are AD822ARZ. The power board does exactly as its meant to do, it supplies the board with +-5V and a midway reference voltage for the INA129, however there is almost a 1.3V difference between my floating virtual ground outputted from my power board and my ground plane on my biosignal board.

The V- and V+ are -+5V with reference to the Vref but are +1.3V and -8.7V when measured from the ground plane in my biosignal board, is there anyway to correct/explain this?

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Best Answer

First off your Instrumentation Amp resistors should be high precision types, 1% or better. Without this your DRL circuit, as connected, is amplifying the gain error and DC offsets. (As a side note, the DRL circuit and connection do not quite look correct.) If this circuit must be used in this form with non-precision resistors consider changing one of the 1.8k parts to a 1.5k in series with a 500 ohm potentiometer, then tune to zero with shorted input signals.

Also, your input signals label V+ and V- are confusing while you discuss power supply voltages of V+ and V-, (obviously your input signals would be better named Left, Right, for biomed connections).

As other are hinting, the vref should be used to create the virtual ground with other input, see info below.

Lastly, for more specific info see the TI app sheet/report SBAA188-July 2011, on CM Rejection using Right=Leg-Drive Amp, at:
http://www.ti.com/lit/an/sbaa188/sbaa188.pdf