The BA3121 is an audio (ground loop) isolation amplifier. In the datasheet there is no simple pinout picture with names of the pins, so I draw this pinout picture by following the block and the circuit operation diagrams. It takes me some time to do this (i am not an expert).

Before I try/continue, I want to be sure my findings are correct. Is this okay?

BA3121 designed pinout

I have also some doubts about the pin order because mostly, the 5th pin is the upper-right pin. What is Vm1 and Vm2 in the schematics, what is "Vm" and why is it called "Vm"?

The pictures I have used to come to this:

BA3121-circuit-diagram BA3121-block-diagram

You can find the full datasheet here:


Your re-labelling of Vm1 and Vm1 is not equivalent to yours so you should not change them.

These are midpoints for DC near the mid point of V+ and gnd and are both AC input from CM gnd noise but DC is created by internal fixed R divider and input by an R to base on pin6 whose emitter controls Vm1 bias.

This clever circuit cancels the influence of chassis ground noise by more than 50 dB. enter image description here

  • \$\begingroup\$ Okay thanks, so my pinout is correct except the naming of the AGND labels? I just wanted to know if my pinout is correct. So Vm mean "Voltage midpoints". The capacitors are only there to decouple inputs and outputs, right? To reduce current noise when input and output are on the same ground rail. However, an audio connector has three terminals, Left, Right, AGND. Maybe the AGND is connected to the chassis or common ground but the connector provides three terminals. That's why I labeled it AGND because I thought it makes more sense in real life. In most cases I use already decoupling capacitors \$\endgroup\$ – Codebeat May 3 '18 at 16:20
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    \$\begingroup\$ Yes but inside they different circuits, connected to a chassis which may not be not 0Vac between these pins, meaning a noisy car ground from preamp location to power amp location. Normally we define “ground” as a 0V for something. e.g. analog, digital, AC, DC which are not same here. The pins are well defined as I pasted in answer. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 3 '18 at 17:26

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