Help identifying circuit and its function...

Thread Starter


Joined Oct 1, 2009
Hi all, I'm currently waiting on parts for another project so I dove into reverse engineering an old A/D converter that had been acting up. While tedious, I find it's a great way to learn. So far the circuit is pretty advanced, it stays differential up to the converter, but there's a unique BJT sub-circuit across an opamp but only on the + signal path. The circuit in question is in the box. ADC Odd Circuit .png

The connection to Q15 comes from the main board, so I assume this is a control voltage to turn Q15 & Q14 on and off. For a second there I thought the part coming from R176 to Q13 was a current source to bias the opamp into class A, but again, there's no identical sub-circuit on the - signal path so I doubt its that. Any clue what this thing does and how it works?