Ext com mode pin 11 on Extension Connector E2

[jmadsenee]

Hi,

Does anyone know exactly what that pin does, what it's connected to, the circuitry around it, anything?

Any help is appreciated!

Cheers!

John

[redpitaya]

Hi,
FPGA has differential inputs (AIPx and AINx x:{1-4}) for the slow analog inputs on E1 connector
(AIx x:{1-4}).
So you are measuring voltage between AIPx and AINx.
Pins from E1 (AI1..AI4) connector are connected to the AIPx of the FPGA.
The AINx pins form FPGA are connected to each other (common mode) and to the internal reference voltage (u_ref).
The pin Ext.com.mode is also directly connected to the AINx and the ref. voltage.
So if you don’t have any voltage on Ext.com.mode pin you are measuring voltage between
AIx and u_ref (which is effectively zero).
If you have any voltage on Ext.com.mode pin you will measure voltage between
AIx and Ext.com.mode.
Because Ext. voltage is applied to all slow analog inputs its “common mode”
Best, Red Pitaya