Hi,
I am working on a PCB design using the AD7195BCPZ ADC, and I would like to use the AC excitation outputs (ACX) to implement an external chopping scheme using an external switch made of 2 NMOS + 2 PMOS transistors.
My setup is the following:
-
Differential input signals:
-
Vline+
-
Vline-
-
-
ADC reference:
-
VREF+ = 4.5 V
-
VREF- = 0 V (GND)
-
The intention of the external MOSFET switch is to periodically swap the ADC analog inputs so that:
-
Vline+ alternates between AIN1 and AIN2
-
Vline- alternates correspondingly between AIN2 and AIN1
So effectively, I am externally inverting the differential input polarity synchronously with the AC excitation/chopping signal.
My question are:
When using this external chopping approach, is it also necessary to swap/invert the reference inputs (VREF+ and VREF-) synchronously with the analog inputs?
Or is it sufficient to keep:
-
VREF+ fixed at 4.5 V
-
VREF- fixed at GND
while only swapping the analog differential inputs?
Also, regarding the ACX outputs: what is their default electrical state when AC excitation/chopping is disabled? Are they driven to a defined logic level, or do they go into a high impedance state?
My system is not a ratiometric bridge measurement; the reference is not part of the excitation path.
I would appreciate any recommendation or application note reference regarding the correct way to implement external chopping with the AD7195.
Thanks.
