AC excitation signals

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.