AD8220 Instability in high impedance measurement

Hi All, 

I am using the AD8220 in a electrochemical sensor measurement application. 

The sensor outputs a DC signal in the -1/+1 range. The signal is created between a ref electrode and a "working" electrode, as a potential difference, creating the voltage  we want to measure (physically, the sensor is like a small battery that we want to measure without discharging).

The key requirement of the circuit is to have an high input impedance, >100Giga-ohms. I selected the AD8220 for its high input impedance.

Below is a snap shot of the circuit we are using. The AD8220 is powered with +5V/-5V.

The VREF is 2V, created through a voltage-divider and then "bufferized" through a simple op-amp. We have built a custom 4 layers PCB with proper ground plane.

Input filtering caps C3/C4/C6 are unpopulated right now (i fear that any leakage at thuis point might disturb the sensor, creating a path for a dc current to flow.

We have an instability problem (i am guessing), we get no signal from the sensor at the output, but if we physically touch the input pins with our hands, the signal is output. This is a very frustrating issue.

1/ Do you see something bad about the design above ?

2/ Since the input bias current is very low, can the instability come from a poor polarization of the input of the AD8220

Thansk a lot for helping

Best Regards from France

  • 0
    •  Analog Employees 
    on Sep 22, 2020 12:50 PM 2 months ago

    Hi bDef38330,

    Thank you for using AD8220 in your design.
    For your circuit, will it be possible to show the input part of the AD8220? 
    How does the sensor connects with the AD8220? and if there are other blocks connected to this input.
    I'm currently interested on the input configuration (nature of the input, levels etc.) on the AD8220, I'd like to see if you're operating within the common-mode voltage of the AD8220.

    We can actually check this out easily using our online Diamond Plot Tool to see if we're using the AD8220 at the its proper operating level.
    I know the response was a little late, but thank you for the patience bDef38330.

    Best Regards,
    Dann