Instrumentation Amplifier AD620 problem: output voltage sudden drop

[resodad]

Hello,
This in amp is showing some unexpected behavior.
The voltage output is normally near zero. After 10 minutes the output still fluctuates with the input, but the output voltage drops to -3 volts offset or so.
Have you ever seen this? What's wrong?
View attachment 60255
Thanks,
John

 

[crutschow]

How much gain do you have? What is the value of R4?

Monitor the voltage at pins 2 and 3 and see if it changes.

 

[Ian Rogers]

Is the Load cell temperature compensated? Sounds to me that the input is dropping off (as Carl implies) read the mV on pin 2 & 3 on startup & 10 minutes running.

If it's a 350 ohm bridge the zero drift due to temperature should be minimal... If it's a 1k bridge then it could be that. -3 volts with a 100 ohm gain resistor is about 6mV drift (a bit high)

 

[resodad]

Thanks Carl and Ian,
Rg is 499.0 ohms; gain is 100.
This was a one-time sudden drop in output voltage, not temperature drift.

AD620 Pin 2 (- input) reads 3.47v
Pin 3 (+ input) reads -7.55v

BUT, when I disconnect the load cell from the AD620, the load cell outputs (top and bottom of bridge in fig.) are both 5.007v, which is correct. I know the load cell is working.
John

 

[ericgibbs]

Thanks Carl and Ian,
Rg is 499.0 ohms; gain is 100.
This was a one-time sudden drop in output voltage, not temperature drift.

AD620 Pin 2 (- input) reads 3.47v
Pin 3 (+ input) reads -7.55v

BUT, when I disconnect the load cell from the AD620, the load cell outputs (top and bottom of bridge in fig.) are both 5.007v, which is correct. I know the load cell is working.
John

hi John,
I would say pin3 input is damaged, I would suggest you try another AD620

 

[resodad]

Thanks Eric,
So I wonder what I did to damage it? It happened to 2 chips.
John

 

[ericgibbs]

Thanks Eric,
So I wonder what I did to damage it? It happened to 2 chips.
John

I would check the wiring etc, before inserting a new AD620.

For reference I have just simulated the AD620 in your circuit and it works as it should 0mV to 10mV in, 0 to1V out.

BTW: I always have a 4k7 in series with both input pins.