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# AD623 input signal offset compensation

I am using an AD623 with a MC33161 voltage monitor to monitor the temperature of a part.

V_SET is around 1.1 V, V_ACT the current temperature monitor voltage being equal to V_SET when the temperature is reached.

Reference voltage for the AD623 is 2.54V from the MC33161. Gain ist set to 213 by R1.
The circuit works well, but some temperature controllers show a difference of up to 6 mV for (V_ACT - V_SET) at stable temperature which is too much for the gain,
an out of valid temperature is indicated by the MC33161 for such an 'offset'.

Hence a compensation of one of the input voltages is needed, which has to be positive or negative, in the range +- 0 to 6 mV.

There is no negative voltage available in the circuit.
A single simple voltage divider at V_SET or V_ACT may not work due the positive as well as negative offset.

Will an OpAmp be the solution to add / subtract a few mV offset or what else can be a stable solution?

Thanks for any hints!!!

• Hi stoccarda,

Good day and thank you for reaching out to us about your query.
If I understood correctly the one giving the offset of 6mV are the temperature controllers  which is connected to V_ACT and 'false' triggering the voltage monitor MC33161D because this is being gained up by the inamp. I just want to make sure that all the AD623 works expectedly and I understand the scenario.

If this is the case will it be okay if you show us the circuitry that connects to V_ACT? This may show us what's causing the offset or if it is inherent with the device/s connected there. The only way I could think off for now is if we can calibrate the effect of the 6mV offset at the output of the AD623 going in to the voltage monitor but this will require doing it for all the temperature controllers.
Let's see first what we have on the initial stage stoccarda then we'll go from there.
Thank you.

Best Regards,
Dann

• Hi Dann !

There is one temperature controller which has two monitor voltage outputs: V_ACT for the actual temperature  and V_SET for the set (desired) temperature.
If the desired temperature is reached, V_ACT should be equal to V_SET. In the real controller V_ACT can be up to 6mV higher or lower than V_ACT  when temperature is reached and stable. It is different for each of them.

As you have written, the gained offset causes the false triggering of the voltage monitor.

There is no negative voltage available on the board.

The controller is a ready circuit and can not be changed, hence an adjustable +- offset is needed to compensate the error.
Hope it clearifies the problem .

• Hi Dann,

• Hello,
will the OPAmp (or different) be a better forum to get ideas how to find a solution?

• Hi Stoccarda,

Sincere apologies for the delay in response.
Since you mentioned that there's no problem with the AD623, I think we have to look at the actual circuit connected to the input pins of the AD623.

As I requested from my previous response, will it be okay to show us the schematic at the input side of the AD623? or give us the actual temperature controller used.
I'm curious to look at the specification sheet of these temperature controllers and how they were connected to the AD623, since if the offset is inherent, we may be able to adjust the trip point of the MC33161 to accommodate the maximum offset we expect from the system. One more way is through system calibration, but since you're not using any microcontroller (based on what you showed here) we have to think about how to actually perform it based on the diagram.

Thanks stoccarda. Apologies again for the delay in response.

Best Regards,
Dann

• Hi Dann,
thanks for coming back !

The temperature controller has two monitor outputs:
ACT: Temperature  voltage  monitor.    Buffered  measurement  of  voltage  across
Sensor+ & Sensor-.  [1 kΩ output impedance]
SET :Setpoint voltage monitor.  Buffered measurement of the setpoint input.
[1 kΩ output impedance].

They are directly connected to pin 2 and 3 of the AD623.

From data sheet:

Setpoint vs. Actual T Accuracy: 5 mV (negative as well as positive difference measured)
with Gain=213 -> more than 1.065 'offset error' at MC33161.

You are right, there is no MCU used in this circuit.

Thanks