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Hi

I've selected AD8226 in my design for self diagnosis of 0-20mA current loop.

I simulated AD8226 for sensing a constant current 1mA across 40E resistor. Below is the result for same.

Below are the parameters set. +Vs=30V , -Vs=-6V G=1 Sense resistor=40E current flowing=1mA Observing output voltage of 40.78933mV(Fig 1 and 2) but Vout=I*R = 1mA*40 = 40mV

Again I simulated AD8226 for sensing a constant current 20mA across 40E resistor. Below is the result for same.

Below are the parameters set. +Vs=30V , -Vs=-6V G=1 Sense resistor=40E current flowing=20mA Observing output voltage of 800.7578mV(Fig. 3 and 4) but Vout=I*R = 20mA*40 = 800mV

It is observed that nearly 0.7mV is additionally added to output, which is 700uV, But in the datasheet it is mentioned that 50uV is the input offset voltage. As gain is 1 output offset also should be 50uV.

Please let me know if I'm missing out anything.

Thanks and Regards

Nidhi P Shetty

• Hi Nidhi P Shetty,

The offset you're seeing at the output of your simulation is the offset voltage of the device plus the offset introduced by your common-mode voltage.
On the datasheet you have a maximum offset (without common-mode) of 700uV or 450uV depending if you have ARZ or BRZ respectively.

On LTspice, AD8226 is modeled with an offset voltage of around ~313uV.

Also, I've checked the CMRR of AD8226's model and it is around ~94dB.
The offset introduced by the common mode voltage is given by:

On your first case, you have a common-mode voltage of 23.98V, therefore you'll have an additional 478.46uV on your output.
Add this to AD8226's offset voltage of 313.36uV. You now have a total offset of around 791.82uV. Also shown in this simulation.

On your second case you have a 23.6V common-mode voltage.
Similar with the first case you'll have an additional offset around 470.882uV, the total offset now will be 784.242uV.

Thanks Nidhi P Shetty.

Best Regards,
Dann

• Hi Dann

Regards

Nidhi P Shetty

• Hi Dann

After seeing practical results given by AD8226 I got another doubt.

Whether total offset error (Sum of internal offset and offset due to common mode voltage) remains same over same common mode voltage.

I will not change common mode voltage. Now if I check this offset voltage in many ICs whether the offset value remains same?

I just wanted know how I can correct this error without adding any additional circuitry(like OP1177). If it is a constant error throughout then I can directly subtract it.

In real time application is this method is feasible.

Operating temperature requirement for my application is  -5 to +60C.

As per my previous query, I got max. possible error for my application around 1mV but practically it is less than that. It is good that it is less. But how I can compensate for this without major hardware change.

Regards

Nidhi P Shetty

• Hi

Any update on my query?

Regards

Nidhi P Shetty

• Hi Dann

Any update?

Please help me out with respect to mentioned query. Without understanding these I'll not be able to proceed with these parts.

Regards

Nidhi P Shetty

• Hi Nidhi P Shetty

Apologies for the delayed response.

Are you referring to the internal offset and CMRR on different AD8226 IC?
These parameters may vary across different IC. Therefore, you want to measure it on your AD8226 to know exactly what offset you have before calibrating it for a more accurate measurement. You may also want to consider the input offset voltage drift of the device, more on it in this article: https://www.analog.com/media/en/training-seminars/tutorials/MT-037.pdf

In terms of offset compensation, since you don't want any additional circuitry, you're right, the easiest way is to subtract this offset through software.