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Hi team,

I had a question when verifying offset by common mode voltage with LTspice.

In the following circuit, the input common mode voltage was swept from 0V to 70V, and the output voltage fluctuation was confirmed. <result> The result shows that the output voltage shifts in proportion to the input common mode voltage.

I think this is due to CMRR, and I did it to confirm it.

However, the CMRR of the data sheet is a minimum of 90dB as shown below.

For example, for Vcm = 10V highlighted in the table above, the output voltage was approximately 1mV.

I think the effect on the true output voltage is Vcm / 90dB = 10V / 31622 = 320uV.

In other words, the result of LTspice is 3 times larger than expected.

Could you explain why? I have one more question.

When the input common mode voltage is 2.5V, the output voltage deviates slightly from 2.5V.

I thought it would be 2.5V ideally, but why is it off?

Is an offset added to the spice model?

Best regards,

T-yoshi

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• Hi team,

I noticed that when the common mode voltage was 2.5V, the output voltage was shifted by 4mV to 2.496V.

Since the offset voltage is 200uV, RTI, from the data sheet, I understand that the model has an offset of 200uV applied.

Is this correct?

Then, it was assumed that all the errors were due to the common mode voltage from the data after deducting the offset.

It looks as if the CMRR is 80dB when calculated as follows. My calculation may be wrong as it is a minimum of 90dB in the datasheet.

Could you please comment on something?

I want to know what CMRR is actually set in this model.

Best regards,

T-yoshi

• Hi T-yoshi,

Good day! I will check this internally and will get back to you as soon as I have relevant information.

Best regards,

Franz

• Hi Franz-san,

I was asking this question thinking that CMRR is Referred to the Input, but now I think that CMRR is Referred to the Output.

If so, it makes sense to think that LTspice's CMRR is set to 106dB.

Best regards,

T-yoshi

• Hi Franz-san,