Hi !

I have a question about AD8220.

Datasheet(P20) says that

"The AD8220 defaults to G = 1 when no gain resistor is used.

Gain accuracy is determined by the absolute tolerance of RG. "

Q1)

Does this means R1 & R2 accuracy is small enough so

we don't have to think about the internal resister accuracy?

Q2)

Can you tell me the absolute accuracy of R1 & R2?

I want to know this because to think about the gain error for the Instrument-Amp

I think R1 & R2 value is very important.

Is there any specification about R1 & R2?

Is this the resister specification?

Best regards.

Kawa

Hi Yusuke,

The AD8220 is a three-opamp inamp and there are 2 different specifications to consider here but in the end it all comes down to the gain equation: G = 1 + 2Rf/Rg.

In a gain of 1 configuration, there is no Rg and the gain equation doesn't depend on the Rf value either. It's like having buffer with a resistor in the feedback and the value of the resistor doesn't affect the gain. Instead, the gain of 1 accuracy depends mainly on the resistors in the difference amplifier and also on the op amps.

For gain >1, you put an Rg resistor and now "2Rf/Rg" term is larger than "1" term so the gain error is determined mainly by the Rf resistors and Rg resistors

Now as you could see on the table below:

For Gain = 1, it shows how close the difference amp gain is to 1 (within 0.06%)

For Gain > 1, it mainly shows the accuracy of the Rf resistor ( but also includes the difference amp gain error). Note that for this configuration, you also need to add in the accuracy of your Rg resistor.

1. No. The R1 and R2 resistor accuracy is important and it is specified in the Gain error of the AD8220.

2. The gain error specifications includes the R1 and R2 accuracy. The Rg resistor accuracy must be added to the AD8220 gain error specification to get a total gain error.

I hope this helps.

Best regards,

Emman