Error with the fixed gain of the internal instrumentation amplifier

Hi,

Good day. We'll look into this and get back to you.

Regards,
Gilbeys

Hi massimoklk ,

Good day. A node should not be connected in the High-Pass and Low-Pass Sections of the circuit. 

This had an effect on the gain value that you have measured

Also, I tried implementing the input side for proper simulation of the gain of the circuit. Please see the revised schematic for reference. It should at least correlate with the expected gain. Thank you.

ad8233-v3.asc

Regards,
Gilbeys

Thank you very much! what a silly mistake...
by the way, why did you put the DRL output to GND instead of simulating the connection with an electrode?

thanks again

Hi massimoklk ,

I was trying to simulate the AC response of the IN-AMP in the circuit. But you can also connect the RLD pin via voltage dependent voltage source

It should have no effect on the response of the output. Thank you.

Regards,
Gilbeys

Hi Gilbeys!
I've got further with my design. Now i have proper filter design and a correct BW for my application. However, I was trying to calculate the CMRR of the circuit, and i did not see what i was expecting. The ad8233 has a typicalCMRR value of 86 db. However, in my simulations i have something like this:

How can I improve the CMRR of this circuit? it is due to the internal opam from the sallen-key filter?
ad8233-v9CMRR.asc

Thank you for your help.

Regards,

Hi massimoklk ,

Good day. The datasheet discusses some ways to improve the circuit's CMRR. Thus, the use of the driven electrode is one of them.

Can you provide how you calculate the CMRR and arrive at your plot above? Thank you.

Regards,
Gilbeys

Hi!
yes of course.
as you can see in my simulations, I have implemented the RLD in my system. However, i do not know how to simulate it.

On the other hand, I calculated the CMRR by simulating the ac response with diff and common mode.
Here I've attached the results in .txt. as you can see, they are in dB, therefore, the cmrrr is the subtraction between gain in diff mode vs gain in common mode.
additionally, i have added the new simulation file.

CMRR_DIF_MODE-ad8233-v9.txtCMRR_COMMON_MODE-ad8233-v9.txt
8562.ad8233-v9CMRR.asc
dif mode:

common mode:

thanks!