Hi

I need help in the design of ECG, and more precisely with the selection of amplifier gain ad623.

I applied scheme as in the Annex but used the amplifier ad623.

I think the value of the resistor Rg is wrong. How to choose the correct value resistors R2, R3 and Rg ?

Sincerely

Hello,

The AD623's on-chip feedback path has about twice the resistance as that of the AD620, so you get more gain from the -623 with the same external resistor. The AD620 has a gain of about 8 in the circuit shown above, but substituting the AD623 results in more gain.

The instrument amplifier's gain-setting resistance is Rg in parallel with the series combination of R2 and R3. R2 and R3 total 44K ohms. Paralleling with Rg = 6.98K ohms yields a composite resistance of:

R = ((R2+R3) x Rg) / (R2+R2+Rg) = 6.02K ohms.

Using the AD620 amplifier, the gain equation is G = 1+ (49.4K / R) = 9.2

The AD623's gain-setting equation is G = 1 + (100K / R) = 17.1

Since you're using the AD623, the second equation is correct in your case. The AD623's gain-setting formula appears in its data sheet like this:

Since R2 and R3 set the gain of the left-leg drive amplifier, it would be best to leave them unchanged initially. These resistors should be matched in value for the best common-mode rejection of your circuit.

Best regards,

Bob