The customer want to place only one resistor at the 7th pin(No Potentiometer),
and he want to know how to select the resistor's value. For example, if the
gain is 15, then how to calculate the resistor value? Whether there is a
formula to calculate the resistor's value at fixed Gain?
the internal resistors of the AD626 are ratio matched for gain of 10 or 100.
The absolute tolerance of the resistors is +/-20% so I think that is why the
datasheet does not specify a formula. The external gain-setting resistor value
would have to be dialed in/adjusted to match the internal resistors. So even a
formula would have +/-20% error, but I’m sure we could figure it out.Assuming
the customer can handle the +/-20% variation, I believe the gain formula should
Gain = 5 * (1+ 10k Ω / (10 k Ω || (555 + Rext)))