[SOLVED] Maximum feedback resistor value for biasing an amplifier input

[c_mitra]

The input bias current in the feedback resistor (and maybe also in its other resistor) causes a voltage shift in the output voltage if the opamp. Usually an opamp has an extremely low input bias current so the effect is very small.
If your circuit needs a 10M feedback resistor and you do not mind having a voltage shift of 0.1V (when the other resistor is capacitor-coupled to ground) then a bias current of 0.1V/10M= 10nA will do it.

In the case of a pH meter (electrometer), current flows through a glass electrode that is typically in the range of pA and use opams with bias currents in the fA range. The price? The electrode response takes 10s of seconds. Again, we use feedback resistors in 10M or higher this is a case of impedance matching (and the voltage does not need much amplification; it is in the range of 100mV).

 

[Audioguru]

Yes, many Cmos opamps like a TLC07x has an input bias (leakage) current of typically 1.5pA. I have never seen an opamp with an input leakage current that is 1000 times less.

 

[KlausST]

Hi,

I have never seen an opamp with an input leakage current that is 1000 times less.

Maybe there are. But it is not specified in the datasheet. Not for cheap OPAMPs.

It´s easier and cheaper to specify picoamperes in the datasheet and measure those picoamperes during production tests.
Measuring in the femtoAmperes range simply takes time. And time is money.
So if they specify femtoamperes they need to test it ... on each single produced OPAMP.
(because "testing time is money" they specifiy the testing time in milliseconds for the manufacturers of testers.)

They won´t do this with a cheap TLC07xx. (And no user cares if it is 1.5pA or 0.1pA.)

But they need to do this with special "low bias current OPAMPs". And thus they are more expensive.

Klaus