Divider Resistor Selection in Opamp

Hi!

If you use two 1MOhm resistors you will have an output resistance of 500kOhm. This output voltage has to "drive" the opamp input pin (offset and bias current, leakage). And this current is very small (<1 nA) for this opamp. Hence, calculating with 1nA, you get a voltage error of 500k*1n=500u=0.5 mV. If you use 2MOhm you will get twice the output resistance and twice the voltage error.

Best regards,
Edwin Krasser 

Thanks Edwin 4 ur reply.

-> What does it mean "This output voltage has to "drive" the opamp input pin (offset and bias current, leakage). And this current is very small (<1 nA) for this opamp". How 1nA value is found?

->I have read that opamnp inv & non-inv pins are very high impedence pins. Now if directly connect pin  to 4V voltage source & in second case via 10K resistor.How do they diff.

-> In open collector opamp, it is fine ot have supply voltage less than voltage at o/p open drain terminal or V(open termianl) <= V(supply)

Aamir;

" What does it mean "This output voltage has to "drive" the opamp input pin (offset and bias current, leakage). And this current is very small (<1 nA) for this opamp".

How 1nA value is found?"

The TLV3401 data sheet shows its input bias current specification on page 4. It is only 80pA typical, 250pA maximum.

Hi,

I did not find the 1 nA somewhere. But as you said, the 250 pA are maximum, but 1500 pA is the max over the whole temperature range. Hence I calculated with 1 nA (just a guess). And this is the current that might flow in one pin.

Best regards
Edwin Krasser