hello,

i built this circuit:

+Vop = 13V
-Vop = -13V

and when i powered it without any input or output connected and measured the potential between BUFIN+/BUFIN- and GND it was almost 3V, then fell gradually and eventually settled around 1.2V. what could cause this circuit to have a non-zero voltage at the inputs and how might i correct it to drop as close to zero as possible?

thanks!

 

Just for kicks, reduce R21 significantly, say to 100k. Check the behavior again.
If the voltage is reduced, it could be the Opamp’s input bias current.

 

Agreed. You can have up to 175 nano amps dribbling out of the input pins (offset this is input bias current) and a 2.2 Meg resistor. 2.2 Meg x 175 na =0.39 volts maximum. Following schmitt trigger's advice would reduce that maximum offset to 1/20 of what is is now. and

 

i didn't have a 100k resistor handy, so i bridged R21 with a 500k pot and that definitely reduced the voltage at the inputs. i could see it vary as i moved the wiper on the pot. what are the consequences of leaking that current into the inputs? would it adversely affect the outputs of upstream components in any way? this circuit sits downstream from another opamp, will having that voltage on its outputs degrade its performance? are there any negatives to decreasing the value of R21 besides the increase in passive power loss to ground from the input terminals of the opamp?

thanks again!

 

i didn't have a 100k resistor handy, so i bridged R21 with a 500k pot and that definitely reduced the voltage at the inputs. i could see it vary as i moved the wiper on the pot. what are the consequences of leaking that current into the inputs? would it adversely affect the outputs of upstream components in any way? this circuit sits downstream from another opamp, will having that voltage on its outputs degrade its performance? are there any negatives to decreasing the value of R21 besides the increase in passive power loss to ground from the input terminals of the opamp?

thanks again!

It depends on what is driving those inputs.
if it is another op-amp with an output capability of ≈20mA it won't even notice 175nA.