We have current to voltage converter opamp which has below schematic. Below is the formula to find TP1 Voltage :

Vout (TP1) = I1 x R2 or R1 (depend on the switch2 and not calculating the C1)

the question that I have is when the switch1 is open and nothing is connected to opamp V+ should Tp1 Voltage be zero doesn't matter what is the gain resistor(SW2), and if I am correct why I am not getting this in a real scenario.

I think it is because of the noise or rail power but not sure.I am using TP1 (after some other amplifier) for my 4-20mA output calculation.


simulate this circuit – Schematic created using CircuitLab


1 Answer 1


When Vin+ = 0V then Vin- must also = 0V

Vout = I*R always if not saturated.

When SW1=closed I source drives the output = Vout/Rf so that Vin-input stays at 0V.

For this to work well with high R values, Op Amp Input bias current Iib = ? must be low. FET types are very low.

Tp1 voltage = Iib * R e.g.

for Iib=10nA, R= 100k

  • 10nA * 100k= 1mV output.
  • \$\begingroup\$ So why when I build this circuit and test Tp1 I am not getting 0V \$\endgroup\$
    – Shahreza
    Commented Jan 8, 2019 at 15:34
  • 1
    \$\begingroup\$ does input bias current * R= Vout? \$\endgroup\$ Commented Jan 8, 2019 at 16:55
  • \$\begingroup\$ Yes, could because of feedback capacitor \$\endgroup\$
    – Shahreza
    Commented Jan 8, 2019 at 17:41
  • \$\begingroup\$ Cap leakage R must be >> R1or R2 but this is controlled by \$I_{IB}\$ not cap \$\endgroup\$ Commented Jan 8, 2019 at 17:42
  • \$\begingroup\$ So how do u cancel the input bias current so you will get the same result no matter what gain you have. \$\endgroup\$
    – Shahreza
    Commented Jan 8, 2019 at 18:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.