Setting LMC6001 offset voltage

Question

I have got two LMC6001 samples. I will use them for pH sensor interfacing. With one LMC6001, I can cope with high input impedance errors. But I want to set offset voltage near by to 2 V. Can I make offset circuit with other LMC6001?

Answer 1

You don't need the second opamp. These are typical circuits for offset control in opamps which don't have offset null inputs, like the LMC6001:

You'll want the lower circuit, because there you can use the fA input current feature of the opamp. The other one has a much lower input impedance.

Without R3 and the potmeter this is just a non-inverting opamp where

\$ \mathrm{ V_{OUT} = \dfrac{R1 + R2}{R1} V_{IN}}\$

Adding the potmeter:

\$ \mathrm{ V_{OUT} = \left(\dfrac{R1 + R2}{R1} + \dfrac{R2}{R3} \right) V_{IN} - \dfrac{R2}{R3} V_{OFFSET} }\$