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schematic

simulate this circuit – Schematic created using CircuitLab

I want to add a DC volatage of 1.25V to a sine wave signal. I am using a DC bias circuit with a voltage divider (R1=R2=100Kohm) connected to a 2.5V source and to the ground. That is giving me the correct 1.25V DC offset that I need in my application.

However I would like to lower the impedance so I am using a voltage follower op amp. I am using the LT6202 op amp which is supposed to be rail-to-rail and unity gain stable. When I connect the voltage follower to the DC bias circuit I mentioned above, the DC offset rises from 1.25V to 1.32V.I have done the simulation also and I am getting the same results.

When I am using R1=R2=10KOhm instead of 100KOhm the DC offset becomes 1.258V, which is closer to the wanted 1.25V value.

I am not sure what is causing that. Any help?

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  • \$\begingroup\$ You need to show us your schematic if we're to have any hope of helping you. \$\endgroup\$ – Nick Johnson Nov 11 '15 at 14:23
  • \$\begingroup\$ Where are you measuring the offset? At the input or the output? \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 11 '15 at 14:37
  • \$\begingroup\$ The offset is measured both in the input off the op amp and at the output and is for both around 1.236V with a real small flactuation between those two points. \$\endgroup\$ – JPa Nov 11 '15 at 14:41
  • \$\begingroup\$ Where's your input signal in the schematic? You say it's 1.23V, which is very close to the desired 1.25V, but don't mention that in your question - which figure is correct? \$\endgroup\$ – Nick Johnson Nov 11 '15 at 14:44
  • \$\begingroup\$ I do not have an input signal on that schematic, I am trying to understand at the moment why I am not getting the expected 1.25V from the above circuit but I am getting a 1.32V DC offset instead. The input signal will be connected at the C1 at the side that is currently connected to the ground. \$\endgroup\$ – JPa Nov 11 '15 at 14:47
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Nonideal opamps have input bias current - that is, they source or sink a small amount of current from their inputs. If you look at the LT6202 datasheet, you can see that for this opamp, with Vcm at half-rail voltage (which is roughly the case here), bias current will be between -7 and -1.3 microamps. This is sourcing current into your resistor divider and affecting the setpoint, equivalent to having a smaller resistor in the top half of your divider.

You can use lower value resistors to reduce the error - at the cost of more wasted current - or find an opamp with smaller input bias current.

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Your LT6202 has an input bias current of -8 uA to + 2.5 uA. That coupled with the 100 k ohm will give you a big offset voltage. Pick another oapmp or reduce the resistor values.

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