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I have a similar problem as javier acha already described here Convert from a specifc range to another one which is to map a (AC, 100Hz) voltage range from [-12V, +12V] to [0V, 4V]. I am currently trying to solve it using the OpAmp approach.

This is why I have (a very basic) question concerning Spehro Pefhanys suggested circuit (see link):

What do I actually need R6 and R7 for? Can I also just leave them away?

Thanks Cristobal

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  • \$\begingroup\$ What supply rails do you have access to? What will you be driving with the output? How much loading can the input tolerate? \$\endgroup\$
    – jonk
    Commented Mar 1, 2017 at 8:08
  • \$\begingroup\$ I have a laboratory power supply which can provide up to +-25V. The circuit shall be used to map the voltage range coming from a photodetector to the range allowed by an ADC (ADS1115). \$\endgroup\$
    – Cristobal
    Commented Mar 1, 2017 at 8:36
  • \$\begingroup\$ I am currently using an OP467 \$\endgroup\$
    – Cristobal
    Commented Mar 1, 2017 at 8:36

1 Answer 1

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R6 and R7 are there to compensate for the input bias current. If you had an ideal op-amp it would have an input current of zero and you could connect the pin directly to 0V, but in the real world their inputs do require a very small amount of current. This current flows through the input resistors and therefore produces a small offset voltage at the inverting input.

By connecting the non-inverting input through a resistor as well, you ensure that a similar offset voltage is seen at both inputs and therefore its effect on the op-amp output is massively reduced.

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  • \$\begingroup\$ Do you also know what C2 and C3 are exactly needed for? \$\endgroup\$
    – Cristobal
    Commented Mar 1, 2017 at 11:34
  • \$\begingroup\$ Am I right in assuming that they act as filters? \$\endgroup\$
    – Cristobal
    Commented Mar 1, 2017 at 11:36
  • \$\begingroup\$ Something else about R6 and R7: Is it important to use certain resistances or is anything similar to the inverting input resistor alright? \$\endgroup\$
    – Cristobal
    Commented Mar 1, 2017 at 11:38
  • \$\begingroup\$ C2 and C3 are decoupling capacitors, yes they act as filters by providing a path to ground for high frequency noise on the supply. R6 and R7 should be about the same as the inverting input resistors, doesn't have to be exact as the currents will vary slightly as well so it will never be perfect. \$\endgroup\$
    – Finbarr
    Commented Mar 1, 2017 at 12:58
  • \$\begingroup\$ How can I do this? \$\endgroup\$
    – Cristobal
    Commented Mar 2, 2017 at 16:11

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