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Opamp input series resistors

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ArticCynda

Hi everyone,

There are a lot of opamp circuits where resistors are used in series with inputs. See for example the design of the µCurrent (https://www.eevblog.com/files/uCurrentRev5schematic.pdf). Here, R12 is a 270 ohm resistor leading to the non inverting input. Since the current flowing into the opamp is so small, and the value of the resistor is also very small, the voltage drop over the resistor is nearly zero. So what is its purpose?

There are a lot of explanations online, but none of them are very clear, and most of them are contradictory. The resistor values also seem to be pretty random.

In the same schematic as above, also series resistors on the output are used (R8 and R10) with the same value. What is their function?

Since this design uses break before make switches from external currents, a voltage transient may exceed input Schottky rail clamp diode current ratings.( Vin is rated at +/-0.3V max outside supply rails.) Although input current max is not obvious but current source is only 1mA but unknown voltage, the 270R offers some current limit for some overvoltage protection.

Hi,

1) the resistor could just act as current limiting resistor. In normal use it has no effect. But in case of overvoltage at the input, then the OPAMP inside protection diodes may become conductive. The energy is dissipated by the resistor..
This is especiallly true when the OPAMP is powered down, but the input signal exists.

2) OPAMPS with BJT input stage sometimes have a lot of input bias current. At the -In there are resistors, where the bias current causes a voltage drop, you will see this as additional input offset voltage. To compensate this you could add a resitor at +In. It should be the same value as the resistor at -In. Then the bias current causes the same voltage drop on both OPAMP inputs and the result is no (low) additional offset volatge.

3) it could build a low pass filter in combination with the OPAMP input capacitance. This is only for high frequencies, or better say high dU/dt. It may prevent the input stage from saturation. Often saturation of the input stage leads to a relatively long time for the OPAMP to get back in stable regulation.

Klaus

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