Op Amp Integrator question

[twigbat]

Hi,

Please could someone point me in the right direction to understand this circuit. From what I can fathom it's an Op Amp Integrator. The circuit is powered by 4 x AA batteries and is used to feed an A/D port on a CPU for the purpose of monitoring the battery voltage. There are two resistors, R2 and R3. R2 appears to be a constant value of 143K. R3 however, varies from circuit to circuit and sits between two test pins - TP1 and TP2. I am guessing that R3 is somehow selected to calibrate each individual circuit so the input to the A/D converter is accurate. If this sounds about right, I'm curious how the value of R3 might be selected and what parameter might be measured in order to determine what value R3 should be?

Any advice, gratefully received.

OPAMP Integrator.png (8.86 KiB) Viewed 11594 times

[twigbat]

Apologies, slight error with previous diagram. Amended one here.

OPAMP Integrator v2.png (9.52 KiB) Viewed 11584 times

[pebe]

Hi there,

The circuit is, in fact, that of a variable stable voltage supply.

R1 feeds current through D1 producing a constant 1.25V across D1 which is fed to the non-inverting input of IC1. If the op-amp is working in its normal linear mode (which this one is), its output voltage will rise until the inverting input matches the 1.25V of the non-inverting input. The output will then equal 1.25V multiplied by (R5 + R2 + R3)/R5. So with the values shown, IC1 output will equal 1.25V x (150k + 143K + 2.2k)/150K = 2.46V.

D1 has a nominal value of 1.25V but is subject to variation within the limits stated in the datasheet. Also the two inputs differ slightly because of the their offset voltage. R3 is therefore adjustable so the output voltage can be finely adjusted to give the required voltage.

C1 will not affect the voltage because its reactance at DC is infinite compared with the resistors. It is there to reduce HF noise or aid in the stability of IC1. Similarly R4, C2 are there to reduce noise.

As the output is fed to the A/D input of a micro, then it will be used as the reference voltage for the A/D conversion.

[twigbat]

Hi pebe, many thanks for taking the time to explain it, very much appreciated. You've been very thorough and I understood your very clear description. Cheers, Tim.

[pebe]

Thanks Tim. I'm glad I was able to help.