Below is the beginning schematic for a circuit I'm working on. We have a previous revision of that product (Rev A), and two sets of recent product (Rev B1 and B2). Rev B1 was made 2 weeks prior to Rev B2. All three products are tested on the same calibration system with the same settings.
When I measure pin 1 from the OpAmp, I get three different values for each product. I've unsoldered the 5.1Mohm resistor and measured it and it is correct.
According to the manufacturer, the components on Rev B1 and B2 are of the same batch but made a different times. Technically, all three revisions should have the same components.
Rev A (Older) - 1.21V/1.20V/1.21V
Rev B1 (Manufactured 2 weeks ago)- 1.15V/1.15V/1.1V
Rev B2 (Manufactured last week) - 1.05V/1.05V/1.05V
My question is: do photodiodes have tolerances? I am inputting the same amount of light into each product, but I get a different output from each version.
Technically, all revisions should be the same. At the very least I would expect B1 and B2 to have the same readings. It just seems very odd to me that every group of product has the same components, but they're all different readings. This is the very first measurement that takes place, and only gets amplified down the circuit.
Any reasons for why this is happening?