Troubleshooting the Precision Current Source circuit in my Fluke 8842A, and I noticed that the supply voltages for LM308A (U404) exceed the max spec given by the datasheet. The positive rail is +30V, negative rail is -5V, specs given by the datasheet are +/- 18V. Obviously it was designed like this, but I was looking into finding a replacement (LM308A is obsolete) but now I'm not sure what spec to go on.
Why would Fluke design the circuit to use supply voltages outside the spec of the op-amp?
what did you find that resulted in the decision to replace the LM308A?
I think the LM308A is fine. The problem I'm seeing is when measuring resistance with the meter. The reading are not accurate at all, and it seems like the reference current is not changing properly with range (ex: In the 20k range, a 10k resistor measures 5.4k. In the 2k range, it measure .54k, in the 200k range, it measures 54k). It's most likely the analog switches (testing them soon), but when I order parts, I wanted to order the opamp just in case to have it on hand, but it's obsolete.
I bypassed the protection circuit and still have the same issue. Measuring the current coming out of the circuit, it measures the same in 200, 2k and 20k range - 0.0545mA (should be 1mA in 200 and 2k, and 0.1mA in 20k). 200k range it measures 0.0565mA (should be 0.01mA, and 2000k and 2M range it measures 0.0452mA (should be 0.005mA and 0.5uA).
I checked the first stage of the precision current source and that seems to check out, so it's either this opamp or the analog switches. Problem is the analog switches are a fluke part, and I cant seem to find anything close to them. I think I will try and use a LTC222 and make an adapter board.