>I am very supprised that you have +/- 30 volt rails for an op amp. What
>kind of device is this? Most op amps would die with more than about +/- 20
>volts, or a total of 40 volts. Yours is more like 60 to 70!
Yes, that's what I've seen on most op amps, but this one is in fact designed
for a higher voltage supply. The op amp in question is a Motorola MC1536,
which I found on a chip directory web site listed "High Voltage Op Amp +/-
40V". Motorola's chip literature web site was unavailable, so I couldn't get
more info.
>If you have a DC present on the input of both pins to the diff amp, over
>the manufactures stated common mode range, the op amp can do all sorts of
>unpleasant things, including failing. What is the DC level at the pins,
>related to power supply ground, on each input (+ and -) of the amplifier.
>Common mode voltages are usually below what the supply rails are, for the
>op amp to have a chance of working.
The voltages are quite benign: 312 mV on the - input pin and 309 mV on the +
input pin. That makes some sense to me. Remember that the shunt resistor has
30 mV across it, which is sampled by what looks like a high impedance input
network to the op amp. Each side of the shunt resistor is connected to a 1M
resistor, which is in turn connected to both a 10K resistor to ground and a
10K resistor to one of the input pins. As a result, I think the 30mV voltage
drop across the shunt resistor is dropped down to 3mV (312mV - 309mV) at the
op amp input. Of course, I have no idea how much amplification the op amp is
set to do. The output is 189 mV, which is cut in half by a divider to 94 mV
before going on to the microprocessor board. My guess is that the op amp
output voltage is supposed to directly represent the plate current, perhaps
plus a small offset to account for losses through the A/D and D/A chips. In
any event, I think the op amp output should be closer 40 mV.
>Finally, is there an offset trim pot on the amplifier? It is possible that
>it is drifting (DC offset) or that it has a differential gain resistor
>changing. If this happens, it will become a common mode amplifier too, and
>offset will show up on the output for even small CM voltage on the inputs.
>Those 10 K resistors around the op amp (should be in pairs that are closely
>matched to be a diff amp) are critical.
Unfortunately, there is no offset trim pot, only fixed resistors and a
couple of bypass caps. I measured them as best I could, and things seem
pretty reasonable (assuming that I've identified all the resistance that's
in parallel with parts of this circuit.) Almost all of the resistors are in
resistor packs, making measurements difficult.
However, I think the input voltage measurements are pretty reasonable, so
I'm beginning to believe that it's the op amp itself.
73, Dick, WC1M
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