[AMPS] strange idling current

[Peter Chadwick]

>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. 

If there's nothing across the shunt, there should be no difference voltage
across the input pins. Try putting a short across them and looking at the
output voltage: it should be zero. If not, change the op-amp.

The high voltage op-amps (there's a similar type available from National)
always worry me slightly about reliability at maximum supply volts. I
sometimes wonder how good the process control is to make sure that they
don't get a long term reliability hazard that would be very hard to find in
a factory test without life testing at full voltage and high temperatrure.
That isn't done unless it has to be, because of cost.

If in doubt, change the op amp. Resolder the joints on the pcb, too. I'm
always surprised at how often that cures problems - I shouldn't be after 35
years in the industry, (20 of them in an IC manufacturing firm) but there
you are.

73

Peter G3RZP


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