Dears,
My customer using OPA2348 to monitor the NTC sensor which is integrated on IGBT module. the input voltage of IN+ is range from1.8V~3.7V. the VCC of OPA2348 is +5V.
Normally, the IN- (connect to Output via a IN4148) will follow the inputs, but sometimes the IN-(connect to Output via a IN4148) is 0.7V. It may happen when testing for days, sometimes is 10 days.
is there any risk of using the below schematic? thanks!
Gavin,
I believe the problem you see is caused by the back-to-back input protection diodes between the input terminals of OPA2171 - see below.
In order to eliminate the issue, please use instead OPA2196 or OPA2191, which do not include those input diodes - see below.
Gavin,
OPA2348 does not have back-to-back input protection diodes but the diodes from OPA2171 still may effect its output-see below.
With all the capacitance around the inputs, the circuit gets into a lock-up state, which gets reset by discharging the cap with the multi-meter.
Gavin,
The circuit is called a half-wave recifier because when the output is loaded in a way to require output current sinking, it will only show positive half of the input waveforem - see below.
I have simulated the circuit for various values of C1 and the results are shown above. What it means is that depending on R1*C1 time constant, the output will have dramatically different shape, which results in a large differential voltage between the inputs of OPA348 – see above. The output will eventually snap back to follow the input but for a large values of C1like 100nF, it may limit the frequency below 1kHz (10k||100nF => 1ms).
