Electronics > Beginners
Limit the output voltage of current sense opamp
npelov:
I want to use negative power supply to ensure the output can go down to zero on a low side current sense opamp. The prolem is if current flows in the oppozitte direction. then we have negative voltage on the output going to the ADC. The ADC is a part of a PIC MCU and it has diode to ground, so I can put current limiting resistor (1k - pic will work with up to 10k voltage source resistance) and it should be fine. But is there any other way to make sure the output voltage doesn't go much below 0V.
P.S. Current can flow in the opposite direction in few ways - if R1 is a battery being charged, if R1 has a big capacitor and the V1 goes down by few volts.
exe:
Try to put small signal diode immediately at opamps output. It will be inside the loop, so diode drop will be compensated. Like on this circuit: http://2.bp.blogspot.com/-a3OkjYEP9lw/U6g0VjyhlnI/AAAAAAAAAPs/aRSS7eaiYmQ/s1600/modified+halfwave+rectifier+print.PNG (not sure if the link works)
piguy101:
The circuit you may be looking for is known as a "precision rectifier." This allows the circuit to behave as an ideal diode (no voltage drop).
https://en.wikipedia.org/wiki/Precision_rectifier
But as OP mentioned, is it always a good idea to use a series current limiting resistor before the ADC input whenever you expect the input voltage to go below 0V or above the ADC supply voltage. This limits the current on the ESD diodes to a safe level.
MasterT:
Resistor is the best, simplest and effective solution to keep an adc safe. Any triks trying to cut-off negative half-wave at the OPA side could not guarantee presence of negative voltage in case of circuits failure.
xavier60:
In my experience, allowing a PIC's input protection diode to be forward biased causes offsets in other ADC channels.
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