Hello,
I need to drive a voltmeter in a project I'm doing without affecting current flow. We are talking about very low currents in the milliamper range, so the only choice I have is to use an op-amp to connect the meter in order to prevent major impact from measurement.
The leftmost three resistors "simulate" the actual device in this mockup. I need to measure the voltage drop over the center resistor, R2, the ohmic value of which is unknown. (Ie. it can be anything from a short circuit to a break.) This essentially means that the measured V2 can be anything between 0V and the supply voltage V+.
My design uses a voltage divider (with appropriately large, megohm resistors) to keep the maximum voltage lower than the top rail.
V+ is rather high (24V), so I may have to use a voltage divider to feed the positive supply leg of the amp as well, since most op amps seem to operate in a much lower voltage range - though this is not noted in the mockup.
My main doubt is about the bottom rail. In order to get a proper measurement of 0 Volts, I was thinking I need to establish a "virtual ground" which is above the actual ground used for the bottom rail in the op-amp, using R3 which is part of the measured circuit anyway...
(As for Rm, it's the resistor to adjust the measured voltage range. This is also where I will have to compensate for the voltage divider on the input.) Does this look like it will work?
Also, which op-amp do you suggest I should use? (Should be cheap, ubiquitous and good enough for the job.)
I need to drive a voltmeter in a project I'm doing without affecting current flow. We are talking about very low currents in the milliamper range, so the only choice I have is to use an op-amp to connect the meter in order to prevent major impact from measurement.
The leftmost three resistors "simulate" the actual device in this mockup. I need to measure the voltage drop over the center resistor, R2, the ohmic value of which is unknown. (Ie. it can be anything from a short circuit to a break.) This essentially means that the measured V2 can be anything between 0V and the supply voltage V+.
My design uses a voltage divider (with appropriately large, megohm resistors) to keep the maximum voltage lower than the top rail.
V+ is rather high (24V), so I may have to use a voltage divider to feed the positive supply leg of the amp as well, since most op amps seem to operate in a much lower voltage range - though this is not noted in the mockup.
My main doubt is about the bottom rail. In order to get a proper measurement of 0 Volts, I was thinking I need to establish a "virtual ground" which is above the actual ground used for the bottom rail in the op-amp, using R3 which is part of the measured circuit anyway...
(As for Rm, it's the resistor to adjust the measured voltage range. This is also where I will have to compensate for the voltage divider on the input.) Does this look like it will work?
Also, which op-amp do you suggest I should use? (Should be cheap, ubiquitous and good enough for the job.)
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