Can you just add an OP amp to give an analog voltmeter high input impedance? - Page 1
Forum section: electronics / beginners

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 10, 2025, 05:28:17 pm
content: Can you just add an OP amp front end to an analog voltmeter to give it high input impedance? And OP amp with decent bandwidth would also good for AC voltage measurement right?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 10, 2025, 05:50:40 pm
content: In most cases it needs more that just 1 OP-amp. The old analog meters usually use a chain of resistors at the front to select the range / attenuation. By adding a high impedance buffer oe would also need some means to change the ranges.It gets even more complicated for really high voltages, high than the voltages the amplifier can handle. Here one would still need a divider up front, though possibly higher impedance than the original.Another point to include is usually some protection, as series resistor and some clamping device with low leakage.For a gain of 1 an amplifier is reasonable simple. For different gain and a bit higher frequencies, the gain/divider is not just set by the resistors, but also parallel capacitors. Above some 10 kHz one usually also needs to get the right capacitor ratio, getting the same gain from the capacitive part.Analog voltmeters may not work well with higher frequency AC anyway - so do and other don't.

author: Laval
author_rank: Regular Contributor
timestamp: February 10, 2025, 06:50:01 pm
content: I don't know the specifics of your use case but generally speaking, like Kleinstein said, you would basically need to build another multimeter frontend over the existing one (if using an op amp). I'm not sure what kind of voltages you want to measure (voltage and frequency) but there are high voltage probes designed for this. They basically form a voltage devider with the input impedance of your meter and you then have to multiply the reading by a factor.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 11, 2025, 06:17:18 pm
content: I concur with Mr Kleinstein too.If you are going to use an opamp to boost the input impedance you might as well build a proper front end. Because the analog meter is a low precsion >1% device it doesn't need to be complicated or expensive. Here's an illustration of what could be done on a shoestring. I'd use a low voltage low cost MOSFET opamp. A 358 won't really cut it, a JFET or CMOS opamp is needed for the high impedance attenuator. Chose your own opamp and battery voltages to suit your build. Its good enough for most purposes given the inaccuracy of AMMs or a meter movement.The input attenuator gives scale factors of V, V/10 and V/100, the output amp can be switched to X1 or X10. The input impedance is 10Meg at LF and DC. So, a four decade range in total. Its not fully bomb proof but it will survive an encounter with 230V AC. Note that circuit ground shown is local to the amplifier, its not mains ground.It can also be used to make a simple AC/DC volt meter if you have a basic meter movement. Its got ouputs for an analog multimeter too. I got up far too early today (old age) for an espresso fueled lash up session. It seems to work quite well with my AVO and my Simpson and with a junk box uA meter too. It runs fine on 8.4V and 9V. You can ditch the second stage meter driver drive U1b etc (and R10,C3 and the AV/RMS switch) if only the AMM output is needed. Other CMOS RR opamps will work but watch out for Vos. Use a class X cap for C1. In the meter movement version it can be switched to read average or RMS for sinusoidal inputs or average. You will have to rescale the current setting resistors (R12, R13, R14) for meter movements other than 100uA FSD.If you use cheapo switches it could be built for under $10 (batteries not included).OK Brunch time!