20-08-2017, 08:42 PM
Some multimeters do indeed have caps in parallel with the 10M, but a lot don't. Many multimeters - including some really quite expensive ones like the Fluke 179 - are only good to low-kHz...
Obviously, it has to cope with the input voltages, and so it ends up being a reasonably expensive part. You often find a trimmer in there too, which also has to take the voltage, and someone has to set up on the production line (and it's an additional calibration step).
IIRC, you have an original Fluke 87? The schematics for that are here: http://assets.fluke.com/manuals/83_85_87smeng0500.pdf - note C37 and C20.
The DTM0660 chipset used by Aneng and countless others implements the true-RMS conversion digitally, and as such has a digital low-pass filter included as part of that. The -3dB point of that filter is 3kHz. The spec is 400Hz or 1kHz.
An AC bandwidth stretching beyond 10kHz or so is considered to be a "premium" feature, and is typically only found on the more expensive meters. The best cheap one I can think of off the top of my head is the Fluke 101, which I measured as -3dB at 11kHz, which implies about 1.5pF of capacitance to ground after the 10M input resistance. Though the spec of that one is only 500Hz.
Bottom line: you have to know your instruments.
Obviously, it has to cope with the input voltages, and so it ends up being a reasonably expensive part. You often find a trimmer in there too, which also has to take the voltage, and someone has to set up on the production line (and it's an additional calibration step).
IIRC, you have an original Fluke 87? The schematics for that are here: http://assets.fluke.com/manuals/83_85_87smeng0500.pdf - note C37 and C20.
The DTM0660 chipset used by Aneng and countless others implements the true-RMS conversion digitally, and as such has a digital low-pass filter included as part of that. The -3dB point of that filter is 3kHz. The spec is 400Hz or 1kHz.
An AC bandwidth stretching beyond 10kHz or so is considered to be a "premium" feature, and is typically only found on the more expensive meters. The best cheap one I can think of off the top of my head is the Fluke 101, which I measured as -3dB at 11kHz, which implies about 1.5pF of capacitance to ground after the 10M input resistance. Though the spec of that one is only 500Hz.
Bottom line: you have to know your instruments.
