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How Auto Ranging on a DMM works

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MikeNye:
Hi Dave,

While you've been on the subject of multimeter input protection circuitry lately, I thought it would be good to explain how auto ranging works on a DMM, why the ADCs don't blow when you measure high voltages / currents, and how the DMM knows how to switch between ranges.

Cheers.

Kind regards,
Mike Nye

altaic:
I'm really curious about this as well. I'd like to see equivalent circuits of common switched attenuators which are designed for safety.

Also, a rundown of the design decisions and gotchas, such as switching speed vs. transients. I.e. if your multimeter is on the mV range, and suddenly switch 200V onto it, what keeps it from blowing?

Are there intrinsically safe designs which bypass attenuation if under voltage is detected, and switch attenuation back on the moment a threshold is crossed?

Also, are there ICs specifically for auto-ranging an external ADC (as opposed to fully discrete or otherwise all-in-one multimeter-on-a-chips)?

There's a really nice service manual (w/ schematics) for the Keysight 34401A 6.5 digit multimeter which employs analog switches, muxers, etc., though there are still some secret sauce ASICs. It's freely available with registration at http://literature.cdn.keysight.com/litweb/pdf/34401-90013.pdf.

hopski:
+1  :-+

timb:
Basically, the reason the buffer/amplifier/ADC don't blow up when you apply 200V while it's in the 2mV range is because of the very high input impedance.

Generally at the input there's a relay to switch between the <2V and >20V ranges.

The <2V range has a few hundred Kohms of input impedance and goes straight into an op-amp configured for unity gain, which then goes to a second op-amp that can select between a few levels of gain (x10, x100 and maybe x1000) in order to amplify the mV and uV signals up to the ADC's native input voltage, which in this example is 2V. Anyway, let's say the input impedance is 600k ohms in the >2V range. Let's also say 1000V is applied to the input in this range. The maximum amount of current the op-amp's input would be subjected to is ~1.7mA, which isn't nearly enough to damage it. Why? Well, the inputs of the op-amp have ESD diodes connected to VCC and GND that will shunt the excess voltage away, and, since it's current limited by the input impedance, will be able to handle it.

Most precision DMMs don't rely on the op-amp's input diodes and instead will use some sort of ultra low leakage (<1pA) diodes, Zeners, JFETs or similar for protection.

Now, for the >2V ranges, the input impedance is 1M-10M so there's even less current to deal with.

Here's two visual examples:

1kV Applied to the 2V Range


1kV Applied to the 1kV Range

ChunkyPastaSauce:

--- Quote from: timb on March 20, 2016, 03:33:25 am ---Here's two visual examples:

1kV Applied to the 2V Range

1kV Applied to the 1kV Range

--- End quote ---

What app is this?

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