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Why "2V voltage drop" is used by VI instruments to measure current?
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desert:
For current measurement on VI (voltage/current) instruments, I found that current sense resistor values are always chosen in this way: R * Max current = 2V. For 1A current range, 2 Ohm high precision resistor is used for current measurement; for 100uA current range, 20K Ohm resistor is used.

There are some drawbacks of using 2V voltage drop:
1, Big voltage drop which requirements amplifier to deliver higher output voltage. 2V is 10% of +/-10V voltage range.
2, Big power consumption and heat on the resistor. For 1A current, the power dissipation on the resistor is 2W. The resistor should have big package to handle the power/heat, and resistor value may drift due to heating which results in measurement accuracy degradation.

Any not using smaller voltage drop, for example 0.2V, for current measurement?

Thanks.
bdunham7:

--- Quote from: desert on December 20, 2022, 05:15:18 am ---For current measurement on VI (voltage/current) instruments

--- End quote ---

Which specific instruments are you referring to?
desert:
I think the design is similar for different kinds of high-precision VI instruments with four-quadrant source and measure capability.
If you want to have a specific model, take National Instruments PXIe-4136 for example.
Kleinstein:
2 V drop at the shunt is convenient for the low currents. It is however a problem for the larger currents, like 1 A or 100 mA.  With many ranges one may have to make comrpomises and they may have decided that a high power resistor could be the smaller problem than switching the voltage / amplifier.  SMU often extend quite far to small currents and for this they need low bias amplfiers that naturally have quite some dirft an low frequency noise. For this reason they may want the larger drop at the resistor.

With the newer instruments they usually don't provide full schematics. Simplified drawings to explain the functionality may not include all the details - the higher current shunts may still use less drop and have an extra amplifier that is hidden.


Many DMMs are more like using a 200 mV maximum voltage at the shunts and for the high currents possibly even less.
Stray Electron:
  I have several dozen V-I instruments around here and at least a dozen devices that use external current shunts to measure voltage and current and none of them uses a 2V system.  I can pull up the specs if you want to see them but I have several meters used for testing bridge wire firing systems and their Maximum Open Circuit voltage is something like 0.005 volts. edit: That was just what I recalled but I went and looked it up and it's actually 18uA on the 1K Ohm range, you do the math. Manual at https://www.artisantg.com/TestMeasurement/64821-4/Valhalla-Scientific-4314A-Digital-Igniter-Tester

   In the US, external current shunts are typically rated as 50 Mv at their full rated current, weather it is 1 Amp or 500 Amps.  Here is an example and you can clearly see the amperage and "50Mv" marking on it.  This is just the first one that popped up on Ebay.  https://www.ebay.com/itm/394105485912
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