Why "2V voltage drop" is used by VI instruments to measure current? - Page 1
Forum section: electronics / general

author: desert
author_rank: Contributor
timestamp: December 20, 2022, 05:15:18 am
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: December 20, 2022, 05:39:47 am
content: Quote from: desert on December 20, 2022, 05:15:18 amWhich specific instruments are you referring to?

author: desert
author_rank: Contributor
timestamp: December 20, 2022, 06:09:24 am
content: 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 20, 2022, 08:47:00 am
content: 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.

author: Stray Electron
author_rank: Super Contributor
timestamp: December 20, 2022, 02:31:35 pm
content: 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 athttps://www.artisantg.com/TestMeasurement/64821-4/Valhalla-Scientific-4314A-Digital-Igniter-TesterIn 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

author: TimFox
author_rank: Super Contributor
timestamp: December 20, 2022, 02:59:13 pm
content: A typical voltage output for external shunts used in the US for remote current measurement was 50 mV (50 MV would be huge).I believe this was due to typical panel meters being 1 mA with 50 ohm resistance.My Fluke A90 switchable shunt resistor has 100 mV full-scale (switchable ranges from 0.1 mA to 10 A full-scale).

author: coromonadalix
author_rank: Super Contributor
timestamp: December 20, 2022, 03:34:09 pm
content: Now you have hall effect based sensors, like Allegro ACS series, voltage drop is almost history, it depends of your needsFor 100 / 200 amps shunts i've seen up to 100mv drop, but at theses high currents you loose sensitivity in the lowest ranges, seen some fluke current coil loop up to 1000amps and give 100mv at the outputYou have clamp meters who don't create losses in the circuitYou have current transformers etc ...Lots of possibilities

author: bdunham7
author_rank: Super Contributor
timestamp: December 20, 2022, 04:22:46 pm
content: Quote from: desert on December 20, 2022, 06:09:24 amI'm not familiar with that model, but the reasons for using a shunt calculated to generate 2V @ full range, if they do, would be to get the greatest precision using a 2.000V ADC setup without an instrument amp in front of it. I can see this being a good solution for low currents, but I'd agree with you that the drawbacks of using 2R for a 1A range would outweigh any benefits over using something like 0.02R and x100 gain. So if they actually are doing this, I've no idea why.

author: Stray Electron
author_rank: Super Contributor
timestamp: December 20, 2022, 04:49:54 pm
content: Quote from: coromonadalix on December 20, 2022, 03:34:09 pmIt mostly depends on if you want to measure AC or DC. A current shunt and a good quality US made analog meter movement, such as Simpson, are tough to beat for simplicity and reliability.

author: Stray Electron
author_rank: Super Contributor
timestamp: December 20, 2022, 05:08:19 pm
content: OP, your post got me to thinking. I have a couple of analog Yokogawa AC Watt meters and AC Amp meters. And FWIW they are Extremely Fine instruments IMO! So I would not hesitate in the least to recommend them.I just looked up the spec for my Yokogawa 2013-14 AC Amp meter and here are the losses for each current range. You can calculate the voltage drop from the loss and the range and you can see that the voltage drop is nowhere near 2 volts. These are transformer coupled meters with different windings for each range so the voltage drop and power loss is different for each range.10 Amp @ .6VA20 Amp @ .9VA50 Amp @ 1.7VA100 Amp @ 2.4VA

author: David Hess
author_rank: Super Contributor
timestamp: December 21, 2022, 01:45:47 am
content: The old standard 3-1/2 digit ADC accept either a 2.000 volt or 0.200 volt input and the 4-1/2 digit ADC accepts 2.000 volts. The change to 0.200 volts is done by dividing the reference by 10, but dynamic range considerations make this difficult with a 4-1/2 digit ADC.For a long time precision ADCs were most often 2000 or 20,000 counts making the most natural input range 2 volts. It was only later that 4000 and 6000 and 10,000 counts became more available, with inputs of 4 volts, 6 volts, and 10 volts.

author: desert
author_rank: Contributor
timestamp: December 21, 2022, 04:02:28 am
content: There are many kinds of VI instruments, to be more specific, the VI instruments I am talking about is SMU (Source and Measurement Unit) for semiconductor wafer and component testing.The voltage source and measurement range could be from ±50V maximum to ±5V minimum.The current source and measurement range could be from ±1A maximum to ±10uA minimum.This kind instrument has tough accuracy requirement, especially for low current range.For ±10uA current range, the accuracy specification is ±(10nA + 0.05% of setting/reading + 0.2nA/V*|V|).I guess using “2V voltage drop” for current measurement is the way to achieve high accuracy for low current range. And keeping the same voltage drop for different current ranges is to reuse the same current measurement circuit.What’s the main problem to achieve good current accuracy while using smaller voltage drop (such as 0.2V)? Any example to show the calculation and lineup?

author: David Hess
author_rank: Super Contributor
timestamp: December 21, 2022, 05:44:41 am
content: Quote from: desert on December 21, 2022, 04:02:28 am2000 counts (3-1/2 digits) at 0.2 volts is 100 microvolt resolution. 20,000 counts (4-1/2 digits) at 0.2 volts is 10 microvolt resolution which was much more difficult to achieve in the past, especially with an integrated CMOS converter. Automatic zero corrects for drift but not flicker noise which might exceed that level. (1)This is why old multimeters which relied on these converters had a minimum 200 millivolt range if 3-1/2 digits and 2 volt range if 4-1/2 digits, for the same best resolution of 100 microvolts.(1) Old datasheets do not give details on the noise levels but excluding flicker noise, those old CMOS designs had peak-to-peak noise of at least several microvolts. The 4-1/2 digit converters might also have had linearity problems at a resolution of 10 microvolts.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2022, 09:46:47 am
content: The general problem with current measurments is measuring relatively small voltages. 5.5 digits with 200 mV burden already requires 1 µV resolution. At that level amplifier drift, noise and thermal EMF can become a problem. One problem here is the heat from the shunt causing thermal gradients near the shunt. So the shunts not only need a low TC but also low thermal EMF values. Modern zero dift amplifier can help quite a bit, but the thermal design to avoid gradients at the wrong place still needs care (e.g. good symmetry).For the SMUs the porblem is that the very low current ranges may not work well the AZ amplifier, but may have to use classic JFET or similar amplifiers. In this case dirft of a few µV is hard to avoid.Using separate amplfiiers for the higher currents is extra efford, but could still be worth it.For the early handheld meters ADC counts directly correspond display steps. 100 µV accuracy was reasonable easy, 10 µV already demanding for a low power design. So a 200 mV range for the ADC and also the drop at the shunt was the natural choice as this can avoid extra amplification.