Using a milliohm shunt resistor in a 3.3V application.

[takacsjd]

I'm trying to measure small currents in a wireless sensor application using a USB6009.

I expect 2uA in normal operation from the device, and have sucessfully measured these on the low side of the device with a 250 ohm resistor. 

This was with a USB6008. Now, we would like to use a calibrated standard shunt (.001ohm) and a USB6009 to get data closer to the actual use case being able to see

150 - 350mS transitions of the field detector circuit cycling on and off (needed the higher sample rate) and hopefully seeing real activity not having several millaohm draw of a 250 ohm shunt.

Granted, the noise of the 6009 is .5mV in the 1V range. I was thinking, (perhaps wrongly) I could accurately see microvolt level voltage data with a differental system.

This may be my issue to begin with. My experiement is below.

I have a .001 ohm shunt resistor standard from http://www.rc-electronics-usa.com/current-shunt.html (A series)

In my first experiement, I had the device (powered by a 3.3V CR232) with the current shunt bridging the low side of the battery, the terminals attached to AL0(+-)

The dynamic range of the measurment was set to 800nA to 10uA

I was then seeing millavolt noise on the line weather the shunt was connected or not, basically no data. 

I then tried this with 1.5Kohm resistors pulling the AL0(+-) lines to a GND contact on the 6009. Same result.

Then, I took a known lab voltage supply at 5V and attached a 1.5Kohm resistor in series with the shunt. AL0(+-) was accross the shunt. 

I calculated that I should see 3.33... uA on the shunt in this configuration, yet saw just the .300 mV ripple on the line which is a much higher current than should be there. 

Any recomendations on doing these small current measruments? I'm thinking buying a SOIC from TI or someone might be a good solution / or just a plain old op amp to get this voltage.

[Hooovahh]

We go the op amp solution here.  We have a custom board that has a switchable gain resistor so two current ranges can be used.

[crossrulz · Accepted solution]

I know there are op amps out there specifically for current shunts.  I'm pretty sure TI has some good ones as well as Analog Devices.

[takacsjd]

Thanks very much for the reply! 

So, the lesson to take home here is do not try to measure data in the 0-1 V range that is less than the system noise - the internal diff. amp will just report that noise. 

I'm fairly certain .001 ohm is a std. level for shunts. I think I'll go with a tailored OpAmp - I suppose that is a wise choice as you would have to have very accurate setting of OpAmp gain meaning tightly tolernaced resistors / or somthing very good to measure them with I don't have somthing that tight around here so thank you for the mention of specific use case products. I'll go with Analog Devices opamps and resistors, usually great stuff there. 

For anyone out there who wants to know, 

The pathway to get to the real measured voltage (I Think)  would be to divide the voltage output of a properly set OPAMP (which amplifies the signal to be greater than the system noise) by the gain of the opamp.

[crossrulz]

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@takacsjd wrote:

The pathway to get to the real measured voltage (I Think)  would be to divide the voltage output of a properly set OPAMP (which amplifies the signal to be greater than the system noise) by the gain of the opamp.

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I usually create a DAQmx scale and set the channel to use that scale.  You just have to supply the slope and y-intercept and DAQmx will do all of the work for you as you read the data.

[SnowMule]

Couple of things I've done to measure µA currents in my devices, none of which involve using a 6008.

One, I've used a Keithley picoammeter.  Shielding is real important here.  Quick and easy way to make those measurements.

Second, use our own shunt resistors and measure the voltage across them using a 6.5-digit multimeter.

When our device is in normal operation, the current range splits the ammeter's ranges so we lose some data as the DMM switches.

By using our own shunts, we can maximize the range of the meter and eliminate the measurement delays.

Another option may be to use a SMU. 

Picoammeter and shunt/DMM is more for engineering testing.

For manufacturing test, I have a shunt resistor across a current amplifier (MAX4372), then feed that output into the 600x.

Our "Sleep current" test commands the DUT to go to sleep, then we check that the reading's below some value.

The failure mode we try to catch isn't µA, it's mA, so the 6008+4372 is capable of making that measurement.

Just remember the 6008 is a *low cost* DAQ, not a high performance DAQ.

Go into it with low expectations, and you'll be pleasantly surprised. 

If you need high performance, you'll have to spend some money.

[johnsold]

The choice of shunt resistor value is always a trade off among size of the voltage signal proportional to currrent, the disturbance to the circuit from adding series resistance, the power dissipated in the shunt resistor, and noise levels.

There is no "standard" value for shunt resistors. The 0.001 ohm value you mention is common for shunts measuring tens of amperes. It makes very little sense for your application: 2 uA * 0.001 ohm = 2 nV. Measuring that will require an expensive specialty meter.  As you have noted the 250 ohm resistor gives you a signal that is down in the noise at 0.5 mV.  If your circuit will not tolerate a higher resistance, then amplification is the appropriate next step.

Lynn

[Henrik_Volkers]

TIA ! TIA !  and TIA!

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