Electronics > Metrology

measuring 100mA current source with a better than Keithley 2000

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Overspeed:
Hello

I need the measure a pulsed current  with a frequency of 2 to  5 Hertz and a max value of 100 mA  ( stable frequency no sweep )

What I need to set as circuit ?

I suppose the circuit will a be a precision resistor with a low noise Op Amp to built a 4 wire measurement setup to built a 4 wire measurement

I suppose I need to control the shunt resistor temperature

I have built several basic shunt measurement circuit  but what is the best setup regarding the fact to reach a better resolution than 100 nA ( the published resolution of a Keithley 2000 )  I have a Hioki DM7275 7.5 digit and Also a Keithley 182

Regards
OS

The Soulman:
Your best bet is the Hioki on the 120mV range in 7,5 digit mode (is the sample rate high enough?) with an external 1 Ohm shunt.
100mA reads 100mv and the least significant digit is 10nA, speed and accuracy may not be great.
Power loss in the shunt 0,1V*0,1A=0,01W or 10mW, easily manageable.

You could add a op amp to the above and amplify the shunt signal 10x or 100x  for 1nA or 0,1nA resolution but you would need some range switching in that case.

What is the duration of your current pulse and how accurately do you need to measure it?
And why do you need that much resolution?

 

Alex Nikitin:
The question is not stated correctly. With the Hioki and a reasonable quality 1 Ohm shunt is not difficult to measure 100mA current better than the Keithley 2000 does it. The reason is very simple, the K2000 is very noisy compared to the Hioki. on 100mA DC range the last digit of the Keithley with be 100nA per unit. However on the fast (=NPLC1) speed (which will be required to get the bandwidth of 10Hz+) the p-p noise of the K2000 will be 30uA or about 300 times 100nA  :palm: . The Hioki has p-p noise on NPLC1 about 0.4uV on 100mV range and about 1.5uV on 1V range. Even with 0.1 Ohm shunt as in K2000 it will translate into 4uA p-p, with 1 Ohm shunt it should be close to 0.4uA and can be pushed to as little as 150nA p-p with 10 Ohm shunt (if you can afford 1V voltage drop). Which is about 200 times better than what the Keithley 2000 can do.

Cheers

Alex

P.S. - I have both Keithley 2015 (which is the same as K2000 in DCI mode) and Hioki DM7275 in my home lab, as well as quality 1 Ohm and 10 Ohm shunts with ppm level of stability, so the numbers above I took from my own measurements. In the attachment the Hioki measures 1V output at NPLC1 ("Fast") from a very low noise source - Fluke 731B, with p-p noise below 0.3ppm (so the noise of the source is well below the noise of the meter).

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