# Measuring fast current

Discussion in 'General Electronics Chat' started by kubeek, Mar 13, 2014.

1. ### kubeek Thread Starter AAC Fanatic!

Sep 20, 2005
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Hi,

I need to measure the configuration current of a CPLD, the datasheet says it should be roughly 7mA over period of 150us.

I am thinking of making an opamp current-voltage converter (transimpedance amp), what would be the GBW product needed to pull this off? I am planning to measure the output with an oscilloscope, so 10mA=2V would probably be ideal, so this if I want it to react in 1us this gives me slew rate of 2V/us which doesn´t seem too demanding.
This gives conversion ratio will need Rf of 200Ω, how would I calculate the bandwidth of this circuit?

The fastest opamp that I have readily available is the TLE2142 wih 27v/us and 5.9Mhz GBW, which seems it should be ok.

2. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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or a Digital storage scope.

3. ### #12 Expert

Nov 30, 2010
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I think you're right. I see 2/3 of a megahurts as your bandwidth for 1% precision and you have the numbers for gain. Personally, my first thought is 10 ohms in series with the load to get 70 millivolts. That's a required voltage gain of 28.57 to get to 2 volts.

math math math...8.4 v/usec

4. ### kubeek Thread Starter AAC Fanatic!

Sep 20, 2005
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The problem with using a simple resistor is that the scope is quite noisy and has only 5mV/div when using 1:1 probe, which for this doesn´t seem adequate. Higher resistance would mean higher drop, and since the circuit is running at 1.8V that would be too much and skew the results.

5. ### #12 Expert

Nov 30, 2010
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Your scope can go down to 5mv per division? Try a 1 ohm resistor in series with the chip and watch for 7 millivolts on the scope. Is that above the noise level?

6. ### kubeek Thread Starter AAC Fanatic!

Sep 20, 2005
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Yep actually it can even go down to 1mV but that is completely useless as the noise without probes is about +/- half a division. It´s lecroy 625zi, quite a nice toy to play with but sadly it is not mine but my employers'.
Also I am planning to do some static current measurement, and that will be somewhere around 100-200uA, so I think the opamp circuit will be better as it doesn't drop any voltage.
Well I guess tomorrow I will build it and see how it goes.

7. ### alfacliff Well-Known Member

Dec 13, 2013
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General Radio has a seres of clamp on current probes that work much higher in frequency than you are going to use. also, Tektronix has a clamp on current probe for their scopes which would work too.

8. ### crutschow Expert

Mar 14, 2008
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A transimpedance amp absorbs current so it would need to be in the ground leg of the circuit and must be be able to carry the full current of the CLPD.

If you current pulse period is 150ns you likely want at least a 15ns rise and fall time for the current detection. This requires a bandwidth of at least 23MHz. You would then need an op amp with at least a 100MHz GBW and and output circuit capability of at least 10mA. A current-feedback op amp would likely be best for that. A current-feedback op amp would likely need a frequency response of no more the 40MHz for your application.

9. ### kubeek Thread Starter AAC Fanatic!

Sep 20, 2005
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This is a one off for doing just a handful of measurements, so hacking one together in an afternoon from supplies at hand is preffered to spending a few hundred bucks on a proper current probe. I have an agilent current probe here, but it has too high range and again has too much noise for low currents.

10. ### kubeek Thread Starter AAC Fanatic!

Sep 20, 2005
4,691
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Thanks for the input crutshow, I will make some simulations and see what they say about this. Yes I am planning to put this at the ground node.
Thankfully it is 150us not 150ns, so I don't think I will need any out of the ordinary parts and that the TLE2142 should suffice.

11. ### #12 Expert

Nov 30, 2010
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Whew! I thought crutschow had embarrassed me again. Merely a misinterpretation of the time period.

12. ### crutschow Expert

Mar 14, 2008
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Yup, it's me with a little egg on the face. Read us as ns. Guess I need stronger glasses.

13. ### #12 Expert

Nov 30, 2010
16,705
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ns?

not seeing/near sighted? nearly senile? noobie stoopie? never sure?

Last edited: Mar 13, 2014