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I am trying to sense very low current (50 - 200 uA) on the high side of a circuit, and was considering LTC6102HV for the job since it can be "floated" to 105 V.

My problem however is that my circuit is not floated to +105 V but to -105 V and the current does not flow to the ground. if I understand it right, I cannot use LTC6102HV.

Here's a picture that illustrates the problem, I am trying to measure I_sense, across R_sense.

I can probably just use a diff amp (e.g. AD629), but since I am measuring microamps, I am not sure if it is the right way to do it.

Any ideas?

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  • \$\begingroup\$ I doubt if I can give you an answer, but you'll probably need to know what's the highest value Rsense your application will allow (or maximum voltage drop across Rsense), and how accurately (and how precisely) do you need to measure the current. \$\endgroup\$ – The Photon Jul 24 '12 at 19:21
  • \$\begingroup\$ The Highest value of R_sense will depend on what I choose to use to measure the current. I can see different possibilities, there are a gazillion differential opamps and a few highside current monitors. It's just that I am not sure what to use. Most highside current monitors I have come across does not really have "high" negative voltages as inputs. And I have not seen any applications with diff amp that measures microamps. I have seen people measuring miliamps. And as far as precision goes, it has to be quite precise I am afraid, since it is a very low current. \$\endgroup\$ – Ender Wiggins Jul 24 '12 at 19:38
  • \$\begingroup\$ In that case, you could use, say, 25 kOhms for Rsense and then use any voltage sensor able to measure 0-5 V. You can float the whole circuit at ~-100V, and isolate the communication back to whatever circuit needs the information back at the ground level. \$\endgroup\$ – The Photon Jul 24 '12 at 20:50
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    \$\begingroup\$ Please don't post large images. Olin scaled it down to less than 1/10th of the original size and it's still more than large enough. \$\endgroup\$ – stevenvh Jul 25 '12 at 5:06
  • \$\begingroup\$ Sorry about that \$\endgroup\$ – Ender Wiggins Jul 25 '12 at 16:39
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Actually the AD629 you already found should work for you. If you look at the web page for it you will see that it is categorized under 'Current Sense Amplifiers'. Its special feature making it so usable is the capability to amplify correctly even with high common mode voltages. This means that both inputs have a voltage with respect to ground, but only a small difference. This is exactly what you need for a current sense amplifier.

When you measure micro amps, the current sense resistor you want to use might be about 1kOhm, which would give you 1mV/1µA (without amplification). You might want to look at Dave Jones uCurrent project as an example for how to measure even nano amps, and for a discussion of how the size of the sense resistor might affect your circuit (though I doubt that even 1V voltage drop will affect your circuit when it is powered by 100V...)

Another option you can consider is using a isolated OpAmp like the AMC1200. You would need to power directly from the rail where you are measuring, so you have no common mode voltage. The isolated output allows you to use it at any other place of your circuit without any level shifting.

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  • \$\begingroup\$ Thank you, I have been leaning towards AD629, since the common mode voltage for it can be +- 260V. The Isolation OpAmp AMC1200 is an interesting solution too. \$\endgroup\$ – Ender Wiggins Jul 24 '12 at 22:28

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