Programmable Electronic Load - Current Sense Circuit

RE: Programmable Electronic Load - Current Sense Circuit

DAB — Fri, 22 May 2020 18:53:40 GMT

Nice update Jan.

There is a lot of good discussions following the post, so I hope people take the time to follow them.

DAB

RE: Programmable Electronic Load - Current Sense Circuit

Jan Cumps — Thu, 21 May 2020 19:45:56 GMT

Advise from Analog Devices on dealing with non-inverting opamp circuit offset.

https://www.analog.com/media/en/training-seminars/tutorials/MT-037.pdf

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RE: Programmable Electronic Load - Current Sense Circuit

Jan Cumps — Thu, 21 May 2020 16:53:14 GMT

I've placed the resistor R32 back:

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As you can see, the board has gone through heavy storms by now.

Input: (chart Y axis in µV)

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Output: (chart Y axis in mV)

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For those who get stress when seeing a bouncy graph:

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RE: Programmable Electronic Load - Current Sense Circuit

jc2048 — Sun, 17 May 2020 14:04:54 GMT

This is your gain curve but with the vertical axis scaled so we can see what it's doing in more detail.

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With the exception of a few very odd measurements [settling time? noise?] it has a clearly defined shape. At higher values, it's clearly asymptotic to a gain of somewhere just below 7.6.

The reason for the shape is that the amplifier has a small voltage offset. At low values, the offset plays a large part in affecting the gain calculation, at high values, much less so.

Here is a plot of incremental gain values. This is generated by taking successive pairs of readings and working out the slope of the curve between the pair. Ignoring the odd measurements, the gain is something like 7.53 or 7.54.

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That lets us see the gain without the offset getting in the way. The op amp looks like it is doing what it should with the gain. It can't take into account the offset because it doesn't know anything about it.

If you were to remove the offset before doing your absolute gain calculation, then your absolute gain curve would flatten until it was horizontally level.

RE: Programmable Electronic Load - Current Sense Circuit

Andrew J — Sat, 16 May 2020 10:59:35 GMT

Interesting. Is this using your compensation approach for the DAC? I presume that the current is being logged by Labview from the Rigol PSU rather than being measured? I think you can do a lot with this data set, particularly analysing in comparison to 'expected' values, 'ideal' values and error margins. I'm filing all these test approaches away for when I get around to it!