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  • Author Author: Jan Cumps
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Programmable Electronic Load - Analyse the Summing Node Zero Point

This blog documents investigates the feedback node of the electronic load that , and are designing.

It's an important spot in the load's design. It measures the set point and the feedback from the output.

When the output is driven to 0, it should be on a potential as close as possible to 0 V.

On the first prototype it's -0.2 V. Not so much off, but the negative value  influences our ADC measurements.

This document checks how we can get this node to 0 V.

image

 

Because this document is evolving, some comments below may be out of sync with the content. That's because the content is adapted based on the conversation.

The measurements taken here are based on the original design, without R32 in place and U3B + tied to ground.

The current sense side of R7 is connected to ground, and a variable negative voltage from 0 V down is applied to the current sense side of R8 to simulate current being sensed.

 

The circuit isn't complex. The set point is driven by a DAC. It's set to 0 for this test.

The second input to this node is OpAmp 3C. It has both inputs tied to ground so should theoretically have 0 V at the output.

On my board I measure a potential of -0.212V at the left side of R33.

I hope to get this closer to 0 V to ease the ADC a bit - its performance degrades with negative voltage at its inputs.

Like the other blogs for the electronic load, this is a working document that will be updated with findings from anyone who wants to chime in.

 

Behaviour at 0V

 

buzy image

  • in reply to Jan Cumps

    If the input voltage is a bench PSU (sorry, I'm a bit slow to catch on at times), aren't you just measuring its noise.

     

    You've got a few millivolts of fluctuation in the readings at the output. Divide that by the gain of eight and that equates to something like half a millivolt at the input. You'd be lucky if your PSU was cleaner than that (particularly if it has a lot of automation stuff inside it).

     

    Anyway, do the test with lots of readings. It will be interesting to see if you get a scatter plot with a wide, fuzzy band stretching across it.

     

    One way to quickly test if you are measuring the PSU noise would be to put a larger resistor in series with the current sense resistor, scale up the voltages appropriately, and then see if the variation drops by the attenuation factor.

  • in reply to Jan Cumps

    I'm preparing the driver board for automated measurements.

    image

     

    image

    I'm going to measure the actual voltage that arrives at the current sense (A). I can't rely on the precision of the voltage that's delivered by my power supply.

    The second masurement is after the opamp with gain of -7.8 (B).

    The last sample is the voltage that goes to the ADC A, after the opamp with -1 gain (C).

    I hope that by having a fixed measurement setup I can avoid the impact from me holding the PCB while measuring and influence of bad probe contacts.

    It'll also allow for long time measurements.

  • in reply to Jan Cumps

    A day of studying and the flow looks significantly cleaner. No loop yet.

    image

    The upper flow is the load, the lower one the PSU.

    The blocks in the load flow are custom made: Programmable Electronic Load - Write a LabVIEW Library part 1: Initialise Block

  • in reply to Jan Cumps

  • I get excited about small things image but the image below is the result of controlling our electronic load and a power supply in a LabVIEW script.

    image

    Two times the PSU is set to a certain voltage to mimic load current, then one of the load's ADCs sampled to validate that.

    First time the output is set to 0 V and the load should report 0 amps. The second time to 0.02V and the load should report 0.4 A.

    There are several reasons why the returned values aren't spot on, but that's not the purpose of the automation. The automation is there to reveal the errors and give a repeatable test case.

     

    This is how it looks like in the LabVIEW run-time.

    image

    I still have to learn how I can make the 'set PSU then sample" block reusable and how to make good loops. Still it's satisfying to know that all tools are up to the game and that it's just skills that are lacking.

  • in reply to Jan Cumps

    Progress:

    image

     

    I'm able to det the PSU to a particular setting, then read from the electronic load, then set the PSU to the next voltage.

    It must be possible to save such a block as a module but I'm not that advanced in LabVIEW yet.

    It does show that I can make this work though - have a PSU and the load talk together in an automated script.

  • in reply to Jan Cumps

    It's been a while but I'm getting the hang of it again.

    image

     

    Upper part is the electronic load. The LabVIEW flow throws a *IDN command to it.

    The lower part is a RIGOL power supply. LabVIEW configures the first channel, then enables it and logs some data.

     

    Now I'll try to combine the two and read some ADC data back from the load ...

     

    The Rigol part is described here: Automate Rigol Power Supply with LabVIEW

  • in reply to Jan Cumps

    My power supply, scopes and this instrument support SCPI. Maybe I should automate this in LabVIEW ...

  • in reply to Jan Cumps

    I redid the measurements today:

     

     

    *-7,8*-1opamp 3Copamp 3BADC BADC BADC Btheory ampsinput- inputU3C out- U3C outU3B outvoltrawabstheory U3B outcompensated ADC measuredelta theory compensatedgain0000,11-0,11-0,214-0,21337511406439900,000625-0,000625#DEL/0!0,4-0,020,02-0,0510,051-0,053-0,0511875282652640,1560,1628125-0,00681258,1406250,8-0,040,04-0,2040,2040,0990,0993755325320,3120,313375-0,0013757,8343751,2-0,060,06-0,3630,3630,2580,258375137513750,4680,472375-0,0043757,872916666666671,6-0,080,08-0,5170,5170,4110,411219421940,6240,625-0,0017,81252-0,10,1-0,6720,6720,5650,566813302330230,780,780813-0,0008137,80813

     

    image

     

    Redoing the measurement after it has been on for a while for -0.02 V input brings it more in line:

     

    *-7,8*-1opamp 3Copamp 3BADC BADC BADC Btheory ampsinput- inputU3C out- U3C outU3B outvoltrawabstheory U3B outcompensated ADC measuredelta theory compensatedgain0,4-0,020,02-0,0470,047-0,056-0,05625303652340,1560,15775-0,001757,8875

     

    that results in this gain graph and this error between expected and measured value at ADC:

    image

  • in reply to jc2048

      wrote:

     

    It might be easier to see what it's doing if you plot the difference between the expected and the measured.

    Here's the data, based on the same measures:

       

    theory U3B outcompensated ADC measuredelta theory compensated0-0,0001880,0001880,1560,1614375-0,00543750,3120,318187-0,0061870,4680,4584380,0095620,6240,627937-0,0039370,780,784312

    -0,004312

     

    image

    Tomorrow I'll try to redo ...