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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

    In case anyone's interested in why the readings look to be out, here's a quick analysis of the situation.

     

    Jan has measured the voltages and we know the resistance values, so we can calculate the currents in each resistor for the two pairs. Like this...

     

    image

     

    The additional current into the node where the resistors join is then the difference between the two. The only place that current can come from is the op-amp, so Jan is doing an experiment to measure the input bias current of the input pins of the actual op-amp that he has on the board - the bias current for this particular op-amp flows out of the pin (shown with a negative sign on the datasheet).

     

    Here's what the datasheet says

     

     

    image

     

    so he's not  too far out, their typical figure is 0.8uA and he is measuring around 1uA. Well inside their maximum value (at 25C) of 2uA. (I've ignored any meter loading in all this.)

  • in reply to Jan Cumps

    It is fun to watch the great minds at work.  Thanks for the plug.

     

    John

  • in reply to jc2048

    Ill live but I wish I had mine up and running too so I can participate a little more than just thinking power image

  • in reply to Jan Cumps

    As this seems to be an issue of overloading the protection diodes in the ADC, im not sure if using a different channel is the best fix, it may work but might just move the problem to something else. One other thought I just had was a diode across the feedback of the inverting amplifier, this way it will have a full gain of -1 in one direction but way less int he other limiting the output to less than 100 or 200mv, or across the inputs to the opamp, I dont have my kit yet so im just thinking in my head till it arrives image

  • in reply to Robert Peter Oakes

    I was thinking about avoiding this particular ADC input influencing (shouting over) the other 3 ones. That would be difficult to handle in firmware.

     

    As long as we can keep the negative voltage above the range that it starts to influence ADC channel B, C and D we are good.

     

    This is not an issue of your design. It's introduced by the ADCs.

  • in reply to Jan Cumps

    Ok. So it's sounding like the real desire is to protect the ADC rather than

    stop an op amp doing its thing. A double shotky may be the answer to

    bolster the internal diodes and to trigger before the internal ones. This

    can be had in some very small packages but would also need a resistor to

    limit the current. Though I think the op amp could handle the load without

    cooking itself. What do you think ??. This is pretty standard practice too

    I believe so I'm dissapointed I did not think of it earlier.

  • in reply to Jan Cumps

    Love the advertisement for "John's excellent probes". It's like one those things from the old days of American TV where the presenter would suddenly turn, look very earnestly at the camera, and start reading out the advert. Perhaps we could see if John would like to sponsor this episode of Circuit Conundrums (starring Jan Cumps as The Man with the Meter).

     

    The interesting figure is the 0.104V at the summing node. If there was only R2 and R4 in circuit, that voltage could only be 0.054V (give or take a little for the resistor tolerance). The fact that it's so high suggests that there's an additional contribution to the current through R2 from what's connected beyond the dotted line.

     

    The second clue is U3B. The op-amp is doing a good job of keeping the virtual earth at pin 6 close to GND, yet it has to position the output at -0.212V to do it (and not the -0.108V you would expect given the 0.108V at the output of U3C).

     

    What do they both suggest?

  • in reply to jc2048

      wrote:

     

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

    You need to measure the output of U3B too - it might be lower. The ADC will have some form of ESD protection and that might clamp the voltage. Only way to know is to measure both ends of R33 and see if they are different.

     

    Then you need to measure the output of U3C. You have two amplifiers in series and you need to know how much of a contribution each is making to whatever offset you measure at the output of U3B. That will give you some useful clues as to what is going on.

     

    image

  • in reply to jc2048

      wrote:

     

    This is unfair on Peter. It's his design yet you're having all the fun of sorting out the prototype.

    yes image.

     

    Then you need to measure the output of U3C. You have two amplifiers in series and you need to know how much of a contribution each is making to whatever offset you measure at the output of U3B. That will give you some useful clues as to what is going on.

     

    I've just measured this - with John's excellent probes, they are very good for probing a live circuit without shorting things out. The value over R33 is 0 V, As a double-check I measured left and right of R33 and it's exact the same voltage.

    Edit: I will also measure the voltage at each significant point in this part of the circuit and list them in this document.

     

    (As Peter points out the meter input resistance will affect the readings, so start with measuring the outputs of the amplifiers because they are the lowest impedance points in the circuit and your meter has the least effect on them - the output impedance will be under a hundred ohms.)

     

    I have used the ADC and Peter's GUI too to get these values, without a meter attached. I get similar results. My meter (it's a Brymen BM235) has an internal impedance of 10MOhm for DC.

  • This is unfair on Peter. It's his design yet you're having all the fun of sorting out the prototype.

     

    But if you're intent on doing this anyway...

     

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

    You need to measure the output of U3B too - it might be lower. The ADC will have some form of ESD protection and that might clamp the voltage. Only way to know is to measure both ends of R33 and see if they are different.

     

    Then you need to measure the output of U3C. You have two amplifiers in series and you need to know how much of a contribution each is making to whatever offset you measure at the output of U3B. That will give you some useful clues as to what is going on.

     

    (As Peter points out the meter input resistance will affect the readings, so start with measuring the outputs of the amplifiers because they are the lowest impedance points in the circuit and your meter has the least effect on them - the output impedance will be under a hundred ohms.)