finding short on motherboards using a shorty with display

[optikon]

Hi there,
I've got the pcb and tested them.
but I have some spurious oscillation at the output of the opamp
and I'm struggling to find the reason for some times now...
as I have changed some parts of the design, it may come from that, but I have to investigate.
more to come soon, thanks for your patience.

Did you want some help troubleshooting this issue? Post a pic of the spurious oscillations and more details..

[kripton2035]

the schematic looka actually like this, with a more stable current source.
I have oscillations at the output of the aop, around 10-20Hz that forbid the precise measure
I've put decoupling capacitors 1-10µF on each IC with no luck.

[Catcatcat]

If initially there was no such problem, then most likely this is due to the layout of the components on the printed circuit board.

[Old_but_Alive]

is there a LC or RC filter into the opamp  +ve supply line ?

it will need decoupling from the rest of the circuit.

suggest something like a ferrite bead or a 10R resistor, and 1UF or 10uF capacitor,

[kreutz]

It will probably benefit if you use some kind of EMI filtering at the input of the opamp. See http://ww1.microchip.com/downloads/en/appnotes/00001767a.pdf (Solutions for Radio Frequency Electromagnetic Interference in Amplifier Circuits by Microchip) for ideas. Or better yet [url]https://www.analog.com/media/en/training-seminars/tutorials/MT-070.pdf/url] by Analog Devices.

[kripton2035]

there is no LC or RC or ferrite bead on the +V line, but as the power supply is a 9v battery is it really necessary ?

no the initial schematic was a little different, there was no zener look at the preivious page of this thread, or the schematic on my web page for the project
the new current source is less dependant of the supply voltage, so it should have a better reproductibility of the measures.

[Catcatcat]

Zener diodes tend to produce white noise, it may be worth analyzing in this direction.

[PM3295]

Did it ever work correctly with the new current source?
What is the amplitude of the oscillations?
Can you show the scope display capture?

[optikon]

Use an oscilloscope to measure your oscillations and post the pic.

[sadman]

Zener diodes tend to produce white noise, it may be worth analyzing in this direction.

or better he used temperature compensated voltage reference like LM336DZ 3.6 volt   

[Old_but_Alive]

kripton2035  wrote

there is no LC or RC or ferrite bead on the +V line, but as the power supply is a 9v battery is it really necessary ?

no the initial schematic was a little different, there was no zener look at the preivious page of this thread, or the schematic on my web page for the project
the new current source is less dependant of the supply voltage, so it should have a better reproductibility of the measures.


I suggest you carefully lift the AD +ve supply pin, and add the RC

Then you can see if it fixes the problem

[optikon]

10 - 20Hz is a strange frequency. Too low to be a loop oscillation directly or mains. But it might be an alias of a much higher frequency aliasing with your scope or whatever it is you are using to measure with.

If you have supply oscillations, you should be able to see them

1) measure your power supply pins. Do they also oscillate?

Do you have a common mode oscillation because the whole thing is floating, sitting on your desk or ESD mat ?

2) Measure your ground, is it also oscillating?

[Checksum8]

I used a LM317 as an adjustable current source. After tweaking the correction factor in code and current adjust, I get repeatable readings.



****corrected schematic attached

[mars01]

Looking at the used Opamp DS, the slew rate is rather low (the AD8628 https://www.analog.com/media/en/technical-documentation/data-sheets/AD8628_8629_8630.pdf opamp being an instrumentation opamp that's not surprising) which should make it rather insensitive, but it might help if you add some small capacitors in parallel with the negative reaction resistor (33k by value): a 10pF to 30pF might help but you can try with a higher value, into the nF range. Like @optikon said, the oscillation you see might be just an envelope of a higher frequency oscillation.
Other than that, given that you have placed the decoupling 0.1uF capacitor between VCC/VSS, the only thing that may be the cause is to have some capacitance connected where is not supposed to

If I were in your place I would disconnect the 1k resistor found on the negative input of the opamp and check if there are still oscillations. This way you can determine where the oscillations are caused. You can also remove the current source transistor (or whatever you have in the new schematic). Bottom line is that you have to isolate the exact location where those oscillations are generated. After that it is much easier to fix the problem by adding some capacitors.

[PM3295]

Here is a cheap modification for the current source that is worth a try maybe.

Replacing the zener with a classic TL431 may provide improved regulation with consuming about 50% of the current required for the existing regulator.

Below shows, the diagram and the result of imposing a 2 Vpp ripple on the  5 VDC for testing purposes. The new current source (AM1) shows a variation of only 0.02 mA while the old one (AM3) varies by a few mA's. The variation of the old source shows some non-linearity while the new one appears more symmetric.

These are just simulation results. I have not tested it in hardware.  

[optikon]

would be good to understand why the osc is occurring. If you want to change the current source, just do it with an opamp and transistor. You can crush supply LF supply noise by nearly 120 dB. it will have superior performance in all regards. That is, if it was the problem to begin with-- which hasn't been established yet.

[PM3295]

would be good to understand why the osc is occurring. If you want to change the current source, just do it with an opamp and transistor.

I totally agree that using an opamp based CS will be superior. However, that needs a complete pcb modification. I was thinking along the lines of minimal modification if there are already pcb's produced. The TL431 can be implemented without much trouble in place of the zener.

It will be useful to see some waveforms and to establish if the design performed correctly after the current source was modified the first time round.

[kripton2035]

thanks guys for all these inspiring ideas.
will try them
but I have a work project that has to be closed in the next weeks and I won't have much time for the shorty project now.
Hope to have more time for this in july. stay tuned.

[sadman]

why not use a linear adjustable regulator as current source which will also safe some space in circuit see attach milliohm metter, in Elektor electronic a nice design was published with op-amp based current source I will search it and share

[koky]

i have found this
http://cappels.org/dproj/dlmom/dlmom.html
lock-in amplifier  can be a good solution

[kripton2035]

no, because it tests AC ohms, I want to test DC ohms.
as there are capacitors on supply rails, if you use such an ac ohm meter, it will always tells you there is zero ohms, the capacitor impedence.
using dc ohms meter, you can find shorted capacitors.

[optikon]

I totally agree that using an opamp based CS will be superior. However, that needs a complete pcb modification. I was thinking along the lines of minimal modification if there are already pcb's produced. The TL431 can be implemented without much trouble in place of the zener.

It will be useful to see some waveforms and to establish if the design performed correctly after the current source was modified the first time round.

Ahhh Yes you have a good point about making it fairly easy to modify an existing PCB. The various flavors of 431 are one of my favorite parts.. You can get a lot of performance from them for very little cost. Nice little parts they are.

Yes we're still waiting for OP to post some waveforms. 10-20Hz has piqued my curiosity because its quite odd to me. Guess Kripton is busy.. I guess there is no hurry.

[PM3295]

establish if the design performed correctly after the current source was modified the first time round.

Simple yes or no will do.

[kripton2035]

I made the prototype on a perfboard, with the original "current source" (in fact it is not one...)
I made a pcb and during that time I thought I would make this new true current source (at least better)
I modify the pcb to be able to have both current sources on the same pcb if anything went wrong I could still use the original one.
I received the pcb, tried the new current source and it never worked dur to these oscillations.

[sadman]

So Finally Found it Elektor Milliohm used two topology PWM pulses and constant current share for reference and an idea

sadman