Comments (11)
Hello,
yes, we are aware of the stability issues and that's why we have unpopulated capacitors that for the caps that Dave recommends in this video:
https://www.youtube.com/watch?v=l2LBkXxN81Y
We found that the issue only happens in very rare cases and these caps reduce the BW so we don't fit them by default.
Regarding the 270 Ohm resistor in the feedback loop: Have you actually tried what you claim here?
I think this resistor is especially required to dampen the bias to oscillate. I even think we have tried to remove it and it was by no means better but I'm not 100% sure.
BG, Thomas
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hmm interesting, according to this:
https://www.analog.com/media/en/analog-dialogue/volume-38/number-2/articles/techniques-to-avoid-instability-capacitive-loading.pdf
both is possible (to have the R in and out-of the loop). The variant out-of the loop however has the clear disadvantage that it has an impact on the DC accuracy (it causes an offset) which is a no-go for this find of device. Secondly according to the simulation, the attenuation at only 1 kHz is already -40 dBm which is also quite tremendous, especially for the use cases I have.
Also as mentioned in the document, the value of the resistor is heavily dependent on the actual amplifier and I don't know how good the spice models of the MAX4239 actually is that you are using.
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Hi there in the original uCurrent R9 is outside the feedback loop.. then in the latter revision R8 is inside the feedback loop... clearly looks like an error GP
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I re checked and they are defiantly different the original and the latest revision with regard to the O/P series resistor, First version outside the feedback path, latest version inside the feedback path...
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We used the latest version when we derived the tinyCurrent, that's why it's inside the loop. But where do you see the "clear error"?
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I just built the original uCurrent. I used an opa335 instead of the max4239 because it was a recommended alternative in a forum (sure its worse) and the max was not available.
When shorting the inputs I can usually voltages up to 3v with my multimeter in dc mode but when going to ac mode it goes down to 0.1v. I guess it is oscilating but I dont have a scope to verify. First of all, just putting a 10pF cap between v- and vgnd did not change anything. I will get a scope soon to actually check what is going on. I am using TI's LMV321IDBVRE4 from mouser.
I want to understand completely how the logic works and I am not sure about the gain logic. To get a x10 gain we need 9k ohms which we get with R3 and R11 or R13 and R14. First, why do we need C3 and C4? Is it just a "for stability"-thing (why there?) or is it doing some opamp-integrator-curcuit magic? Next, I do not get at all, why R8 was changed to be inside the feedback curcuit. Does it not change the gain to x10.25? Does it not remove the purpose of R8 to decouple from the output because the output is now directly connected to the feedback? I bridged R8 and put a resistor in front of my scope but it did not solve my issue.
Regarding the added C5/C6 in tinycurrent, there was a comment by TMM "Connecting bypass caps to both rails is ill-advised as it creates a PSRR issue" (https://www.eevblog.com/forum/blog/eevblog-1057-current-murphy/). I dont know what it means but maybe somebody of you knows.
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I believe the best option for stable capacitive loads is to keep the R8 within the feedback loop, but to change the C4 from the right side of R8 to the left side of R8 (directly to U4p1). Or perhaps just an additional capacitor from U4p1 to U4p4. I haven't tried it yet - but to be honest, I don't think driving capacitive loads is a concern for me/my use.
On a separate note - although the noise level is pretty decent - I'm considering substituting an OPA388 although the current draw is much higher, the noise should be much lower.
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@nonplussed let us know if swapping out the Opamp brought any improvement please.
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