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Current source feedback capacitor
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OM222O:
I tried it and even without R8 and C1 it never passes -180 degrees which I assume is the point at which the op amp starts to oscillate?
worst case was about -165 degrees at about 13MHz


but with the suggested values of 10k and 100nF it seems to never go below 80 degrees:


My intention is to also eliminate the op amp characteristics should not show up but I'm not sure if there is a way around the phase margin of the amp?
I'm not sure how to interpret the graphs but I assume if at any point the phase is lower than phase margin of the op amp I will have oscillations and the components add a pole to gain back some phase?
D Straney:
Good loop analysis going on here.


--- Quote from: OM222O on May 20, 2019, 08:22:00 pm ---The ADC inputs are all protected via 1k resistors (RN1 & RN2), then they have a 1nF X2Y (feed through) capacitor which suppresses noise to ground and a 100nF capacitor which suppresses differential noise (both are C0G caps and the X2Y is capacitance matched) although I screwed up the resistor network purchase and there is no matching (they are 1% tolerance) but I didn't have issues with differential noise in the few prototypes I have made and I really don't want to re order 250 resistors if they're not causing issues  :palm:
--- End quote ---

Oops yeah my mistake, I completely missed that resistor network somehow.  Wouldn't worry about matching, even if one is +1% with +5 nA and the other is -1% with -5 nA, the error is still only 0.1 uV.



--- Quote from: OM222O on May 20, 2019, 08:22:00 pm ---For the reverse polarity I actually plan on using a P-Channel fet (DMP3099L-7 ) since I also have a bunch of them ordered from V2 (it was used as soft power but the new LDO already has that option) which means I can re-use the same parts and it also adds battery life since there isn't a 0.7V diode drop. win-win if you ask me   :-DMM
--- End quote ---

Great, even better!


--- Quote from: OM222O on May 20, 2019, 08:22:00 pm ---for the resistors I honestly couldn't get anything better unless approaching insane territory (>1$ per resistor) which would obviously blow the budget so I'm happy with these. I would have liked a better 1ohm resistor however they only come in massive packages since I chose a high power rating, >2W to be exact to give some margin, I don't like to run a 1W resistor at 1W, even if it's a pulsed load) and they are less optimized for tempco / absolute tolerance. I can calibrate out the constant errors using a milliohm meter (funny how it's always so much easier if you had one of the things you were making, to help you with making it   :-DD) which I don't have. maybe someone can calibrate one unit for me and I'll be able to use that for the rest, but that will have to wait for later.
--- End quote ---

Yeah I'd agree on all of that: the parts you have seem like the right choice for what you're doing - going an order of magnitude better in accuracy would need a whole next level of complexity and cost in parts (plus calibration!).



--- Quote from: OM222O on May 20, 2019, 08:22:00 pm ---I really appreciate the time you put into these posts and it feels nice to have confirmation that I'm not doing things totally wrong from an experienced engineer. Best of luck with the tests and I hope you have more of them  ;) that way I will be able to get more valuable information  :-+

--- End quote ---

Ha, glad to hear it.  No more long tests for the moment, unluckily / luckily, but sounds like it's all going well and the design's solid!  Just one last note, on the stability: what you're trying to avoid is not the phase dropping below the phase margin of the op-amp (that's more of a "derived" parameter, when using the op-amp closed-loop with a gain of +1 I think), but instead you're trying to keep the loop's phase from getting too close to 0 degrees (around the whole loop) or 180 degrees (ignoring the negative feedback).  This article probably explains better, although you can ignore the "Limitations" section for the first pass: https://en.wikipedia.org/wiki/Barkhausen_stability_criterion
Jay_Diddy_B:
Hi,

I have modified the model a little so that it will step through the four current ranges and produce a Bode plot for each range.





I have attached the LTspice model.

For this opamp these values of C1 and R8 are pretty good.

Regards,
Jay_Diddy_B
OM222O:
I tried running the simulation with all 6 ranges and the spice model of max4238 which I got from maxim website. with a capacitor value of 1nF the test would not get past run 1 and would be stuck on run 2!

I increased the capacitor to 100nF and it finished very quickly. I'm not sure what that means.does taking way too long to finish a run mean an error in the circuit?
I also tried increasing the points per decade to 1000 and the frequency range to 100Meg and the test again got stuck for a long time and I canceled it. I'm not sure if I'm doing something wrong
anyways, here is the result I received:


I'm still not sure what should I look for on this graph? in the AC analysis what is the indication of oscillations? a phase of 180 degrees? what is the frequency range that I have to simulate over?

The schematic and maxim file is attached.
Jay_Diddy_B:
Hi OM2220 and the group,

First the V4, injection source should be moved to the output of the opamp:



The requirement for a low impedance on one side and a high impedance on the other side was not met when the value of the current sense resistor is high.

If I do this:



I have replaced the opamp with an ideal opamp, gain 120dB, infinite GBW.

I get these results:




If I take the idealization further and replace all the parts with idealized models I have:




The results I get are:



This can be useful to see how the real components impact the loop response.

I have attached the LTspice models.

Regards,
Jay_Diddy_B
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