Resistance comparator idea check - Page 1
Forum section: electronics / general

author: doktor pyta
author_rank: Frequent Contributor
timestamp: November 20, 2021, 09:56:40 pm
content: I came up with an idea of relatively simple circuit for resistance comparison.Full schematic in the pdf below.Here is the illustration of the principle:A photo of the board:A test setup with 0.33A current source and two 1 ohm resistors:U diff and U ref can be sampled by sigma-delta ADC with VRef input - or for example by 34401A in ratio mode.The circuit includes two bistable relays: one to change the direction of current flow; second for zero calibration (both input pairs connected together to the R ref) - allow to compensate for charge injection and amplifier's Vos.At this point I'm able to distinguish values of 1ppm.I would appreciate comments, ideas of improvements.

author: Andreas
author_rank: Super Contributor
timestamp: November 20, 2021, 10:12:24 pm
content: Hello,nice Idea.I think it will work good for relative low ohmic resistors.The question for me is how will it work above >= 10 K Ohm where leakage currents and charge injection may dominate?Some (marginal) improvement (of leakage) could be made by using poly propylene capacitors instead of poly ethylene types.I would also set Pin 17 to a slightly negative voltage. (Otherwise you have to take care that Rx is always smaller than Rref).with best regardsAndreas

author: doktor pyta
author_rank: Frequent Contributor
timestamp: November 20, 2021, 10:34:34 pm
content: Andreas,forgot to mention that the circuit is powered symmetrically +/-5V.IC2A is responsible for keeping midpoint of Rref and Rx at 0V.

author: Andreas
author_rank: Super Contributor
timestamp: November 21, 2021, 07:18:53 am
content: Hello,I missed that the full cirquit diagram is attached as .pdfSo I saw only the principle cirquit.with best regardsAndreas

author: ch_scr
author_rank: Frequent Contributor
timestamp: November 21, 2021, 09:09:18 am
content: Interesting Idea! I'll just throw into the ring what comes to my mind when looking at the schematic:You did go for an active ground so the current would not flow to the "real ground" and make it shift? Neat!(I now understood your comment it also keeps the center point to gnd not the lower most point. Very neat!)I think there are a few points that make layout challenging,for one star point layout for the current source v+ on the "primary side" and "secondary side" dc ground connections.(Not sure I like the V+ as reference point in general, somehow I'd rather have it referenced to a star gnd as well)Even more challenging, with the RL2 you have uneven amount of solder joints for both inputs.RL2 is in a good place tho, for thermal gradients. Not sure how big the effects are.That brings my mind to thermal gradients - with a current source that big, how warm does it get? Maybe it would profit to havetwo small pcb to isolate thermals better? There are not that many necessary interconnects between the two halves...Or at least, not have solid infill?How much does the measurement shift when you blow across the pcb?Regarding the output circuit: Somehow L5 pokes my eye.To me it looks like the impedance (to gnd) is uneven on the two U1 outputs.The different gains (=different bandwidth, cmrr) might make the two amplifiers behave different for pulses on the input?The inputs look well filtered, just putting it out there. Also, the gain OPA has a feedback gnd connection,the buffer one not. Seems it might behave different in all kinds of ways?I can see why you want to have ratiometric output,But I think an equal impedance and circuit on the U1 output would be better suited.Maybe with both U1 outputs referenced to gnd on one side and going to a instrumentation amplifierthat gives a amplified difference voltage?Add an buffered output for the voltage across the ref resistor to get your second output back andan "buffer opa input impedance simulation" across the dut side of U1 to try keep it equal?

author: doktor pyta
author_rank: Frequent Contributor
timestamp: November 22, 2021, 03:12:00 pm
content: ch_scr,Thanks for Your comments!You guessed correctly that thermal gradients are the limiting factor.I'm, going to share some results when I will have more time.As I have some spare boards I will try to separate heat sink with power components from the sensitive circuit.Regarding relays: these are bistable ones with coils not powered under normal conditions.I have tested this particular type to introduce less than 10nV of thermal EMF when used in symmetrical way.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 22, 2021, 04:39:23 pm
content: IT probably does not make a big difference: with AZ OPs (AD8639) and no longer the BJT based OP2227, there is no more need to get the same resistance on both the inverting and non inverting inputs. With AZ OPs the input currents are not symmetric - more opposite sign. So R12 mainly adds noise.For the supply to the AZ OPs it may help to also have low value resistors in series to have supply filtering and not just the decoupling caps.At the LTC1043 I find it a bit odd to have ground at the ouput side goint to a point where the flying capacitor is otherwise not at ground. So it may make sense to swap pins 2 and 3 of connectors at the resistors. I think this would reduce the voltage swing for the flying capacitors (C5 and C24).Chances are one could do the test with the cables connected both ways. There may be a small difference from parasitic capacitance around C5 and C24.The choice of PP versus mylar caps is a balance: PP caps have less leakage, but larger size and thus higher parasitic capacitance.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: November 22, 2021, 04:49:11 pm
content: Yup, AD8639 is used at the moment.EDIT: if You find the results inconsistent You are right.I've changed R9 to 33k to have more clear readout.