ADC differential input termination - Page 1
Forum section: electronics / projects

author: OM222O
author_rank: Frequent Contributor
timestamp: March 04, 2019, 01:25:11 pm
content: Hey guysI'm using an ADC with 2 differential channels. The capacitors on the input filter stay charged and I still get readings when I disconnect the leads. I really don't want to add an op amp to the front end as the circuit is very sensitive and low offset op amps cost a fortune (about 2 bucks a pop). my next best option is a resistor across either the input or the filter caps (not sure which one is better). again the system is very sensitive (10uV accuracy, 500nV resolution) so I'm not sure if adding a termination resistor will add noise or offset. The input to the differential channel is the voltage across a resistor which is 200kOhm at maximum. even with a 1Gohm termination resistance, the parallel resistance is about 199.96k which is not bad but still not great ... If there isn't a good way to discharge the caps without introducing errors, I think I'll just leave it as it is because accuracy is more important than random readings when the leads are not connected in my application, it's just a minor issue which I hope to solve

author: ejeffrey
author_rank: Super Contributor
timestamp: March 04, 2019, 02:50:29 pm
content: A 1 G load on a 200k source is .0002. so a 100 mV input will have a 20 uV error. Or said another way that is only about 12 bit accuracy. If your source impedance is constant and known you can calibrate that which I assume you are going to have to do anyway with such a high source impedance.If you have some out of band way to detect input presence I would look into using a switch (CMOS or mechanical) instead of a load resistor. That will avoid the long time constants of big resistors and not load the signal source.

author: OM222O
author_rank: Frequent Contributor
timestamp: March 04, 2019, 04:21:21 pm
content: It's used as a part of a 4 wire measurement device. I can tell when the the load is connected and disconnected but the test resistor value would range from a few milliohms to 200k, I assumed worst case scenario in that calculation. The ADC is 24 bit which is why I said it's not too bad, but it's not great either. That also means there is some way of calibrating out the error caused by the termination resistance (first calculating the total resistance seen on the input, then reverse engineering the parallel combination because we know the sum and one of the resistors ... although to do this I have to find a way to do floating point operations to an insane level of accuracy as that would probably cause more error than the current through the 1G resistor itself). My reasoning for 1G is also anything above that is basically open circuit and won't solve anything for me ...I also would like to avoid any kind of manual or mechanical switch as that would become the first obvious weak point. a mosfet can be used but it has to work bidirectionally somehow (ADC can read + and - voltages, and you wouldn't want your device to stop working just because you connected the leads backwards). The leakage through a mosfet can also be a lot higher than a high value resistor (usually a few uAs) If you know a way to solve this please let me know.P.S: the ADC has a function to short Ain+ and Ain- to VDD/2 ... I'm not sure if it actually shorts anything physically to VDD/2 or it's just an internal reference node or something.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 04, 2019, 05:36:15 pm
content: The mode to connect the inputs to VDD/2 could be a good option to discharge a capacitor at the input.The SD ADCs tend to be not very high input impedance - so more like in the 100 K range, when actually used. A much higher impedance usually requires an extra buffer - so ADCs have that internal, but not many.If connected to the outside world the ADC input would likely need some protection from ESD or similar. This can be a difficult part for high resistance sources.

author: 2N3055
author_rank: Super Contributor
timestamp: March 04, 2019, 06:57:34 pm
content: H11 photocoupler maybe?

author: OM222O
author_rank: Frequent Contributor
timestamp: March 04, 2019, 07:33:20 pm
content: can you please explain how an coupler will fix any of those issues?Quote from: Kleinstein on March 04, 2019, 05:36:15 pmUnfortunately the "shorted to VDD/2" mode is just an internal node ... no change on the capacitor!The one I'm using is the ADS1219 nad has pretty high input impedance, both in differential and common mode inputs.The bias current is 5nA which is great! I think it has an internal buffer tbh.as for the ESD: it also includes protection diodes and I have 1k resistors on the inputs as a part of the filter. that shouldn't be a problem at all. I will try to botch solder a 10meg resistor I have on hand and see how accurate the approximation method in software is ... if it wasn't too bad, then I might go ahead with that, otherwise I will leave a note that there are no termination resistors in order to improve the accuracy and that the input capacitors stay charged even after disconnecting, so the readings are meaningless at that point.

author: 2N3055
author_rank: Super Contributor
timestamp: March 04, 2019, 08:03:18 pm
content: As nonmechanical switch. Still the leakage though.

author: OM222O
author_rank: Frequent Contributor
timestamp: March 04, 2019, 08:10:24 pm
content: Quote from: 2N3055 on March 04, 2019, 08:03:18 pmYes, I would use a BJT or N channel fet, or a combination of N channel and P channel called an "Analog switch IC" but the leakage is the main problem! not the isolation!

author: magic
author_rank: Super Contributor
timestamp: March 04, 2019, 10:08:11 pm
content: Are you talking about that ohmmeter project of yours?I think the solution is simple: if you can tell that no resistor is connected (presumably by monitoring the shunt?), simply ignore the ADC and display 'overrange'.And by the way, even if you regulate the current to get 1V across the shunt, you still shouldn't just assume it's 1V and instead measure the exact value and do the corrective math. Then, if no current flows and voltage across the shunt is zero, you get 'infinity' automatically, regardless of voltage at the sense terminals.That's how it's solved in all those $10 DMMs.

author: OM222O
author_rank: Frequent Contributor
timestamp: March 04, 2019, 10:40:01 pm
content: Yes magic, thanks for the suggestionMy problem is when Force is connected before sense is connectedthen the reading is junk unless there is proper measurement across the D.U.T and yes, I am monitoring the voltage across shunt and using the 10uA range to see when the voltage across the shunt goes to 1V which means the load is connectedI had a look at proper 1.024V voltage references (low drift and low noise) and they would be a bit more expensive than two resistors and a digital pot, although not that big of a difference. I really want this device to be extremely accurate at low cost and sell the first batch for very small profit margins (about 10% or so). This project was mainly for learning purposes and not as a commercial product as it has very limited functionality (low resistance measurement device) and I have learnt a lot thanks to this awesome community and people like you! I will try to talk to a professor at our university as well and ask for his opinion on the subject.What do you think of it so far? I wanted to offer them for no profit (just the cost of parts and PCB + shipping) to people on the forum, let me know if you would like a unit yourself.