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ADC differential input termination

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OM222O:
can you please explain how an coupler will fix any of those issues?


--- Quote from: Kleinstein on March 04, 2019, 05:36:15 pm ---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.

--- End quote ---

Unfortunately 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.

2N3055:
As nonmechanical switch. Still the leakage though.

OM222O:

--- Quote from: 2N3055 on March 04, 2019, 08:03:18 pm ---As nonmechanical switch. Still the leakage though.

--- End quote ---

Yes, 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!

magic:
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.

OM222O:
Yes magic, thanks for the suggestion :D

My problem is when Force is connected before sense is connected  :-DD then 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 connected  :-+ I 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.

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