Differential ADC source resistance

RE: Differential ADC source resistance

WoutZenso — Thu, 18 Jul 2019 07:42:06 GMT

Thank you haakonsh for the suggestion though I think that this might be a little overkill for our application. The change in resistance is quite significant, it is more like a switch principle (either it is open, or it is closed, but with high resistance). As far as I understand, Wheatstone bridges are excellent to see small changes in resistance with high accuracy. That is not really our concern. The extra cost of one resistor is so small that it won't justify the additional design/testing time to get everything tuned perfectly (with contact resistance at the interface and so on).

RE: Differential ADC source resistance

haakonsh — Wed, 17 Jul 2019 11:22:27 GMT

They both have their own sampling capacitors.

Btw, if you use a Wheatstone Bridge you can reduce your component count by one:

If you keep R2 constant there will be a voltage differential between D and B that can be used to calculate Rx. R2 also sets the 0V point, where R2 = Rx.

RE: Differential ADC source resistance

WoutZenso — Tue, 16 Jul 2019 14:17:52 GMT

Thank haakonsh for your fast reply! I am still a bit in the dark about how the differential ADC works internally. Should I see it as both the positive and negative input of the ADC having a sampling capacitor to the ground, or is there just one sampling capacitor between both inputs? My calculation assumes the first approach, but if it is the second approach, I'm not entirely sure on how to estimate the source resistance. Would it still be the same?

RE: Differential ADC source resistance

haakonsh — Tue, 16 Jul 2019 11:24:49 GMT

The maximum source resistance limit is due to the sample-and-hold circuit, we need to charge the sampling capacitor before the Aquisition timer runs out and samples the capacitor. If the source resistance is too great the current flowing into the sampling capacitor will not be able to fully charge the capacitor before the voltage across it is sampled.

As long as the maximum source resistance is less than 800kohm you're good, so your calculation of 150k+R+30k < 800kohm is correct. It does not matter how many inputs you have connected, they all got the same limit of 800kohm.