sigma delta adc

[danadak]

We know you want to measure a Meg Ohm resistance, but we do not
know :

1) The range of R's you want to measure
2) Measurement to be absolute or relative accuracy ?
3) Accuracy of measurement ?
4) Any special considerations for the DUT, the R under test, as discussed in the analog devices
ap note ?
5) Resolution of measurement ?
6) Environmentals, like temperature design must work in ?
7) What is the R ? Type of R ?
8) Latency start to to finish for the measurement ?

To name a few.....

Regards, Dana.

 

[maduba]

Thank you all for the reply. do you think we need U12B for threshold comparator for input of the sigma delta adc. because in this circuit is adding offset on the U15.

 

[rjenkinsgb]

do you think we need U12B for threshold comparator

It is NOT a comparator - it's an amplifier with some filtering.

This is a single-supply circuit; for it to be able to handle both positive and negative signals, the "zero" reference point needs to be around half the supply voltage.
The ADC negative signal in would connect to the 1.65V ref, then the positive in can swing either positive or negative from that.

If it were made for a dual supply, eg. + and - 15V, all the 1.65V points would then be grounded; 0V.
That's what the output signa is centred around.

 

[maduba]

It is NOT a comparator - it's an amplifier with some filtering.

This is a single-supply circuit; for it to be able to handle both positive and negative signals, the "zero" reference point needs to be around half the supply voltage.
The ADC negative signal in would connect to the 1.65V ref, then the positive in can swing either positive or negative from that.

If it were made for a dual supply, eg. + and - 15V, all the 1.65V points would then be grounded; 0V.
That's what the output signa is centred around.

thank you for the reply. I am bit confused here. So this U12B is not needed for input to the sigma delta ADC. The output coming from U10 can be connected sigma delta adc is that right?

 

[rjenkinsgb]

The output coming from U10 can be connected sigma delta adc is that right?

Yes, assuming the earlier programmable gain amp has adequate gain to suit the signal input.

 

[maduba]

Yes, assuming the earlier programmable gain amp has adequate gain to suit the signal input.

But U10 is -15V to +15V rail. does it not needed attenuate to +5V because sigma delta adc +5V.

 

[rjenkinsgb]

Sorry, I was not being clear, I was referring to the ADC this appears to be intended to feed; one with a single 3.3V supply.

Hence the 1.65V = 3.3V/2, half supply, so it can vary equally both positive and negative from that.


You have still not stated exactly what you are trying to measure and how, so the input signal levels and gain requirements are still a mystery!

Provide the circuit being measured and things may make more sense.