Hi,

I have to digitise a differential O/P of a subcircuit  using dual slope A/D converter.  But my dual slope A/D converter i am designing is single ended. How can I convert differential O/P  to single ended?

Raja - that might work but you would get the common mode noise including DC. The normal way to do it is to use an instrumentation amplifier like this:


If you can drive resistance you don't need the two buffers up front.

There are many variations that might fit the OP's needs.

The typical thing to do is to gain this pat signal up and use a sigma delta ADC. You will need an accurate voltage reference regardless of what type of ADC you use.

What kind of accuracy and resolution are you designing for?

Just use a fully differential system and this will be pretty easy. An incremental is just a Delta-Sigma that is reset before each conversion and may work a little better for your app:
click here

If can oversample the signal by a lot then use the simple circuit in the second paper (1987, 16-bits).

Hi Dan clement,

I am designig for an uncalibrated inaccuracy of 1C and a resolution of 0.1C with an on current of just 3uA (sensor core+ADC). I know its difficult but thats my goal. The two hard constraints are uncalibrated inaccuracy and low power.

If I use sigma delta  and chopper circuits to minimise the offsets (for uncalibrated inaccuracy) then the power increase. One good thing is that my sensing range is small -20 to 30C and I have a low sampling rate of 1 sample per  minute.

Thanks

mixed_signal wrote on Jan 29^th, 2012, 8:46am:


A Delta-Sigma or incremental is going to use FAR less power than a dual slope and more robust. As Jerry said, nobody uses dual slopes anymore. I doubt if you could even meet 10x that power with a dual slope -- MAYBE if you use an external capacitor, and you can't get away with a cheap capacitor on a dual slope.

I mentioned an incremental ADC. If you are continuously sampling the same input a delta-sigma is fine too, except every once it will give you an answer 1lsb different. With an single order incremental the decode is just a counter and you have so much time that you could get 16 bits with a 1kHz clock.

These are the types of circuits I love to design ))

Do some searching on google scholar. It wouldn't surprise me if there is a published sensor design to meet your needs.

Thanks everyone for your valuable suggestions!

Incremental ADC is basically a delta-sigma except that it is reset before each conversion. This allows it to be used for multi-channel high resolution ADCs since the current result doesn't depend on the past results. Also good for single-event upset environments for the same reason. The decimation filters are easier - for a first order it is just a counter, but you need a higher OSR than a delta-sigma.

edit - I just saw that you saw my references  ;D