Successive approximation register SAR design

[Junus2012]

Dear friends,

I am trying to implement the concept of digital offset calibration technique presented in this image

I would like to ask your help for the SAR unit, do you have a good suggestion of it ? I have seen different designs, it is not unique

Thank you very much

 

[sutapanaki]

That SAR block is just digital, implementing the binary search algoritm.

 

[Junus2012]

Dear Suta,

That is what I am looking for, where I can find a SAR circuit that I can pick it directly to use in the design, I need 8-bit SAR

 

[Dominik Przyborowski]

It is simple.
Set MSB high, compare, set value of comparator as MSB, repeat with every other bit in descending order.

 

[sutapanaki]

Dear Suta,

That is what I am looking for, where I can find a SAR circuit that I can pick it directly to use in the design, I need 8-bit SAR

You should not rely on picking up a ready circuit for that. Just make the algorithm (you can even firdt write a verilogA code for that to test) and implement it to digital. It is not difficult. Plus, this is for calibration, so it does not require high speed and thus you are free to do it anyway you like.

 

[Junus2012]

Dear friends,

Does every SAR has a different algorithm?

--- Updated Oct 15, 2020 ---

For example, I found this from the technology, can I use it

--- Updated Oct 15, 2020 ---

by the way,

I see some people talking about hysteresis comparator used in the successive ADC or in the scheme above,

For me I see the hysteresis effect just like offset effect, and perfectly we should have comparator with no offset, then how we could use the hysteresis

 

[sutapanaki]

Looks like this one is from someone from Melexis. Anyone, just simulate and see if it fits your needs.
SAR uses a binary search or often a sub-radix 2 kind of a search. I don't think hysteresis is good thing in this case.

 

[Junus2012]

Dear Suta, I understand that hysteresis is needed in case of comparesion to a noisy reference, but in the context of auto zeroing I don't understand it why people referring to use it.

Could you please suggest me simulation setup without using DAC to test the SAR functionality?

--- Updated Oct 15, 2020 ---

By the way, I have test the SAR functionality by connecting R 2R and comparator as shown in the circuit below, I think it is the only way for testing SAR ?

I set the voltage to 2 V, so ideally the SAR must reach 2 V, and he succeed as shown in the figure below

Here another question I have, insted of 2 V he is getting 2.0108 V, so is it due to resolution or an offset of the comparator ?

 

[sutapanaki]

It can be both resolution and comparator offset. Do you have comparator offset in your circuit? If not you can add some artificial offset and see the effect. You can probably also introduce hysteresis and also see how it affects the search.