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Current DAC for Continuous Time Delta Sigma ADC (Read 3023 times)
cheap_salary
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Current DAC for Continuous Time Delta Sigma ADC
Apr 22nd, 2016, 6:55am
 
Attached circuit shows Feedback DAC of CT-DSM-ADC(Continuous Time Delta Sigma ADC).

What type of DAC do you think this DAC is, Current(Fig.4.5) or Resistive(Fig.4.3) ?

I think this DAC is current type same as Fig.4.5.
Here I have two questions.

What purpose do you think circuits inside red rectangular have ?

What purpose do you think resistors inside blue rectangular have ?
I think these resistors are not necessary for Current DAC.
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DanielLam
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Re: Current DAC for Continuous Time Delta Sigma ADC
Reply #1 - Apr 24th, 2016, 11:56pm
 
The resistors in the blue box might set the output common mode voltage.
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cheap_salary
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Re: Current DAC for Continuous Time Delta Sigma ADC
Reply #2 - Apr 25th, 2016, 3:32am
 
DanielLam wrote on Apr 24th, 2016, 11:56pm:
The resistors in the blue box might set the output common mode voltage.
Thanks for responding.
However, I don't think so.
Common mode voltage is fed by former stage.
See Fig.4.5.

I think both red and blue boxes are for alleviation of glitch noise of current DAC.
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« Last Edit: Apr 25th, 2016, 5:21am by cheap_salary »  
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DanielLam
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Re: Current DAC for Continuous Time Delta Sigma ADC
Reply #3 - Apr 29th, 2016, 11:38pm
 
That circuit looks like the Resistive DAC 4.3. I see the cross-coupled switches, and the voltages are switched between the rails. The blue box resistors would serve as the voltage divider.

For current DACs, I would be expecting differential pairs and current sources.
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cheap_salary
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Re: Current DAC for Continuous Time Delta Sigma ADC
Reply #4 - Apr 30th, 2016, 3:26am
 
DanielLam wrote on Apr 29th, 2016, 11:38pm:
That circuit looks like the Resistive DAC 4.3.
I see the cross-coupled switches, and the voltages are switched between the rails.
"H59" is not Gnd. And "H58" is not Vdd.

DanielLam wrote on Apr 29th, 2016, 11:38pm:
For current DACs, I would be expecting differential pairs and current sources.
PMOS where its gate is fed by "H59" consitute source current.
NMOS where its gate is fed by "H58" consitute sink current.

Cross-coupled switches constitute differential pairs.

Basic topology is completely same as attached one, if we remove additional circuits inside red and blue boxes.
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Frank_Heart
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Re: Current DAC for Continuous Time Delta Sigma ADC
Reply #5 - Nov 16th, 2017, 3:05pm
 
Hi, CS,

 Did you figure out the usage of circuits in red and blue box? I think they are for de-glitch purpose, but not sure how much better they will do. And it seems this is the DAC for very high SNR ADC, such that they have to minimize the ISL error?

Thanks
Feng
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