The sigma-delta ADC type

[kuohsi]

type of adc

What kinds of sigma-delta ADC to suit for DC-band application??
Can conventional sigma-delta apply?

 

[arunkumar446]

accuracy of sigma delta adc

do you mean very low frequecies for biomedical applications or for instumentation some thing like that. if so sigma delta is best suited as it is suited for low frequecy applications with high sampling and high resolution.

 

[kuohsi]

limit cycles sigma delta

do you mean very low frequecies for biomedical applications or for instumentation some thing like that. if so sigma delta is best suited as it is suited for low frequecy applications with high sampling and high resolution.

Thanks for your reply!
I mean it! Can I ask another question?
What is the effect that DC-signal input to conventional sigma-delta modulator ??
I survey many paper that apply 2K~22KHz input signal to ADC, but no DC-input paper.
Can I input DC-signal to conventional sigma-delta modulator ? Thank you!

 

[arunkumar446]

calculating order of sigma delta adc

when a dc input is applied the sigma delta ,it tries to converge to the average value, so it produces a sequence of 1's and 0's depending on the input relative to the DAC output in the loop, but the sequence of 1's and 0's repeat themselves but the point is the periodicity also depends on the input level.

Added after 2 minutes:

in practical implementation of sigma delta modulator we need to have common mode signal to bias the circutry which makes the sigma delta modulator, so it can be considered as a dc input, at output we will have a dc component and some periodic components depending on the input.

 

[analoggg]

understanding sigma delta adc

hi arun , I hav a question ....
I am trying to design audio 2nd order sigma delta.In that how should I scale the coefficients, i mean how can I select gain factors in front of integrators.Should I first design the opamp and see the output voltage swing and should plug the saturation voltages in behavioural model? Please answer if you know about this.....
thank you

 

[arunkumar446]

dc sigma delta application

for a standard 2nd order sigma delta there is a paper by author boser, u can look in to it. he uses 0.5 , 0.5 as coefficients and for full scale input 2nd intergrator output goes way high, but any way when we give full scale range we have overloading, so any way we wont operate at that range so, for reasonalbe inputs the swing is perfectly suitable.

 

[analoggg]

sigma delta adc types

thnx arun...
I had already seen that paper.Those standard coefficients are for his own integrator right.I mean if we different opamp,(I mean may be in different technology) the input range after which opamp saturates is different right..? or do you think generally these 0.5 coefficients work for all opamps,I mean integrators.
I wanted to know how boser scaled those coefficients(procedure) .If you know pls let me know.......

I first used delta sigma tool box by shrier in matlab, but there was some error in generating these coeeficients.If you are familiar with this delta sigma took box could you pls tell me how to run mex version......I am encountering an error in doing that too.....

Thank you.

 

[arunkumar446]

type adc

the models which he presented are system level simulations, so they are good for any technology. how to calculate them? .... you have to use simulink for doing such simulations and verify.

 

[analoggg]

sigma delta adc coefficients

can you please tell me which CMFB structure is good for full diff folded cascode for SC integrator for audio........capacitor one or mos one

 

[arunkumar446]

sigma delta type adc

i prefer 4 capacitor 4 switch one which is simple, if you are thinking of dual phase architecture, then dual phase SC-CMFB which has uniform loading effect on common mode feed back can be thought of ( but more area).

 

[xdy83]

sfunpsd

maybe just mex simulate* .dsm
i forgot the name of the file.

thnx arun...
I had already seen that paper.Those standard coefficients are for his own integrator right.I mean if we different opamp,(I mean may be in different technology) the input range after which opamp saturates is different right..? or do you think generally these 0.5 coefficients work for all opamps,I mean integrators.
I wanted to know how boser scaled those coefficients(procedure) .If you know pls let me know.......

I first used delta sigma tool box by shrier in matlab, but there was some error in generating these coeeficients.If you are familiar with this delta sigma took box could you pls tell me how to run mex version......I am encountering an error in doing that too.....

Thank you.

 

[analoggg]

sigma-delta adc limitation

Hi thanq again.....
one more thing wen u simulated in matlab simulink, how to get power in db .I mean psd.In default psd block power is not mentioned in db.should we change the matlab programme in the sfunpsd file.is tht d only way.....?
pls tell me........pls.

 

[arunkumar446]

pratical sigma delta mdl

HI analogg,

the psd block which is in simulink wont have in db, some take the output bitstream and use a matlab code for fft and calculate the sndr, you can get that fig. but simple approach is you get tool box from matlab simulink which has psd functions as a simulink model for sigma delta.

 

[analoggg]

dc signal sigma delta

hi again one more question...srry for so many questions.....
should gain spec be derived from accuracy requirement(closed loop gain accuracy)or static error requirement.Can u pls tel me what you derived it from....

 

[arunkumar446]

integrator overloading in sigma delta adc

as you change the gain of the amp you can see in the psd of the sigma delta as gain decreases the SNDR decreases so based on how much you need the SNDR to be you can decide the gain of the OTA. but keep in mind slew rate, band width and other parameters also show effect on this SNDR so be careful in deciding gian, so keep some margin.

 

[icecell]

sigma delta adc matlab

hi arun ,I hav a question.
delta-sigma Modulator is suitable for low fre input(such as AD7755,is 50Hz),but maybe not for DC input,as dc input can result in some extra noise. common mode
will disappear at ouput in full diff circuit which is more familiar. Isn't it?

 

[analoggg]

implementation procedure of sigma delta adc

-> no, sigma delta modulator does giv output for dc.
-> At first even though you are applying DC,after a while input to the inttegrator is DC value - DAC output right(which is non zero after it get its 1st input from the quantizer and keeps on changing later on).So it keeps on changing.it produces series of 1's and 0's.
-> depending whether input dc is a rational number or not it produces periodic sequence of 1's and 0's with period depending on that rational number.
-> above 1V dc modulator wont produce any limit cycles.

Added after 24 minutes:

50Hz you mentioned might be bandwidth or something.ADC s are not generally designed for a single feruqncy right....So Wen we say 50 Hz bandwidth .its from 0-50 Hz.All low frquency applications include DC.So DC is also to be processed.

 

[kuohsi]

delta sigma adc type 4

-> depending whether input dc is a rational number or not it produces periodic sequence of 1's and 0's with period depending on that rational number.
-> above 1V dc modulator wont produce any limit cycles.

Hi analoggg:
What method can I prevent the idle tone(limit cycles) with DC input??
And what is the mean "-> above 1V dc modulator wont produce any limit cycles."??
Is the input value above 1V??
Thank you!

 

[analoggg]

adc sigma delta type

ditherng helps to prevent limit cycles

 

[kuohsi]

sigma delta adc design procedure

ditherng helps to prevent limit cycles

Hi analoggg:
Thanks for your reply!
Dose the dither circuit be difficult to implement?
Do you have dither circuit paper or data?

If I use high-order sigma-delta , is there no idle tone in spectrum?

Thank you very much!