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Maybe silly question about first-order sigma-delta modulator

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skyjam

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I'm designing a simple first-order sigma-delta modulator with RC
integrator.

Now I want to define the minimume and maximum sampling frequency of
this modulator. Of course, minimume fs can be found by investigating
the integrator output voltage --- if the fs is too low, the output
voltage will be too high to implement.

However, the maximum fs is hard to achieve. Will the system be
unstable when fs is too high? Is the maximum sampling frequency
related to the bandwidth or gain of the integrator? And How to predict
the frequency?

I'm a rookie in this field and maybe it's a silly question:)

But still hope to see your opinions. Thanks.
 

Re: Maybe silly question about first-order sigma-delta modul

I've been designing also a 1st- order sigma-delta modulator but with switch capacitors technique. The maximum frequency is limited by the opamp you use int he integrator. In the Jhons & Martin Integrated circuit design book, says that the GBW must be at least 5 times the frequency of the clock.In your case you should check this book:

Continuous-Time Sigma-Delta A/D Conversion
Fundamentals, Performance Limits and Robust Implementations
by M. Ortmanns F. Gerfers
 

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    skyjam

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Re: Maybe silly question about first-order sigma-delta modul

i think CT sigma delta must be quite different from DT ones, except the GBW, may the feedback time of quantizer should be considered.
 

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    skyjam

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Re: Maybe silly question about first-order sigma-delta modul

Thanks for your help!

I'm reading the book and have found sth useful.
 

Re: Maybe silly question about first-order sigma-delta modul

Hi,

As far as i know, the excess loop delay is an important factor to be considered in a continuous time SD modulator. Excess loop delay is the delay in the rise time of the sampling clock to the ouptu of the dac in the feedback. this causes the modulator parameters to change and hence causes instability. so the upper limit for ur sampling frequency may be decided based on this issue....
 

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