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Design >> Analog Design >> sigma-delta adc
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Message started by ravi on Nov 6^th, 2003, 4:27am

Title: sigma-delta adc
Post by ravi on Nov 6^th, 2003, 4:27am

Hi,
If I want to use a sigma-delta modulator for a higher sampling frequency, what parameters should I check for. I mean, if I have a sigma-delta modulator that is working fine for 2 MHz sampling and I want to see whether it is working fine for 4 MHz. What parameters should I take care of? I want to do so because I want to have inputs with more frequency but I want to maintain the same OSR. Theoritically, OSR gives the resolution. I don't want to lose the resolution.

Thanks
Ravi

Title: Re: sigma-delta adc
Post by Paul on Nov 6^th, 2003, 6:53am

Hi Ravi,

as you mention, the theoretical resolution, given by the quantization noise, of the modulator depends on the OSR, the order and the number of levels of the quantizer.
You have to check if the amplifiers (and switches, in case of a switched-cap implementation) offer sufficient BW to work at a higher frequency. You can typically tune the amplifier BW adjusting biasing currents (up to some extent at least), but take care of switches on-resistance. It may add to the thermal noise budget.

Paul

Title: Re: sigma-delta adc
Post by ravi on Nov 7^th, 2003, 3:19am

Thanks a lot for your reply Paul, I too thought the same thing about the amps but I completely forgot the switches.
Is it ok to find THD of the switches at the required sampling rate. Typically how much THD is a good figure. If I am wrong, can you tell me about how to check if my switches are adding any thermal noise. I mean what simulations can I run.

Regards,
Ravi

Title: Re: sigma-delta adc
Post by Paul on Nov 8^th, 2003, 12:52pm

In order to have a good thermal noise result, the contribution of the switches should be neglegible compared to the noise of the amplifiers. In that case their THD is neglegible too. You can run AC noise simulations around your operating point and check for the noise contributions in the simulation outputs.
If you know how to do thermal noise calculations, you can also verify it that way.

Paul

Title: Re: sigma-delta adc
Post by wolf on Jun 30^th, 2004, 1:57am

Hi all,

If you go through the math you can see that you can add the contribution to the thermal noise of Ron and Req of the OTA with some weights which depend on the capacitor ratios. However, as Ron is max a few hundred of Ohms it is usually negligible compared to Req=8/(3gm) which is a few kOhms.
Concerning the OSR, i don't know if you keep your bandwidth and just double fs. If this is the case, you could divide your cap by 2 and keep your ota as it is.
Indeed, the SNR is roughly given by:
SNR=((vin^2)/2)/(4B0*2kT/(fs*Cs))
You have to check if the new bandwidth is still what you need (3-5 time fs). But usually, it is not too far.