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sck236

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single to differential amp before audio DS ADC
Aug 22^nd, 2007, 9:54am

I want to design a low noise single to differential amplifier for the audio delta sigma modulator.

To reduce thermal noise, I increased the gm of the input transistors and I achieved about integrated 8uV noise from 20 to 20khz.

But when I sampled the noise using PNOISE, the integrated noise becomes 30uV due to noise folding.

Then, I reduced the bandwidth of the amp by increasing the compensation cap to 8pF. Now, the sampled noise is 20uV.

How can I reduce the sampled noise further? Do I need to reduce the bandwidth(Now, the bandwidth is 10MHz) more?

Is increaing the compensation cap only the way to reduce the bandwidth?

I am also concerned about decreasing the bandwidth since the next delta sigma modulator samples the signal at the speed of 3MHz.

When the sampling frequency of the modulator is 3Mhz and the sampling cap is 3pF,
how much should the bandwidth of the single to differential amp?

Could anyone suggest the procedure to design a low noise(10uV) amp before the sampled system ?

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HdrChopper

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Re: single to differential amp before audio DS ADC
Reply #1 - Aug 22^nd, 2007, 2:49pm

Hi,

the only way I know of to reduce folding is to use and AAF. Actually, your amplifier is acting as a "poor" AAF, and I say poor since the cut-off frequency should be half of your sampling frequency in order to totally avoid folding. So, if further lowering the BW in your amp is a concern due to other aspects of the circuit (as you suggested in your post), then the only option is to make a better (lower noise) amplifier.

To lower the noise, you have to increase the power / area of your circut. By increasing gm1 you certainly decrease the noise, but also increase the BW (BW is proportional to gm/Ccomp). You'll come to the conclusion that the only way to lower the amp noise is by increasing Ccomp (thus by reducing its BW).

Regards
tosei

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sck236 Junior Member Offline Posts: 15	Re: single to differential amp before audio DS ADC Reply #2 - Aug 24^th, 2007, 7:14am Thank you for your suggestions. Another question is what OSR today's audio ADC(low power portable) uses. 64 or 128 or 256? Can anyone explain advantage and disadvantage between 64, 128 and 256 OSRs with respect to SNR and implementation of S2D amp.?
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