Anti-Alias Filter for Delta Sigma ADC

[Sonnenmann]

I am looking for an low-power architecture of an anti-alias Filter.
Signalbandwidth=8KHz , Clk,adc=1-4Mhz.Resolution > 12 Bit There are no extern elements allowed.

Can somebody show me a basic architecture/schematic/papers of such a componet?

 

[philcorb]

12 bit ~= 72 dB SNR. Theerfore it would make sense to have at least this attenuation at the aliasing frequencies. A single pole filter, providing -20 dB/dec will only give 60 dB attenuation at 8MHz. A two pole filter, providing -40 dB/dec will give 80 dB attenuation at 800KHz. As a starter, how about a 2 pole passive RC filter and go from there. Maybe an opamp-RC filter would be a suitable solution if you require an active filter -- perhaps using multiple feedback or sallen-key topology.

 

[joseMiguel]

Hi,

If i understand your question, you want a microcontroller with in built 12 bits ADC.
Is there such kind of ADC in ARM cortex M0 based microcontroller?
(I am agree with the above anti aliasing filter)
regards
JoseMiguel

 

[Sonnenmann]

Thank you for your answers.

The system is a fully integrated sensor system in a 350nm technology.
So i have to control the sizes of cap and resistors because of minimal chip size.

Is there any architecture to minimize cap sizes? or to avoid values >50pF or should i use Mosfets for the big resistors?

 

[FvM]

The question is unclear, in my opinion. The signal bandwidth of the SD ADC doesn't play a role for an anti-alias filter, because it's achieved in the digital domain. It's only important in so far, that the lowest frequency component that has to be supressed by the anti alias filter is fs - 8 kHz. Furthermore, harmonics of fs have to considered as well.

So the problem is about supressing fs components (and it's multiples) from the input signal. Usually SD ADCs have external filters for it, but for a rather low resolution of 12 bit, an integrated analog lowpass filter should be feasible without breaking the noise specification, I hope.

 

[philcorb]

You could use Mosfets as resistors or capacitors. Both add distortion so it depends on your linearity spec. The resistors and capacitors can probably be traded-off against one another. The larger the resistors, the more noise but less area (as the caps can be reduced in size).