24-Bit & 32-Bit ADC input filter
24-Bit & 32-Bit ADC input filter
(OP)
Hi friends I'm working on 24-Bit & 32-Bit ADC's. I need to design filters at the input of these ADC's. Which type of filter will work the best here? My reference voltage is 2.5V and the input span is -2.5 to 2.5V. I want the output count fluctuation of <5.
Thanks...
Thanks...





RE: 24-Bit & 32-Bit ADC input filter
Please get real. Fewer requirements given, means lousier solution.
TTFN
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RE: 24-Bit & 32-Bit ADC input filter
RE: 24-Bit & 32-Bit ADC input filter
Your channel impedance and the voltage range give you the power in your channel. Signal to noise ratios (SNR) is defined in units of power. You can use Johnson Noise to figure out your absolute lowest noise floor, before systematic effects start popping up (like Schott noise, flicker noise, and 1/f noise).
The ratio between your sample rate and your signal bandwidth lets you start to figure out how complex of a filter you need. A first order filter has a slope of 20dB/decade. 5 counts out of 2^32 is -179 dB, so that is nine decades (1,000,000,000:1) or a very high order filter (2nd order filters are 40dB/dec, third order are 60dB/dec...).
Is you SNR and SFDR specification for your A/D greater than 179 dB? If not you will need digital filtering (averaging) to approach that floor. For example, I have a 16-bit A/D (96dB) with a SNR of 80dB, so I don't even see -96 dB until after averaging 40 samples.
Welcome to channel design.
Z
RE: 24-Bit & 32-Bit ADC input filter
Is it even a solvable problem?
TTFN
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RE: 24-Bit & 32-Bit ADC input filter
I might need some more help in understanding these concepts I'll keep posting my doubts.
Thanks a lot....
RE: 24-Bit & 32-Bit ADC input filter
TTFN
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RE: 24-Bit & 32-Bit ADC input filter
RE: 24-Bit & 32-Bit ADC input filter
Glenn
RE: 24-Bit & 32-Bit ADC input filter
Dan - Owner

http://www.Hi-TecDesigns.com
RE: 24-Bit & 32-Bit ADC input filter
If true, it's a lot easier to let the ADC 'eat dust' and code a filter in firmware. A digital FIR filter with a few coeffs. does just fine even in 8-bit processors, with no need of hand-tuning a lot of coils and caps.
Unless you system knows in advance the input, an anti-aliasing filter should be added anyway.
RE: 24-Bit & 32-Bit ADC input filter
Start with the basics and keep asking questions as they come up.
Z
RE: 24-Bit & 32-Bit ADC input filter
RE: 24-Bit & 32-Bit ADC input filter
That's about 29 bits, so if we assume another 3 bits that are averaged away, we're at 32.
Note that the OP's output fluctuation of <5 results in about 29 stable bits.
TTFN
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RE: 24-Bit & 32-Bit ADC input filter
YMMV - this doesn't take other noise sources into account (Schott noise, 1/f noise, etc.).
Z
RE: 24-Bit & 32-Bit ADC input filter