1. Home
  2. Electronics Design
  3. an analog to digital converter problem

an analog to digital converter problem

I have a linear ccd signal input, the ccd analog output shift out pixel's voltage at max frequency of 1MHz. According Nyquist the raw signal's bw is 500kHz. check page 6 in below:

formatting link

I need to build an afe + adc circuits.

I want to increase the dynamic range. The spec says noise is 2mV. Who knows the measurement bandwidth? whatever, just say, 2mV.

I picked an adc, super good, with bw of 16MHz. I want to do an AC coupled, bandpass afe. I calculated the variable gain should be 0-60dB or less. I want to limit my afe's bw, or limited harmonic for the ccd analog inputs. Remove the inband white noise as much as possible. but I want to consider the in band ripple will generate the uncertainty. I want to control the in band analog uncertainty within 0.1%. How many harmonics should I choose? To do a reverse Fourier transform might sovle this problem but I am far from the textbooks.

I guess I need 500kHz * 15th = 8MHz. how about the ripple introduced by the 1st to 15th harmonics for square wave?

Reply to
Power boy
Loading thread data ...

Congratulations. You have made an erroneous "According to Nyquist" claim that I hadn't thought to refute:

formatting link

Nyquist's limit says absolutely nothing about a signal's bandwidth going into a sampler: it only says what the bandwidth must be for perfect reconstruction.

In fact, the ultimate input signal to your CCD is spatial, and is as bandlimited (or not) as the focus is blurry (or sharp). In pixel-starved applications folks usually want to have a blur spot that's smaller than a pixel, to lose as little information as possible, even if it means some aliasing.

Each pixel, taken as an individual thing, is 1us long and has 2mV or less of noise. The 1us sets some pretty firm bounds on the measurement bandwidth.

What the heck do you mean by AFE?

If you want to sample the output of your CCD, you want to make the ADC synchronous with the pixel clock. You want to either sample at the one best point in the pixel or you want to sample a burst around that point and average. What you don't want to do is to munge up the CCD signal -- for that you don't need to worry about harmonics, you just need to worry about your amplifier/buffer's step response, to make sure that you're getting plenty of the current pixel, and not much at all of the previous one.

For 16 bits and 1us, I get about a 1.8MHz bandwidth, assuming one dominant pole (11 time constants = 1us, if someone wants to check my math).

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Reply to
Tim Wescott

im

pling/sampling.pdf.

ed

e

by

-

thanks but you still didn't point out the bandwidth even this is spatial. if this is spatial, I treated as sqare wave impulse, the bw for this is still 1/(2*delta_T). To sample this ccd with big dc offset, pink noise and white noise I want to remove the noise , dc offset. However, a HPF introduces too much distortion due to the in band phase delay. I can only use an AC coupled lpf. the pink noise is still there. I couldn't just directly send it to adc which includes too much noise. the white noise's spectrum density is flat that's why I narrow the input bandwidth. I tested a little bit if I use ac coupled, bessel or butterworth with cutoff at 16MHz it just introduced rising edge delay. if I use HPF the signal is totally distorted, within 1us the voltage level is tilting due to the phase delay. I know the pink noise is still there.

my signal is about 5 uv with the dc offset 3V. noise is about 2mV. I want to do 100 times averaging which improve 10 times snr. and I will do other kind of filtering after digitized the signal.

Reply to
Power boy

laim

ampling/sampling.pdf.

g

rved

a

ss

B
I

vle

d by

ne

--

y
s

I just changed detector and design approach after research a few days.

My current solution will be dual slope integration plus correlated double s= ampling technique. I figured out the noise source (readout, photon, dark) a= nd some methods to remove them. thank you all.

Reply to
power boy

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.