Intrinsic 1/f noise in ADCs and DACs
in [Design]
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From: Phil Hobbs on 8 Jul 2010 12:22 Having spent my career trying to keep clear of the low baseband, I now find myself needing to do very precise measurements of acceleration at very low frequencies--like 1 nano g (1 microgal, 10**-8 m/s**2) per root hertz at frequencies from 10**-4 Hz to about 100 Hz. This is an interesting ride, and will be generating a few discussions here, I hope. [I bought myself a brass plaque for the wall that says, DC: The Final Frontier ] My noise budget is currently dominated by the white noise of a 16-bit ADC (AD7699), running at 100 kHz to spread the noise out, and subsequently filtered. (We may add some high frequency dither if it turns out to be needed.) The DC levels of the signals can be anywhere in the ADC range, but any large changes will be very slow. I'm therefore looking at a subranging strategy, with a DAC providing an offset that gets subtracted off before digitizing, to allow the steps to be effectively 32x smaller, say. That sets up today's question: The low-frequency noise behaviour of most ADC and DAC circuits is dominated by the noise of the voltage reference, which is almost always really horrible. Using a ratiometric measurement I can get rid of this, ideally, so I'm left with the intrinsic 1/f noise of the ADC and DAC. Does anybody have any wisdom about the intrinsic 1/f noise of ADCs and DACs? Thanks Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: Joerg on 8 Jul 2010 12:33 Phil Hobbs wrote: > Having spent my career trying to keep clear of the low baseband, I now > find myself needing to do very precise measurements of acceleration at > very low frequencies--like 1 nano g (1 microgal, 10**-8 m/s**2) per root > hertz at frequencies from 10**-4 Hz to about 100 Hz. This is an > interesting ride, and will be generating a few discussions here, I hope. > > [I bought myself a brass plaque for the wall that says, > > DC: The Final Frontier > I know an RF guy who isn't into plaques but his would read "It's all just jittery DC". > ] > > My noise budget is currently dominated by the white noise of a 16-bit > ADC (AD7699), running at 100 kHz to spread the noise out, and > subsequently filtered. (We may add some high frequency dither if it > turns out to be needed.) The DC levels of the signals can be anywhere > in the ADC range, but any large changes will be very slow. I'm > therefore looking at a subranging strategy, with a DAC providing an > offset that gets subtracted off before digitizing, to allow the steps to > be effectively 32x smaller, say. > > That sets up today's question: The low-frequency noise behaviour of > most ADC and DAC circuits is dominated by the noise of the voltage > reference, which is almost always really horrible. Using a ratiometric > measurement I can get rid of this, ideally, so I'm left with the > intrinsic 1/f noise of the ADC and DAC. > > Does anybody have any wisdom about the intrinsic 1/f noise of ADCs and > DACs? > I don't, but an off-the-cuff question: Can you modulate the input source so the baseband information rides on a carrier of a few kilohoitzes to get you out of the 1/f on the receive side? -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 8 Jul 2010 12:44 On Thu, 08 Jul 2010 12:22:52 -0400, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >Having spent my career trying to keep clear of the low baseband, I now >find myself needing to do very precise measurements of acceleration at >very low frequencies--like 1 nano g (1 microgal, 10**-8 m/s**2) per root >hertz at frequencies from 10**-4 Hz to about 100 Hz. This is an >interesting ride, and will be generating a few discussions here, I hope. > >[I bought myself a brass plaque for the wall that says, > >DC: The Final Frontier > >] > >My noise budget is currently dominated by the white noise of a 16-bit >ADC (AD7699), running at 100 kHz to spread the noise out, and >subsequently filtered. (We may add some high frequency dither if it >turns out to be needed.) The DC levels of the signals can be anywhere >in the ADC range, but any large changes will be very slow. I'm >therefore looking at a subranging strategy, with a DAC providing an >offset that gets subtracted off before digitizing, to allow the steps to >be effectively 32x smaller, say. > >That sets up today's question: The low-frequency noise behaviour of >most ADC and DAC circuits is dominated by the noise of the voltage >reference, which is almost always really horrible. Using a ratiometric >measurement I can get rid of this, ideally, so I'm left with the >intrinsic 1/f noise of the ADC and DAC. > >Does anybody have any wisdom about the intrinsic 1/f noise of ADCs and DACs? Actually, no. I do have the name of the guy at ADI who really knows the AD7699, and he's been willing to go into the lab and and try stuff when I had problems. The 7699 is a charge-balance part so should be pretty good. We're seeing a couple of LSBs RMS noise out of ours, down to about 0.4 LSB when digitally filtered at 1 KHz, averaged over a second. It's a 500 KHz, mux'd ADC. I wonder if digitizing another, reference sort of channel, would provide any numbers that would be useful in processing the real samples. Note that ADC power supply noise will be important. Even the electrical quality of the SPI lines will start to matter at some point... everything is on one chip. I wouldn't clock it at all while it's busy digitizing; you can afford that at 100 KHz. Thinking about my Audi transmission, you could use three DACs for the zoom offset: The current one, the next one up, and the next one down. Lowpass the hell out of all three and switch between them as needed. The next logical step would be to make your own multiple-output, say, 16-step string dac, with just low-value resistors and a few of mux's, to make the basic steps per above, and fine-tune them with low noise dacs. Hey, make all three signals available (current zoomed signal, next one up, next one down) and let the ADC mux switch between them all the time. Then you can splice the crossovers perfectly. Analog Devices has a new 20-bit DAC that's designed for MRI gradient driver apps. It probably has low 1/f noise, because that matters for imaging. I've seen ghastly popcorn noise, tens of PPM, from supposedly audiophool-quality CMOS dacs. That can be a nightmare. Some parts will sit for hours and just occasionally make a few pops. John
From: Vladimir Vassilevsky on 8 Jul 2010 13:00 Phil Hobbs wrote: > Having spent my career trying to keep clear of the low baseband, I now > find myself needing to do very precise measurements of acceleration at > very low frequencies--like 1 nano g (1 microgal, 10**-8 m/s**2) per root > hertz at frequencies from 10**-4 Hz to about 100 Hz. This is an > interesting ride, and will be generating a few discussions here, I hope. I design seismic data acqusition equipment. The frequency range is about the same as yours. http://www.abvolt.com/projects_and_solutions/data_acquisition_board.htm > My noise budget is currently dominated by the white noise of a 16-bit > ADC (AD7699), Junk. > running at 100 kHz to spread the noise out, and > subsequently filtered. (We may add some high frequency dither if it > turns out to be needed.) The DC levels of the signals can be anywhere > in the ADC range, but any large changes will be very slow. I'm > therefore looking at a subranging strategy, with a DAC providing an > offset that gets subtracted off before digitizing, to allow the steps to > be effectively 32x smaller, say. This is equvalent to highpass filter. There could be better ways for doing that. With good ADC, there should be enough of linearity and dynamic range to make the offset step unnecessary. > That sets up today's question: The low-frequency noise behaviour of > most ADC and DAC circuits is dominated by the noise of the voltage > reference, which is almost always really horrible. It depends. If the reference is external, you can fix the noise. > Using a ratiometric > measurement I can get rid of this, ideally, so I'm left with the > intrinsic 1/f noise of the ADC and DAC. There is no 1/F in the chopper stabilized ADCs. > Does anybody have any wisdom about the intrinsic 1/f noise of ADCs and > DACs? Check ADS1282 from TI. This state of the art ADC has the noise floor of ~5nv/root(Hz) all the way down to DC. Total dynamic range ~ 133dB (yes, thus is true number). Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Joerg on 8 Jul 2010 13:01 John Larkin wrote: [...] > I've seen ghastly popcorn noise, tens of PPM, from supposedly > audiophool-quality CMOS dacs. That can be a nightmare. Some parts will > sit for hours and just occasionally make a few pops. > When diagnosing this sort of stuff I found that one needs a really quiet environment. A screen room may be enough (most of the time). One company had such "pops", every so many seconds. Frustrating, couldn't find it because how do you trigger anything on that? Gazing out the window, thinking hard ... "What's that occasional reflection on that hill towards the horizon way over there?" ... "Oh, that's nothing, just some military radar, oh wait, s..t!" -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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