Intrinsic 1/f noise in ADCs and DACs
in [Design]
|
From: Joerg on 8 Jul 2010 16:02 Chieftain of the Carpet Crawlers wrote: > On Thu, 08 Jul 2010 09:33:23 -0700, Joerg <invalid(a)invalid.invalid> > wrote: > >> 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? > > Called chopping? Not necessarily. Depends on the sensor. Sometimes an excitation with a clean sine wave is preferable to the usual DC excitation. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 8 Jul 2010 16:03 Phil Hobbs wrote: > Joerg wrote: >> 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? >> > > I'd love to do that, and we'll probably have to eventually. It's a > little difficult since it involves changing the sign of the acceleration > periodically (i.e. chopping the sample), meaning mechanical motion, > which brings in a whole lot of other issues. > Yeah, can be iffy. But without knowing what your sensor is that's hard to say for me. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 8 Jul 2010 16:05 Phil Hobbs wrote: > Vladimir Vassilevsky wrote: >> >> >> Phil Hobbs wrote: >> [...] >>> 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). >> >> [...] > I've seen the ADS1282, which is a very nice part, but (a) it's too slow, > and (b) we can't afford 8 of them per sensor anyway, either from a cost > or board space point of view. We may be better off in the end using > more than one board--we have a bit of space to play with in the axial > direction. > Can't you mux 8:1, into one ADC? -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Phil Hobbs on 8 Jul 2010 16:32 Joerg wrote: > Phil Hobbs wrote: >> Joerg wrote: >>> 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? >>> >> I'd love to do that, and we'll probably have to eventually. It's a >> little difficult since it involves changing the sign of the acceleration >> periodically (i.e. chopping the sample), meaning mechanical motion, >> which brings in a whole lot of other issues. >> > > Yeah, can be iffy. But without knowing what your sensor is that's hard > to say for me. > If 'twere mine, I'd be a bit more forthcoming, but it's client work--sorry to be all cloak-and-dagger about it. Cheers 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: Phil Hobbs on 8 Jul 2010 16:34
Joerg wrote: > Phil Hobbs wrote: >> Vladimir Vassilevsky wrote: >>> >>> Phil Hobbs wrote: >>> > > [...] > > >>>> 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). >>> >>> > > [...] > >> I've seen the ADS1282, which is a very nice part, but (a) it's too slow, >> and (b) we can't afford 8 of them per sensor anyway, either from a cost >> or board space point of view. We may be better off in the end using >> more than one board--we have a bit of space to play with in the axial >> direction. >> > > Can't you mux 8:1, into one ADC? > That's what we're doing now, with the SAR ADC, but it doesn't work as well with delta-sigmas. We need at least 1 kHz bandwidth on each channel, which would need a dedicated delta-sigma apiece. Cheers 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 |