Intrinsic 1/f noise in ADCs and DACs
in [Design]
|
From: Joerg on 8 Jul 2010 16:42 Phil Hobbs wrote: > 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. > Yeah, know what you mean. Same with a few projects here, wrestling with transducer issues. Hey, if anyone knows a good hands-on transducer engineer (PZT, dicing saws, flex bonding, production) let me know. Location would be near West Palm Beach, Florida. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Jim Thompson on 8 Jul 2010 17:04 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 plaque over the office door says, "Some days you eat the bear and some days the bear eats you" ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Obama isn't going to raise your taxes...it's Bush' fault: Not re- newing the Bush tax cuts will increase the bottom tier rate by 50%
From: Joerg on 8 Jul 2010 17:12 Jim Thompson wrote: > 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 plaque over the office door says, "Some days you eat the bear and > some days the bear eats you" ;-) > No, you can only rent beer :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: whit3rd on 8 Jul 2010 18:33 On Jul 8, 9:22 am, Phil Hobbs <pcdhSpamMeSensel...(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. That's seismometer or Mossbauer drive territory. There's a lot of literature available, I suspect. Most will be op amps and magnet/coil mechanisms. Are you doing an interferometry exercise? With a HeNe laser and a reflector moving circa 10**-8 m/s your fringes will dance around at 0.02 Hz. Wouldn't you be using a video digitizer and software, instead of an ADC, though?
From: Phil Hobbs on 8 Jul 2010 18:42
On 7/8/2010 6:33 PM, whit3rd wrote: > On Jul 8, 9:22 am, Phil Hobbs<pcdhSpamMeSensel...(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. > > That's seismometer or Mossbauer drive territory. There's a lot > of literature available, I suspect. Most will be op amps and > magnet/coil mechanisms. Yup. Or piezo balances. (Spehro and Co. are using SQUIDs for measuring gravity, but they're wild men.) We're doing the measurement interferometrically, which has a lot of advantages, including small size. > > Are you doing an interferometry exercise? With a HeNe laser > and a reflector moving circa 10**-8 m/s your fringes will > dance around at 0.02 Hz. Wouldn't you be using > a video digitizer and software, instead of an ADC, though? That's 10**-8 m/s**2, not m/s. It's the acceleration accuracy we need--a tiny fraction of a fringe, at DC. (Hence the plaque.) 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 |