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Intrinsic 1/f noise in ADCs and DACs

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
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Reply to
Phil Hobbs
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I know an RF guy who isn't into plaques but his would read "It's all just jittery DC".

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?

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Regards, Joerg

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Reply to
Joerg

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

Reply to
John Larkin

I design seismic data acqusition equipment. The frequency range is about the same as yours.

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Junk.

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.

It depends. If the reference is external, you can fix the noise.

There is no 1/F in the chopper stabilized ADCs.

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

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Reply to
Vladimir Vassilevsky
[...]

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

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Reply to
Joerg

Noisey bastard! :-)

You have disturbed the field!

Is your name really "Jon Luc Plaquard? :-)

Reply to
Chieftain of the Carpet Crawle

Called chopping?

Reply to
Chieftain of the Carpet Crawle

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.

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
Reply to
Phil Hobbs

Good idea about the homemade DAC--we can use a MF resistor string and switch between them with a mux. With zero volts across the mux, there should be zero 1/f noise. An old thin-film-resistor DAC like a DAC08 might do more or less the same.

This gizmo has 8 channels in a miniature package--it's on both sides of a 20mm diameter circular board, with a compound TIA for each channel. Not a lot of room for manoeuvre.

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
Reply to
Phil Hobbs

Thanks, that's great info. I agree that the ADC isn't the greatest for the application. It does have to fit 8 channels' worth on a 20-mm diameter board, so a delta-sigma is a bit problematical.

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.

This sensor uses an interferometer rather than coils or piezos, so we start out with a sensitivity advantage--even with the AD7699, our sensitivity is quite adequate in the white noise region, so if the 1/f can be controlled, we should be in good shape.

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
Reply to
Phil Hobbs

We'll see.. getting some in soon. Nice power supply range.

Reply to
Spehro Pefhany

Not necessarily. Depends on the sensor. Sometimes an excitation with a clean sine wave is preferable to the usual DC excitation.

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Regards, Joerg

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Reply to
Joerg

Yeah, can be iffy. But without knowing what your sensor is that's hard to say for me.

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Regards, Joerg

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Reply to
Joerg

[...]

[...]

Can't you mux 8:1, into one ADC?

--
Regards, Joerg

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Reply to
Joerg

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
Reply to
Phil Hobbs

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
Reply to
Phil Hobbs

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

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Joerg

My plaque over the office door says, "Some days you eat the bear and some days the bear eats you" ;-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
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No, you can only rent beer :-)

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Regards, Joerg

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Reply to
Joerg

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?

Reply to
whit3rd

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