[EE] 24-bit A/D. Are We in the Twilite Zone Here?
Dan Michaels email (remove spam text)
>I use Max641, 586 or 587 at my devices (the first one has an internal
>catch diode). EMI? yes, but not enough to cause any problems at the 22.5
>bits ADC installed less than 20 mm from the coil. Just for info, this
>ADC resolution is about 0.12uV (120 nano Volts), the interference should
>exist but it is not noticeable.
Yeah, 0.12 uV. 22.5 bits. Right.
Wagner, I noticed your mention of using the LTC2400 a couple of weeks
ago, and downloaded the datasheet. I'm still digesting it. I've built
amps for amplifying 2-5 uV before, and it's not all that easy to get
good results at those levels. Now, you and Linear Tech come along and
say, hey, we got 0.12 uV sensitivity here, and it works with a buck
regulator ckt, too. Oy, yoy, yoy.
I've got one board with opAmps and a 12-bit LTC1400 running up to
400 Ksps [with 20 Mhz PIC on-board, of course], and damn if I can
get the noise level below about 2 mV, no matter what I do. I've
read everything ever written on low-noise analog and high-speed
digital design, and tried 100 different things, and it's still
2 mV [that's **MILLI-volts**].
On a thread a few weeks ago, I asked a simple question of someone
about how they got their Vref to be noise-free at 12-bits, got
no response back from that guy, but did get some other clowns
telling me "... seems some people are re-learning things the hard
way ...." and ".... yes folks ohm's law is a reality ...", or
some such blather.
Forget the switcher. 0.12 uV. And with digital circuitry within
200 *yards*. Are you kidding?
I realize that the LTC2400 has differential input and 3 hz
bandwidth, etc/etc, whereas I have single-ended input, opamp ckt
with Rin = 1M and BW = 1 Mhz, plus A/D SPI running at 6.4 Mhz,
but still -- 2 mV noise. What's a poor fella to do? [beyond the
100 things he's already tried].
Am I at the tradeoff point in my design? Is there some magical
Noise-vs-??something?? formula here I forgot about [like trading
gain for bandwidth]? Also, they are putting 16-bit sound cards
into PCs nowadays. What the heck do those guys know, that they
can accomplish that?
- Dan Michaels
See also: www.piclist.com/techref/io/atod.htm?key=a%2Fd
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