Flash ADC selection, or other approach?

From: Peter C. Wallace <pcw_at_mesanet.com>
Date: Mon Oct 18 23:00:04 2004

On Mon, 18 Oct 2004, Tom Jennings wrote:

> I'm a fair designer, but could use a hand at parts selection,
> to build a device to suck data off an old rotating memory.
>
> Generally-speaking, I need a little box (black box) that
> can slurp serial data off five parallel serial streams,
> synchronously. The native data rate is 80KHz, there are 128
> sectors (sector = 32-bit word) per track, 4096 bits/track.
>
> Our previous discussion imposing 16X (say) sample rate on
> 12.5uS/bit is 1.28Msamples/sec, five channels. 4 bits vertically
> seems adequate.
>
> All five tracks must be sampled at the same time; internal
> machine timing is derived by random logic hung off the timing
> tracks. Channel-to-channel skew or offset would render the
> data worthless. Can't read one track, then the next, etc.
>
> (That said, it would be possible, if troublesome, to read only
> two channels at a time, where one channel is always the clock
> track, but that presupposes the clock track is perfect. I think
> with all five simultanously sampled I could "fix" a glitch in the
> clock track and re-write it, but that process will degrade if not
> simultaneous. I don't think the hardware savings are worth it.)
>
> With that assumption, it's 65536 samples/track, five tracks.
> With a trivial architecture of one 4-bit sample stored per
> memory byte, it's 320K bytes. Seeing how even little SBCs have
> a megabyte this will do.
>
> As I see it, my two practical (read: lazy) choices are build
> something around a little SBC, or some kludge around a "PC".
>
> The SBC approach: There are many $99 single board computers,
> many with A/D, but most of them are obsessed with bit depth (10
> to 16) and not samples/sec in the lower-cost range. But sticking
> a five 4-bit ADCs on parallel ports, and every sample-interval,
> strobing all ADCs and reading 5 nybbles/bytes into local RAM, and
> a separate command to dump sampled data to a PC via serial port.
>
> The PC approach is to somehow wangle the five ADCs into the PC
> such that it will accept the 65536 5-byte (5-nybble) samples
> with ZERO LATENCY. There's plenty of memory, but my guess
> is simply adapting 5 ADCs onto the PC parallel port, or funny
> hardware to talk USB (and I don't think USB will guarnetee me my
> latency) is as much work as the SBC solution, which, if coded
> in assembly, will be plenty fast enough and trivial to build
> (and easy to test).
>
> I favor the SBC approach, as it would make a useful tool for
> others and it's platform independent.
>
> I can't find 4-bit flash ADCs! The smallest ADC on digikey is
> 8 bits, which is of course fine, but Analog Device's idea of
> fast is 2.5uS conversion, and they're $15/each.
>
> Anyone know of 4-bit ADCs? Or am I barking up the wrong tree
> here?
>

AD's AD7822 is $3.77 in 100s so should not be bad in low quans

This is 8 bit 2 M sample/second, available in DIP.

If you dont mind a surface mount part with SPI interface, the AD7478A is only
$.95 for 8 bits, 1.2 MSPS

Peter Wallace
Received on Mon Oct 18 2004 - 23:00:04 BST