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Simple multi-channel serial ADC (8-ch)?

Discussion in 'Electronic Design' started by Joerg, Feb 15, 2007.

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  1. Joerg

    Joerg Guest

    Hello Folks,

    As an analog guy I get goose pimples when I see converters like the
    TLV1508. Impressive performance but, boy, do they need a complicated
    liturgy to get data out of them. Many of them must be told via SPI that
    I want to read this, that and the other value. Then you must patiently
    wait x clock cycles, wasting away bus time. The guy who designs the host
    processing will throw his coffee mug at me because there'll be a dozen
    of these.

    Here is what I am looking for: 10 bits or so, 8 channels, bone-simple
    readout, can very very slow, low kHz range. All I need is to read DC
    values and it doesn't matter when the exact sampling time was. SPI, I2C,
    anything serial works. I mean, with a FIFO in there they should be able
    to do continuous autoscan and just dump the last valid set of values
    upon request.

    Any ideas? I am also looking for 10-12 bit DAC with lots of channels but
    that seems to be much simpler.
  2. mkaras

    mkaras Guest

    If you intend to interface into a microcontroller they why not design
    in one of the microcontrollers that has 8 channels of A/D built in.
    Very simple internal parallel interface. Examples are SiLabs C8051F126
    with 8 channels of 12-bit A/D, Atmel ATMega16 with 8 channels of 10-
    bit A/D or OnSemi MC68HC908GP32 with 8 channels of 8-bit A/D. Of
    course there are many others.

    - mkaras
  3. Joerg,
    I'd employ a controller such as the AVR Tiny26 with 8
    differential or 11 single ended 10 bit channels.
    Since you need a dozend of them just hook them together
    with an SPI or an USI.
    Linear Tech has a a bunch of multichannel DACs with the
    LTC2600 : 8x 12 bits for 9$ as example.

  4. Joerg

    Joerg Guest

    Unfortunately there isn't going to be a uC on those cards. Also, I found
    that many built-in converters are noisy no matter how clean the supply
    pins are kept.
  5. Joerg

    Joerg Guest

    I wanted to avoid a uC on there because it wouldn't be needed for any
    other purpose. They are too slow for the analog stuff I am doing on the
    boards. The ADCs on uC also tend to be noisier than stand-alone chips.
    Of course I could set up the uC to do autoscan and chips like the
    MSP430F2xxx have that feature in hardware. All you need to do is set one
    bit in a register upon initialize. Beats me why stand-alone ADCs don't
    have that. That way the host wouldn't have to talk, just enable CS via
    the correct address on the bus and then read the stream.

    Thanks, that could be a good one. I am currently leaning towards the
    TLV56xx series. It all depends what's in stock because that is an
    indicator of popularity, and sometimes of longevity as well.
  6. linnix

    linnix Guest

    Micros are sometimes cheaper than standalone ADCs.
    You don't need to use them for anything else.
    How fast do you need?
    Is 500KSPS too slow for you?
    How about 1MSPS?
    That's true. You have to be careful in layouts.
    Some have interval timer based conversions and auto averaging.
  7. Max1202 - simple SPI interface and internal reference. No waits -
    data comes out as you shift the command in.

    Peter Bennett, VE7CEI Vancouver BC, Canada
    peterbb4 (at)
    new newsgroup users info :
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  8. Joerg

    Joerg Guest

    But they need to be flash programmed and are still more noisy.
    The ADC can be slow, a few kHz at the most. Not critical. What I meant
    is that I can't use a uC for any of the control gear tasks. That would
    have justified the hassle of in circuit programming. Unless you buy gang
    programmers many manufacturers do not offer file dumps. They give you a
    nice and cozy design environment but, for example, TI does not offer a
    feature where you connect an EZ430 pod or the like and just download a
    file right from the OS or from a web browser. You have to do that from
    the IAR console. Not such a good thing in this case since the engineers
    at my client are all specialized in optics and process control.
    Even then they are noisier. Bond wires only provide so much in
    conductivity and because form factor is a marketing concern they often
    spring for a single VDD connection, two at the most.
    That would almost suffice. But chips like the MSP430F2012 even have a
    nice SAR converter in there with autoscan. I still don't get why there
    are no stand-alone converters with this autoscan feature. All the ones I
    checked so far still needed an "invocation speech" to them via an SPI
    write or were plain out of stock everywhere. The latter is a huge red flag.
  9. Joerg

    Joerg Guest

    Thanks. But according to its datasheet (page 11) you must still clock a
    control byte into DIN. Seems just like all the others. Man, why do they
    do this?

    Also, that vendor isn't really my favorite. Got too many black eyes WRT
    availability. Digikey only has 12 of these in SSOP, not a good sign and
    not enough for even one unit in our case.

    BTW, I clicked on their site for SSOP: Leadtime 16 wks. Ouch!
  10. linnix

    linnix Guest

    Yes, I understand your concern. Programming environment is one reason
    I am avoiding the MSP. At least with ARM, JTAG is possible. For our
    particular project, we need to be able to JTAG one micro from another.

    If flash is reliable, then I would not care about the programming
    I am not scared by the initial programming, but the possiblity of
    re-programming in the field makes me nervous.

    For example, flash is very likely the problem for the AT91Sam7S256
    failing in the field. Failure is Failure, even if you can re-program
  11. Joerg

    Joerg Guest

    I told them that, and certainly others did. All they'd have to do is
    maybe hire an intern who can write a simple download routine so you can
    dump data via SBW into the device without the design suite open.
    Preferably without even needing to have a clue of what's happening. Just
    by selecting a file, connecting the cable and hitting "Program Now".
    It's not rocket science. But so far, nada.

    The topper would be a wee sub-program that allows one-pin programming so
    you could connect a little IR photodiode and then "beam it over". With
    good encryption to avoid data errors in ambient light. Look, ma, no
    cable! While it's no big deal to write such code for the MSP430 the PC
    part would be a challenge, at least for an analog guy like me.

    Ouch, that would not be good.
  12. linnix

    linnix Guest

    But you are assuming that the bootloader is reliable, I would not rely
    on that unless the bootloader is in hardware rom, not flash and not
    If the spec is open, someone would write it for you.
    Problem is that most micro makers are protecting their programming
    interfaces like trade secrets.
  13. Joerg

    Joerg Guest

    The original one is hard-coded in but I don't remember whether that
    ain't just a write-protected flash area.
    Yep, and that is a rather silly business behavior. For TI it has already
    cost them sales in my case. I had bugged them numerous times about some
    more data about their MSP430 devices, then gave up on it. Went analog,

    Just like a vendor who refused to disclose the schematic of a laser
    controller, a schematic I only needed to find out what the plant
    parameters would be. So, I designed it out. Slam bam, door shut. Even if
    they came back now and repented it would be two days too late.
  14. We use Texas TLV2543 a lot. 11 channels, 12 bit. Yes you need to send
    a command to set up what channel you want to read, but the SPI gets the
    result of the last conversion as you clock in the next command, so its
    only one 16 bit transfer to read the last result and set the next one up.


    Adrian Jansen adrianjansen at internode dot on dot net
    Design Engineer J & K Micro Systems
    Microcomputer solutions for industrial control
    Note reply address is invalid, convert address above to machine form.
  15. Joerg

    Joerg Guest

    I can't use that one because the digital folks decided to go 3.3V. But
    TI has similar ones that go down to 3V or even 2.7V. I guess I'll have
    to use one of them, looks like no other choice.

    Still I think it would be smart to market an SPI AD converter that has
    it's own little clock, totally asynchronous from the SPI bus. Just a
    simple RC or some cheap resonator. Then you could run that at a sample
    rate that is low enough for part tolerance margins. Now, instead of
    getting a first value that was acquired who knows when you could be sure
    that it'll be rather recent, max x clock cycles ago. And it'll be there
    almost instantly.
  16. Hi Jörg,
    I love the ADuC702x 12 bits resolution and not bad at all. Try it and love

  17. Hi Jörg,
    again, why should a manufactor do this. Most people will use µC for this
    operation. The Core for this is easy to write, every µC has RAM for a FIFO,
    a Timer... and they are cheap...

  18. Joerg

    Joerg Guest

    But then I have to learn a new uC ;-)

    And around $8 for just the ADC function would be a hard pill to swallow.
    Plus when you look at Digikey stock is rather low for that series. I
    guess I'll go with one of those SPI varieties from TI. Don't know what
    National would have, can't see it after they managed to mangle up their
    web site.
  19. Joerg

    Joerg Guest

    Because I believe there is a market. Here's the current situation: There
    is a host system, a powerful computer. But the connection to the rack I
    am developing for this client will be via a USB bridge. Sure, we could
    add another layer of complexity by having a uC somewhere in between and
    I seriously thought about that. But if there was an auto-scan converter
    none of this is needed. Of course, then there is my favorite "fix" for
    situations like that: Build a poor-man's autoscan. Figure out what the
    ADC wants to hear, generate that somehow with a digital concoction and
    trigger this from ye olde 74HC4060. Works, done it before, has caused
    goose pimples in design reviews.

    Other scenario (that one is coming down the pike here soon): A mote. It
    only needs to measure one thing, once in a while, then send that info
    off to somewhere via a one-way serial link. Why should this need a uC
    and have to undergo all that costly EMC agency rigamaroo?
  20. Hi Jörg,
    I find it very easy to learn. With winarm package no additional cost, only
    $30 Minikit with serial download cable or simply samples, ready to start
    with... Only the I2C I didn't understood clearly.
    7022 is with $4 delivering 10 AD ports with 12 bit.

    the ADuC7022 may not be the cheapest solution, Atmel may have cheaper ones,
    or Reneas, or...
    The Analog MicroConverters do really have excelent AD-units. That's what I
    know, this is what you didn't guess, me too, but we tested it.

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