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Connecting D/A converter

  1. Jul 23, 2007 #1
    Hello,

    I want to connect a d/a converter [e.g, MAX515] to RS232.
    I need a 0-5v output.


    assuming that I know how to write a code to output binaric signal,
    how do I connect the converter?
    I don't really know what to do with the clk, sync and Vres legs at the DAC.


    Liad.
     
  2. jcsd
  3. Jul 23, 2007 #2

    berkeman

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    Staff: Mentor

    There are a lot of datasheets and application notes at the MAXIM website that will show you how to connect up a DAC in many applications. Can you please point us to one that shows a similar connection to what you are doing, and ask us some specific questions about it?

    Also, keep in mind that RS-232 does not use 0V/+5V signals. It uses +/- split power supplies. You may want to consider using a MAX232 chip to convert your 5V signals to the RS-232 voltages.

    What are you using to create the the digital codes that you are converting for the RS-232 signals? And why don't you just do it all digitally, if you already have digital information? Why the intermediate analog step?
     
  4. Jul 23, 2007 #3

    chroot

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    Staff Emeritus
    Science Advisor
    Gold Member

    You're definitely going to need some kind of RS-232 transciever chip, in addition to your DAC. After you choose one, we can help you figure out how to connect them.

    Berkeman, I think Liad is trying to connect a DAC to an RS-232 bus so he can send digital data to it via RS-232, and have it converted to analog.

    - Warren
     
  5. Jul 23, 2007 #4

    xez

    User Avatar

    You *could* use multiple chips -- RS232 to TTL level
    converters, a SPI DAC, and an RS232 port with
    controllable flow control and data lines in the direction of
    the PC to the DAC to do this, but you wouldn't end up
    with a very nice system.

    You'd have to pick three state controllable RS232 lines
    that are outputs from your PC or control device
    e.g. RTS, TXD, DTR and use them through a level
    converter to SCLK, CS, DIN, and then you'd have to
    program the RS232 lines to toggle up and down a couple
    of times for every *BIT* of data you want to send to the
    DAC. You would NOT be able to just send a string of
    serial data in the form of serial characters over the
    RS232, you'd have to sit there and control the bits on
    and off one by one in explicit software.

    You might as well be using a parallel port to interface the
    thing; it'd be easier than doing it with RS232 in the
    way I've described.

    The nicest solution to interfacing the thing to RS232
    (or for that matter a parallel printer port) would be
    to use:
    a) your SPI DAC
    b) RS232 level converters (line receiver RS232 to TTL)
    c) a small microcontroller that has a UART port and
    a SPI port and which is programmed to receive
    and data from the RS232 port over its UART and then send
    16 bits of data corresponding to your DAC values out to
    the DAC over its SPI port. That way you get an efficient
    use of RS232 serial transmissions the way they're intended
    to be used, and you could have the whole thing run more
    smoothly, quickly, and easily in terms of software
    complexity. Actually if you look there are some
    microcontrollers with UARTs and DACs built in to them
    so you might only need the microcontroller and
    a RS232 level converting line receiver chip.
     
  6. Jul 24, 2007 #5
    First of all, thanks for the quick responses...


    I need to control a 0-5V output via C# code [or any other language for that matter].

    I thought on a 8bit-serial-DAC that will be connected to a serial port [RS232].
    In the code I'll put a 0-100% scroll bar that will control the output.

    8bit = 256 stpes. that's aprox. 2 steps per 1% in the scroll bar, meaning - aprox. 0.02V per 1%.

    any other ways to accomplish the mission will be superb.

    I don't really know much on DAC's :confused:, so any help will be useful.


    Liad.
     
  7. Jul 24, 2007 #6

    berkeman

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    Staff: Mentor

    Ahh, DAC output out of a PC. Got it.

    You can buy relatively inexpensive serial or USB interface widgets to do ADC and DAC for your PC. Here's an example from B&B Electronics:

    http://www.bb-elec.com/product.asp?sku=UD128A8D&TrailType=Sub (price is $270 USD)

    National Instruments also makes these kind of modules.
     
  8. Jul 24, 2007 #7
    And what about building this kind of circuit?
    what shall i need then?
     
  9. Jul 24, 2007 #8

    berkeman

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    Staff: Mentor

    Well, the challenge will be to convert the RS-232 data format to something that you can feed into a serial DAC chip. Here are a couple links with related information, in these cases it is for reading an ADC over the PC's RS-232 serial port:

    Serial A/D example tutorial: http://www.techlib.com/electronics/serialport.htm

    Serial A/D example circuit from MAXIM: http://www.maxim-ic.com/appnotes.cfm/appnote_number/827

    The MAX7547 looks like it might work as the serial DAC, but I'm not sure what-all would be needed for glue logic between the serial data from the RS-232 port and the serial input control lines for the DAC.

    A better way to go might be to use USB, and to get a cheap Basic Stamp or PIC eval board that has a USB interface on it and some prototype area.

    Or keep looking for cheaper pre-made RS-232 DAC widgets. Here's one from StateLogic for $120 USD:

    http://www.statelogic.com/
     
  10. Jul 24, 2007 #9
    I'll start looking for a possibility to build a USB based circuit.
    I'll look for a scheme of a pc-input circuit to analog...

    or...I'll try to make the: suitable for my needs...making it d/a from the serial port...

    anyone with any idea is more then welcome...
    :)
     
    Last edited: Jul 24, 2007
  11. Jul 25, 2007 #10
  12. Jul 25, 2007 #11

    xez

    User Avatar

    Serial as in SPI as in SERIAL PERIPHERAL INTERFACE
    DACs are great, if you have an SPI port. SPI just isn't
    the same serial protocol as what comes out of an
    RS232 port, that's all.

    You didn't mention what frequency you're trying to
    update this at; I don't know if the manually controlled
    scroll bar thing is your REAL application, or if you'll be
    using the DAC for more automated programmable type
    uses.

    If you need static slowly changing DC values, you should
    use a real DAC.

    If in the end you're trying to generate an AC waveform
    at a moderately high frequency and NOT EVER DC, you
    could just program a PC sound card to output the
    analog amplitudes you want. But they have a built in
    200 Hertz or so high pass filter, so you could only
    generate waveforms between 200Hz and 22000 Hz
    that way. But they have '16 bit' DACs, at least two
    of them, in fact. Though really their quality may be
    closer to 12 bits, depending on the card you get. :)
    Actually if you wanted to 'hack up' a sould card you
    MIGHT be able to find a really basic / cheap one and
    take off the output coupling capacitor and get DC values
    out of it, but that's something you'd have to experiment
    with. I'd check REALTEK's data sheets to see if it's
    possible with their common chips and then maybe look
    for something with that chip.

    If you want DC output you could do one of two sorts of
    'clever hacks' to get DAC like function out of a PC with
    very little added hardware:

    a) PWM output -- if you have an integrator circuit,
    e.g. a resistor and capacitor to filter the output and
    eliminate most of the fluctuations / ripple of doing this,
    you can use a single software controlled digital bit
    as a DAC. For 8 bit resolution, generate a string of 256
    bits. Take your desired 8 bit DAC output value and
    set that many bits in your string of 256 bits to '1'.
    Clear the rest of the bits in your string to '0.
    At a constant frequency, successively output each bit in
    the string to your digital output port bit (you could use
    one of the serial flow control bits or parallel port bits).
    The resistor/capacitor filter will generate an 'DC' output
    equal to the average voltage in the waveform which
    will of course be N / 256th of the peak voltage since
    N of the 256 bits are set.

    b) R-2R DAC: configure the parallel printer port in
    basic PS/2 mode. You'll be able to output 8 data bits
    to the port's data bits in software with the right
    software. At the LSB connect resistor value 256R.
    At the next more significant bit connect a resistor
    of value 128R. etc. etc. so you end up with 8 resistors:
    256R, 128R, 64R, 32R, 16R, 8R, 4R, 2R, 1R. LSB to MSB.
    Tie the opposite ends of each of those resistors together.
    Use 1% resistors with the smallest one being 2K or so.
    Actually if you want best 8 bit accuracy pick your resistors
    so they're actually around 0.5%. The current you get
    out of the resistor set will be equal to your DAC output;
    scale the current with a transimpedance op-amp if you
    want to convert that to 0V to 5V easily.

    Otherwise just hook up a SPI DAC like the one
    you mentioned to the PC parallel port if you're ONLY
    controlling it from your slider application. If you need
    fast response, use a microcontroller connected to the
    PC parallel port.


     
  13. Jul 25, 2007 #12
    I can't use a parallel port because that in the end, the system should have more than 1 circuit.
    The same apply for using sound card.

    about the other possibilities, can you explain more?
    any piece of information will be useful. :rofl:

    How to connect such DAC, and how to calculate what should the program output?
     
  14. Jul 25, 2007 #13
    you need to look at the datasheet. try to understand it.

    sclk = clock.
    cs = chip select
    din = data input digital
    clr = clear like a reset

    if you see the timing waveform in the max515, in order to operate you need to toggle the cs low, sclk low high low high ..., etc

    when you have many chips in the circuit, you need to know which is in control. that is why you have the chip select. also the chip cannot operate unless it has a clock. the faster your clock, the faster your output DAC. some DAC has Vref input, some the Vref is the Vdd itself. That measn if it is 8 bit DAC, 255 = Max Vref/VddV, 0 = 0V.

    To me you'll be better off getting a microcontroller, and program it through RS232 (depending how much time you have). Mic. have DACs, A/D etc all built and relatively cheap like $5. Some of the circuit shown in the datasheet (SPI Port to Max 515) they use MISO, MOSI which is Master IN Slave OUT or Master out Slave IN. These are SPI interface to talk to devices.

    So if i have 3 devices, i can talk to all three devices getting data in and out using MISO and MOSI. The trick to selecting which device to talk one at a time is to use the chip select

    Hope this helps
     
  15. Jul 25, 2007 #14

    xez

    User Avatar

    You could order one of the Tini2131_DKit products below,
    and then you'll have a serial interface and a
    microcontroller you can program to take serial data and
    send it to the SPI port. Then you could wire the SPI
    DACs from MAXIM to the SPI interface on the
    microcontroller and use some of the spare GPIO bits
    as chip selects so you could have a few additional DACs
    connected.

    I guess you could use the I2C port to hook up even more
    I2C DACS since they're addressable more easily.

    Just depends on the quantity of DACs you need etc.

    The drawback is that it'd involve some moderately
    complex microcontroller programming, but the
    advantage would be you would not need to do too much
    electrical wiring to get it all working, and the price isn't
    a bad compromise between completely doing it yourself
    and making PC boards, etc., versus the high(er) typical
    cost of "off the shelf ready to use" RS-232 DAC products.

    Actually you could use a single PC parallel port to hook up
    several SPI DAC chips, you'd need two output bits for
    SCLK and DIN, shared by all the DACs.
    Also you'd need one CS chip select for EACH DAC chip,
    so you could actually interface at least seven SPI DACs
    to one parallel port directly. But then you'd have to
    do some fairly complex PC programming at the hardware
    port I/O bit control level to send out the data; it's not at
    all hard, or a lot of code, it's just hard if you're not a low
    level programmer that's used to doing bit manipulation
    logic I/O coding....



    http://www.newmicros.com/
    http://www.newmicros.com/cgi-bin/store/order.cgi?form=prod_detail&part=Tini2131_DKit
    http://www.newmicros.com/cgi-bin/store/order.cgi?form=prod_detail&part=Tini2131
     
  16. Jul 26, 2007 #15
    Hey again!


    thanks for all the comments...:)

    and back to work...what about those three:

    AT89C52
    DAC0800
    MAX232

    can i manage to output my 0-5V with them?
    and...how? how to connect them?

    never-giving-up-Liad. :grumpy:
     
  17. Jul 27, 2007 #16

    xez

    User Avatar

    ATMEL says the AT89C52 is not recommended for new
    designs, and suggests the AT89S52 instead.

    The AT89S52 doesn't look like it has both a SPI
    port (I didn't see one at all) and a UART, so it doesn't
    seem suitable.

    Anyway what's wrong with the development kit I
    mentioned before? It is all wired for you except for
    adding in a few DAC chips.

    There are plenty of microcontrollers out there that have
    both SPI and UARTs (though you'd almost always need
    to use a chip like the MAX232 to interface the UART pins
    to a RS232 port).

    Yes, using a microcontroller with both UART and SPI is
    technically by far the cleanest most well performing and
    most straightforward solution.

    But, the problems with just using "any old Microcontroller"
    are:
    a) How are you going to program it? Does it need special
    programming devices / cables / interfaces?

    b) How are you going to write the software? Do they have
    simple to use freely or inexpensively available software
    for an assembler or C compiler or FORTH, BASIC, or
    whatever?

    c) How are you going to build it? If you're just wiring it
    yourself with no printed circuit board, you'd need
    something that's easy to use in a DIP package which
    doesn't require too many connections to get it connected
    for programming, interfacing, and general use. If you
    use a surface mount package you'll need a PCB.
    Where will you get the PCB? Will you design one? Can
    you find a free template on the internet or in a book?
    You'll also need relatively advanced soldering skills, unless
    you get someone to assemble it for you.

    d) How many of these units total are you definitely going
    to build within the next few months? If you're only looking
    for one or a few, it's probably not worth the time and
    engineering effort to do something that will be really
    time consuming and difficult unless you're really committed
    to it as an educational process / hobby. Otherwise,
    financially you'd be better off spending $75 or $200 or
    whatever for a relatively off the shelf solution rather than
    spending dozens of hours trying to build/program a
    few of these.

    e) How many DACs do you really need attached to ONE
    PC? If you need somewhere between one and sixteen
    you could easily just interface them all to a parallel port
    without the complexity of using a microcontroller, though
    the PC side of the software will be more complex, it's
    probably worth it rather than trying to buy/build/program
    microcontrollers that you're not familiar with.
    This is assuming that you don't have any high performance
    fast update / synchronization needs that'd make it hard,
    though within reason one can get relatively simultaneous
    updates pretty easily for several DACs.

    f) Why are you doing this? Hobby? Business? School?
    The 'right' answer to approach this probably depends on
    why you're doing it as well as more detail about what you
    need the thing for technically. You said 0-5V, but that
    doesn't even get into things like output impedance,
    output current capability, connectorization, initialization,
    and other details that are sometimes relevant to choosing
    an approach.

    If you're looking for a uC with SPI *and* a UART, look
    at some of the TI MSP430F2xxx series parts; there are
    parts in (if I recall correctly) a 38 pin or larger surface
    mount package that have multiple USART/SPI interfaces
    so you can use one SPI and one UART mode simultaneously. The programming software is pretty much
    free, and the programming hardware isn't that expensive,
    but you'd really need a custom PC board.

    If you just want to "get it done and over with", look at
    something that's either an off the shelf RS232 or USB
    DAC system, or a simple to use microcontroller board
    with commercially available DAC modules like some of the
    BASIC STAMP stuff or RABBIT's stuff or whatever.

    If you want to do some programming and some hardware,
    just get one of the uC development boards like the one
    I mentioned that you can just solder your DACs onto
    and program the thing.

     
  18. Oct 8, 2007 #17
    easy da

    Velleman has an usb-board with 2 8-bit D/A's.

    You could fake a d/a by using opamps as integrators.
    Using the parallel port (8 bits) you can use one to sink the integrator and one to raise,
    thus 4 da's. Not fast, nor accurate, but that's the most easy way I can think of.
     
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