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adc reading errors

Discussion in 'Electronic Design' started by [email protected], Nov 24, 2007.

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  1. Guest

    Dear all,

    I have problems on reading the ADS8326 16bit ADC, 250ksps output. The
    uP reads the ADC Dout continuously at the moment, and each time the
    reading is different although the input is a constant DC voltage.
    There is a Instrument amp at the front end of the ADC input, and I put
    some of the decouple caps in,e.g., the In+ , Vref, Vcc, etc. The Vref
    is connected to the Opamp output.

    I am not sure if it the the ground bouncing noise cause problem or
    not. All components are on the breadboard, but they have digital
    ground and analog gound seperately.

    Can someone give me some ideas?

    Many Thanks
  2. John Larkin

    John Larkin Guest

    How much variation are you seeing? In real life, a fast 16 bit ADC
    will never deliver constant codes.

    What kind of amp are you using, and what's its gain?

    Lowpass filtering between the amp and the adc will help, if you can
    stand some reduction in signal bandwidth.

    What do you mean by "The Vref is connected to the Opamp output." ?

    Is this one of those plastic breadboards? They're bad for this sort of
    thing. And separating analog and digital grounds is usually bad, too.

  3. Guest

    Hello! John,
    Thanks for your reply.

    The amp is the instrument amplifier INA128. And I just put the +1.6V
    signal to its in+, and the in- connected to the analog V-. The Ref pin
    of INA128 is connected to the +2.5V after a Opamp. The Vout of the
    INA128 goes to +4.1V, which I measured via a multimeter to confirm it
    too. Gain=1. I did put 10Ohm and 1nF low pass filter for the ADC
    input signal.

    Lots of variations I see, not just 1-3 LSB variations, even the MSB
    too. not sure if the uP read the code correctly or because of the
    circuit noise etc.

    What do you mean by "The Vref is connected to the Opamp output." ?
    --- The Vref of the ADC need a low impedance.

    They are plastic breadboards. What should I do? Make a PCB instead of
    working on the breadboards?

    Would the noise make such big difference on every bits?

  4. John Larkin

    John Larkin Guest

    B-B claims 3 lsb's p-p noise, which is pretty impressive. The INA128
    will add seome of its own, but nothing radical. If you're seeing codes
    all over the place, something is bad wrong.

    Try grounding the ADC inputs and see what you get. You may have a
    clocking problem or something. How are you getting the codes out of
    the adc?

    I usually lay out a board to do anything complex, but this case is
    simple enough that you could still do it by hand. I like to get a
    sheet of bare copperclad FR4 and build breadboards directly on the
    (single!) ground plane, with axial passives, wire, and little
    commercial adapter boards to break out surface-mount parts. That can
    do very low noise stuff.

  5. You could be reading the serial interface incorrectly, for example
    reading each bit on the wrong edge of the clock so that the data is
    changing at that instant. SPI interfaces have a "clock polarity" and a
    "clock phase" setting; both these must match the ADC.

    Look at the serial output on a scope and see if you still see the MSB
    transitions then. I bet you don't. But if you do, remove the amplifier
    and try shorting the ADC inputs to ground. Check you have the correct
    supplies and reference voltage (you could connect the reference to the
    positive supply if unsure).
    I would use the "dead bug" prototyping method on plain copper clad PCB
    material. (Or make a real PCB). You may also be able to buy an
    evaluation board for the chip from the manufacturer, use it as a
    starting point.
  6. Guest

    Thanks, John.
    I tried a ground plane on a sheet metal, which is actually the base of
    one of the breadboard I have. Now what uP got from ADC were almost all
    zeros, as I had before.
    I got all the ADC input zero when I use the C compiler build in SPI
    function, and I thought it might be something wrong on the code, I
    then changed the code to something like the following, not using any
    built-in function. Afterwards I was able to read in different wrong
    ADC input every time. Then I seperate the analog and digital ground,
    not much help. Now I added in a metal plane as a ground plane, and the
    readings are almost all zero.

    Each instruction takes about 0.25usec, i.e., the ADC sampling rate is
    about 4M, below its max. 6Mhz.

    output_low(ADC_CS); //Enable ADC
    for (i=0; i<6; i++){ //ADC conversion time, for 5 clock +
    for (i=0; i<16; i++){ // read in ADC data

    output_high(ADC_CS); //Disable ADC
  7. John Larkin

    John Larkin Guest

    OK, but what's the electrical setup? Is this a PC or a pic or what?
    What sort of electrical port are you using, and how is it physically
    wired to the adc? A little clock ringing or such could be bad. Have
    you scoped the digital lines?

  8. Guest

    Hi! John,
    It is a PIC18F452, and the PIN_C3 and C4 (SPI/SCLK/Din) are used for
    sclk and data input. PIN-B7 is used for CS pin. Flying leads are used
    to physically connect them.
    I haven't got a scope on hand now.
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