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LTC1298 12-bit A/D converter DIP — Parallax Forums

LTC1298 12-bit A/D converter DIP

yzcheyzche Posts: 35
edited 2004-08-16 22:00 in BASIC Stamp
I use converters on a prototying board and a bread board. Converters show different result on different board.·I attached pictures of them on the bread board and prototying board.

on prototyping board with wire wrap socket:
channel 1: 253
channel 0: 266
channel 1: 260
channel 0: 279
channel 1: 254
channel 0: 276
(I need to measure voltage of battery and I need·to connect wire to pin of converter first. After I connect wire to the pin,without battery, voltage went up)
channel 1: 1808
channel 0: 1200
channel 1: 1100
channel 0: 1032

on bread board:
channel 1: 100
channel 0: 120
channel 1: 107
channel 0: 125
channel 1: 109
channel 0: 138
(after touching wire, nothing happen)
channel 0: 120
channel 1: 107
channel 0: 125
channel 1: 109
channel 0: 138

Anyone knows why. shakehead.gif· Anyway to fix it?
bread.jpg
352 x 288 - 24K
solder.jpg
352 x 288 - 27K

Comments

  • DonDon Posts: 34
    edited 2004-08-15 20:55
    The analog input impedance of this A/D converter is quite high as is typical of such devices.· If you connect a wire to one of the inputs with nothing connected to the wire it functions quite well as an antenna.· It will exhibit small voltage fluctuations due to the electromagnetic radiation in its vicinity, e.g. radio waves, EMI from motors and other devices, etc.

    One way to reduce the voltage fluctuations is to lower the effective input impedance.· You can do this by connecting a resistor from the input to ground - the smaller the better for noise reduction but if you make it too small, it will load the circuit that you're trying to sample.· The value that you choose will depend on what's driving it.· If you're sampling a battery voltage, you'll want to choose the resistor so that the current it draws from the battery is small enough to be tolerable.

    Another way to reduce the noise on the A/D input is to place a capacitor between the input and ground.· A capacitor looks like a low resistance to higher frequency AC signals but is an open circuit (in steady state) to DC signals.· The drawback to using a capacitor is that it reduces the ability to track a rapidly changing signal.· If you're sampling a battery, that is probably not a problem at all as the battery voltage will not likely be changing very fast.· You can imagine, though, that with some other signals, you would not want to add very much capacitance to the A/D input.

    As a starting point, you might want to try a 1uF capacitor and see how it affects your results.· Then, change it to a higher or lower value and check again.



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    Don Kinzer

    ·
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-08-15 22:51
    Whenever I test an A/D Converter, I always connected the inputs to 50K or 100K POTS...That way I can test the full range of input values,·and the circuit is pretty much immune to the above mentioned problem.· Just an idea.

    Simply connect one end of the POT to Ground, the other end to VCC (if that's your voltage input range) and the wiper lead to the input of the A/D converter.· If you have any questions I will get you a schematic.


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    Chris Savage

    Knight Designs
    324 West Main Street
    P.O. Box 97
    Montour Falls, NY 14865
    (607) 535-6777

    Business Page:·· http://www.knightdesigns.com
    Personal Page:··· http://www.lightlink.com/dream/chris
    ·
  • yzcheyzche Posts: 35
    edited 2004-08-16 02:05
    Is this what you mean?
    schematic.JPG
    440 x 259 - 11K
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-08-16 03:29
    yzche said...
    Is this what you mean?

    If you mean my comment about the POT, no, I meant like the attached schematic...




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    Chris Savage

    Knight Designs
    324 West Main Street
    P.O. Box 97
    Montour Falls, NY 14865
    (607) 535-6777

    Business Page:·· http://www.knightdesigns.com
    Personal Page:··· http://www.lightlink.com/dream/chris
    bmp
    bmp
    schematic.bmp
    9K
  • yzcheyzche Posts: 35
    edited 2004-08-16 17:17
    I use 680 ohms resister and interference was fix(see attachment). If I remember correctly,· now I can not·use converter's pins because pins·are connected to ground and battery's power can not be connect to ground. Correct me if I am wrong. If am I right, is there any way to fix interference ans still be able to use 2 pins.
    chemactic.GIF
    344 x 363 - 4K
  • DonDon Posts: 34
    edited 2004-08-16 22:00
    The converter's pins are connected to ground via the 680 ohm resistor. If the box at the top of your image is the battery you're trying to measure, note that the 680 ohm load will draw about 7.5ma from the battery. If this is too much, use a bigger resistor. With a 68K resistor, the current draw wil only be 75 uA.

    There is no free lunch. Low current draw requires high input impedance which results in higher noise susceptibility.

    Another option, of course is to add a shielding around your circuit. Search for "Faraday Cage".

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    Don Kinzer

    ·
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