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ads1231 input impedance

stanc
Prodigy 100 points
Other Parts Discussed in Thread: ADS1231

Does anyone know what the input impedance of the ads1231 analog input lines is? the data sheet does not mention this. I would like to put some filtering and protection on the input line ...

Thanks

  • 0
    TI__Guru** 113765 points

    Hi Stanc,

    Unfortunately the input impedance was not specifically characterized so I can't give you a precise number for the ADS1231.  However the input is high impedance as the input becomes buffered by the input amplifier.  A block diagram and description of the amplifier is shown on page 9 of the datasheet in Figure 13.  As the amplifiers are chopper-stabilized there will be a very small input bias current but is usually not an issue as this current is on the order of a nA or so worst case which still allows for an effective input impedance in the giga ohms.  Generally heavy filtering will have a greater issue with analog settling time.

    What type of filtering, components and cut-off frequencies are you considering?  What are you biggest concerns regarding filtering (general noise, ESD, over-voltage, EMI, RFI, etc.)?  Is this a bridge/load cell application?

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 100 points
    Hi Bob, thanks for the reply.
    The application is a resistance measurement where the transducer is installed at the end of a long cable (more than 7 meter cable). The resistance changes very slowly though.
  • 0 in reply to stanc
    TI__Guru** 113765 points

    Hi Stanc,

    Your basic requirement is an anti-aliasing filter.  This can be accomplished with a simple RC filter.  I would start with input resistors in the range of 2.2k to 4.7k ohm.  The differential cap is most important.  Here you could start with 100nF.  If you do wish to use common-mode caps as well, they should be 1/10th the value or less of the diff cap (diff cap 100nF has a max value of the common-mode cap of 10nF).

    You may wish or need to add additional ESD, EMI/RFI filtering.  With that long of cable you will probably need to use a shielded cable or it will become a pretty good antenna.  Proper termination will be key here as well.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 100 points
    Hi Bob

    Thanks again for the reply.
    The shielding is something I was wondering about. It bumps up the cost a lot. I don't have a good sense of what kind of frequencies and amplitudes can be picked up without shielding and what it would then take to filter them out.


    Is there some rule of thumb for this sort of thing?

  • 0 in reply to stanc
    TI__Guru** 113765 points

    Hi Stanc,

    This is not quite so easy to answer.  Basically you want an anti-aliasing filter which can be first order RC.  I generally start in the range of 1k to 4.7k series input resistance for each input line and 10 to 100nF capacitor across the inputs.  This will care for many issues with higher frequency components.  If you run at 10sps, then the digital filter should notch most of any power-line cycle noise.  However, remember that there is gain of 128, so any noise coming from cabling can dominate the signal being measured if you are not careful.  If your system is prone to large transients you may need to provide some sort of transient suppression.  This may be in the form of a diode, but you will need to make sure it is low leakage.  For high frequencies you may need to add some additional filtering prior to the RC filter using ferrite beads loading at the frequencies that will be most problematic.

    Normally what I try to do with my designs is to put placeholders in the schematic for various filters to include ferrite beads and extra capacitors.  I then start out simple using the minimal RC filters and populating any series signal traces with 0 ohm resistors.  I can then easily add more filtering by replacing the 0 ohm resistors as necessary.  Overall the best method is to keep noise out of the signal path entirely by properly shielding and terminating any cable bringing the sensor input to the ADC.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 100 points

    ok, thanks for the input