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ADS1251 Data Offset

Roy Hein1

Other Parts Discussed in Thread: ADS1251

Using an ADS1251 A-D Converter in the application that measures pressure. I use an analog filter amplifier to buffer and amplifier the sensor voltages but I am losing the four or five least significant bits in the converter output. I studied the data sheet several times and cannot figure this out. The input to converter is correct and the Vref level is correct. I need a better explanation of the input configuration on the chip as well as a better detailed description of the effects that the Vref voltage has on these inputs. This is very confusing to me and ANY help would be appreciated since we are redisgning this portion of our application. Thank you for all your help.

  • 0
    TI__Guru** 113765 points

    Hi Roy,

    Welcome to the forum!  I want to be clear on what you are asking.  The topic is offset, but you are talking about losing LSBs.  Even though this is a 24-bit converter, you will not see the effective resolution come close to that.  19-bits is the best this converter can do with respect to noise.  If your reference is noisy then it will be even worse.  If the amplifier gains any noise, that will also follow through.  If you do not have an anti-aliasing filter, some higher frequency noise (from the op amps for example) it can also alias back through.

    The input is a switched capacitor sampling the input at the modulator rate.  The op amp needs to be able to deliver charge appropriately.  This is usually done by having a capacitor across the ADC inputs.  However, most op amps do not like a capacitive load and can start to oscillate.  Usually you need to add a small series resistance which will also help with anti-aliasing.  There can be some kick back current back through the inputs as well, so op amp stability is really important.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin

    Thank You for your answer but everything you identified in your reply has been examined and dressed but we still get an off set. I am attempting to examine another application using the same configuration and see what differences I can find. It may result in a new board layout rather than a component change. It is not a crucial item at this time since out design is already more accurate than any other design being used in our field.  We still want to improve it if possible.

    Thanks again for the response.

    Roy

  • 0 in reply to Roy Hein1
    TI__Guru** 113765 points

    Hi Roy,

    Maximum device offset is +/-100ppm of full-scale with the typical at +/-30ppm.  This is factory verified.  If you short the inputs of the ADC at the pins and measure the results for this case you will see the actual offset of the ADS1251 itself.  This offset could be subtracted (or added depending on the direction of offset) to the result to correct for the error.  I suspect that you have some additional error on your input stage.  I haven't seen your schematic so it is hard for me to calculate possible error.  Are you using a precision meter (like an Agilent 3458A) to measure the voltage at the input pins.  These are really small voltages you are trying to measure and without a calibrated precision voltage source it is hard to quantify the error.

    Best regards,

    Bob B