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Deriving ADS1230 Reference

Rabbit
Prodigy 50 points
Other Parts Discussed in Thread: ADS1230, ADS1248

Please see attached schematic of the analogue I/Ps to the ADS1230.  The operation of the circuit is as follows: 

A constant current source provides current through R4 and a PRT connected at P2.  The reference for the ADS1230 is developed across R4.  The Analogue signal to be measured comes from the PRT.  AINN is connected to Vss.  The gain of the ADS1230 is 64.  AINP and REFN are effectively at the same potential relative to 0v.  According to the ADS1230 datasheet the reference voltage is purely determined by the voltage difference from REFP and REFN with no respect to 0v, is this correct?

This is circuit is not currently giving the correct output from the ADS1230, the output from the ADC is around 30 times smaller than expected.  Is this something to with the configuration of the analogue section.  If not, do you have any suggestions of what the problem may be?  Note, for clarity I have only shown the analogue connections to the ADC.

Many Thanks.

 

  • 0
    TI__Mastermind 24345 points

    Hi Rabbit,

    welcome to our forum.

    You are correct about the reference inputs. The reference voltage is purely determined by the voltage difference between REFP and REFN with no respect to 0V. However you should make sure that the differential voltage is larger than 1.5V as specified in the datasheet.

    However the major problem I see with your implementation is that the signal across the PRT does not meet the common-mode voltage requirement of the PGA. You will have to shift it to ~mid supply. You could do this for example by making R3 equal to R4. You also should make sure that your current source has still enough headroom to operate.
    Alternatively you could generate that reference for the ADC by taking the voltage across R3, if you make it none-zero.

    You may also want to consider the ADS1248, which already offers integrated current sources.

    Regards,

  • 0 in reply to Joachim Wuerker
    Prodigy 50 points

    Hi Joachim,

    Many thanks for your help and very speedy response.

    I have adjusted my circuit accordingly and it now appears to be operating correctly.

    I had overlooked the analogue common-mode criteria on this occasion.

    Regards,