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ADS1147: Can ADS1147 support 4 independent single end input

Sunny Qin
Expert 4890 points
Part Number: ADS1147
Other Parts Discussed in Thread: ADS1148, ADS124S08, ADS1247, ADS1248, ADS114S08

Hi Team,

 

From datasheet I found that positive/negative input of ADS1147 must be IN1~IN4, thus I doubt that this device can`t support 4 independent single end input, because we need to use one of the input to be bias for negative input.

Could you help to confirm it?

Thanks very much.

 

Regards, Sunny

  • 0
    TI__Guru 50921 points

    Hi Sunny,

    You are correct. Thank you for checking! 

    Please see 9.3.2 Low-Noise PGA in the DS for additional information. 

  • 0 in reply to Alexander Smith
    TI__Expert 4890 points

    Hi Alex,

    So do you mean ADS1147 can only support 3 channels single end input, while ADS1148 can support 7 channels?

    And for 3 single end input, we can choose either of 3 of IN0~IN3, then how to deal with the un-used one? float or connect to GND?

    And how about ADS124S08, can this device support 12 single end input?

    Thanks very much.

    Regards, Sunny

  • 0 in reply to Sunny Qin
    TI__Guru** 113765 points

    Hi Sunny,

    The ADS1147 and ADS1148 (and the 24-bit devices ADS1247 and ADS1248) cannot make single-ended measured with respect to ground due to the common-mode input restriction of the PGA when using a unipolar analog supply.  And as you have stated, these devices are restricted to 3 inputs with respect to a common input for the ADS1147 (ADS1247) and 7 inputs with respect to a common input for the ADS1148 (ADS1248).  You need both an + input and a - input for the measurement, so the remaining input for the - would need to connect to some voltage mid-scale.  You cannot connect the input to ground in the unipolar supply case.  Also, the measurement range is limited as VREF also has a limited input range and cannot go to supply limiting the full-scale range.  So these devices do not work well for single-ended measurements.

    The ADS124S08 (and ADS114S08 16-bit version) does allow for 12 single-ended measurements (AIN0-AIN11) with respect to ground by using the AINCOM as the common input to ground and the PGA in bypass mode with a unipolar analog supply.  Also, full-scale range to the analog supply can be achieved.  So these devices work well for single-ended measurements.

    Best regards,

    Bob B

  • 0 in reply to Sunny Qin
    TI__Guru 50921 points

    Hi Sunny,

    All measurements will have to be taken in a differential configuration, so for the ADS1147 you can put a known voltage onto one channel (Mid supply or REFOUT) and then measure the other 3 channels "single-ended" against this voltage. Same thing for the ADS1148, but you would have 7 "single-ended" channels.

    If you are using bipolar supplies the known voltage channel can be connected to GND as it is mid-supply. It should not be left to float.

    Please see 10.1.4 Establishing a Proper Common-Mode Input Voltage of the ADS1147 datasheet. 

    ADS124S08 can support 12 single-ended inputs since it has the additional AINCOM channel for single-ended measurements. 

     

  • 0 in reply to Alexander Smith
    TI__Expert 4890 points

    Hi Alex,

    Thanks a lot.

    Sunny

  • 0 in reply to Bob Benjamin
    TI__Expert 4890 points

    Hi Bob,

    Thanks a lot.

    Regards,Sunny