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ADS1120: Can ADS1120 measure negative differential input sigal

Miles Li1
Intellectual 1640 points
Part Number: ADS1120

Team,

Customer is using ADS1120 in the battery pack, and they need to use ADS1120 to measure bi-directional current.  Now they are wondering whether ADS1120 could measure negative differential input signal? Customer  have found that negative differential input signal can be measured while AVSS is connected to GND. Datasheet does not describe it.

If it can, please tell us how it measure negative differential signal and how it convert this into ADC? Could you help to give the formula of the negative differential convertion?

Wish to get your reply, thanks!

  • 0
    TI__Guru** 112905 points

    Hi Miles,

    For the ADS1120 the measurement will always be differential, that is AINP with respect to AINN.  If AINP is greater in voltage to AINN, then the result will be positive code values (0h through 7FFFFFh).  If AINN is greater than AINP, then the result will be negative output codes (FFFFFFh to 800000h).  If AINN is tied to AGND and the supply is unipolar where AVSS is also tied to AGND, you would normally only see 1/2 of the full-scale range and positive codes only.  There are some exceptions to this as there could be a negative offset within the ADS1120 which could produce a negative code when AINP=AINN=AGND and PGA disabled and bypassed.

    The absolute input range in the Absolute Maximum Ratings table limit the voltage to a range of AVSS-0.3V to AVDD+0.3V.  This is the input range that limits the device from damage (ESD diode conduction takes place beyond those limits).  If the PGA is disabled, the input range for a valid measurement result will be AVSS-0.1V to AVDD+0.1V.  This means that there are some allowable conditions where the input could measure slightly below AGND in a system where AVSS is also connected to AGND.

    If bidirectional current measurement is required, then the easiest method would be to use bipolar supplies (for example +/-2.5V) where the PGA can also be enabled.  Another possible approach would be to offset the measurement in some way where the absolute input voltage is within the allowable measurement range of the ADS1120.

    Best regards,

    Bob B