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ADS124S08: ADS124S08 Analog Input Range

Gokhan Sahin
Prodigy 210 points

Part Number: ADS124S08

Hi everyone,

I'm using ADS124S08 and have a differential analog input in order to measure a load cell  (AIN0 - AIN1)

The reference voltage is 5V, the supply of load cell is 10V since datasheet says that excitation voltage should be between 10 and 20V in its datasheet.

Therefore, unfortunately, I can't measure it as ratiometric.

ADS124S08 measures correctly up to around 12mV but after 12mV remain the output voltage stable, so not changing, as if reached saturation.

Since the output sensitivity of the load cell is 2.0mV/V, must reach the 20mV for 10V supply in full scale. However, ADS124S08 can't measure. 

I have calibrated the gain and offset for not loaded and full-scale load.

Although I can measure the output voltage via measurement instrument correctly, ADS124S08 always measure 12mV. Why?

  • 0
    TI__Guru** 100735 points
    Gokhan,


    There's a lot of people out of the office right now, so I thought I'd jump in and help.

    I think the problem in your measurement is that the input voltage is outside the input range of the PGA. The ADC has a PGA that is built like the front end of an instrumentation amplifier. If you use the PGA in higher gain, the input must be near the mid-supply voltage to operate. Even in lower gains, you will still likely need to be near mid-supply. The PGA input range of the ADS124S08 is described in the datasheet is described in pages 33 to 34.

    If the ADC is running from a single +5V supply, then AIN0 to GND and AIN1 to GND should be near 2.5V. If you are using a load cell of 10V configured as a bridge, the output voltages are likely near 5V. You can measure the outputs of the load cell with a multimeter to check.

    If you are running with a +5V supply for the ADS124S08, you can set the inputs near +2.5V by using +7.5V and -2.5V supplies to drive the load cell. Note that you never want to drive the input voltages beyond the ADC supply rails.


    Joseph Wu
  • 0
    TI__Guru** 113765 points
    Hi Gokhan,

    If you are using the same ground reference for both the load cell supply and the ADS124S08, the input voltage for the ADS124S08 will have a common-mode voltage that is 5V for the load cell (1/2 of the load cell excitation). This will be outside of the measurement range of the ADS124S08 with the PGA enabled.

    I would suggest attempting to power the load cell with 5V. I'm not aware of any load cells that actually require 10 or more Volts to function correctly. The extra voltage may be recommended and will help with increasing the output voltage range, but should not be an actual requirement. If you think of the load cell as a Wheatstone bridge, the change in excitation voltage does not change the operation of the bridge. If you use 5V, then the input voltage will be in the correct range for the PGA at a gain of 128, and the measurement can also be ratiometric if the same 5V supply is used for both the excitation and the reference.

    Best regards,
    Bob B
  • 0 in reply to Bob Benjamin
    Prodigy 210 points

    Thank you, Bob,

    It works correctly after supplied 5V but the range of it decreased.

    Best regards.