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ADS124S08: Input range vs VREF

GREG SHIPLEY
Intellectual 270 points
Part Number: ADS124S08

I am using the ADS124S08 for a strain gauge design,  things are going good, I can accurately read a typical 350 ohm strain wheatstone bridge sensor that is powered off of 5VDC with great performance with a VREF of 1.25V and a gain of 64.

Problem I am having is one of my sensor is a MEMS type bridge sensor that wants a constant current source.  I am going the source but it results in a voltage between the Differential + and GND of 5V and a voltage between the Differential - and GND of 5V.  I am not getting much count response from the ADC.  The sensor is varying nearly 6 mV over the range and with a reference of 5V and a gain of 64 I should be getting allot of 23b counts.  I am not I am only getting 100 or so.  Am I missing something here on how this constant current sensor works?

Just making sure I am not violating an input rule of the ADC.

CMJ1500 TR PBFREE Central Semiconductor Corp | Integrated Circuits (ICs) | DigiKey is the link to the constant current diode I am using to provide the 1.5mA constant current supply.

ME501-5_DS_D-Metallux-SA-17062019.pdf   is a link to the sensor I am using.  it is a MEM501-A-0500-0-1-31-1

  • 0
    TI__Guru** 112725 points

    Hi Greg,

    It appears that the way you have your sensor connected is producing an output voltage that is outside of the measurement range of the input to the ADS124S08.  It would be helpful to see a schematic as to how you have connected the various components.

    From the sensor datasheet it would appear that the sensor should be connected to a voltage source and not a current source.  Perhaps there is a different datasheet than the link you provided.  Based on the linked datasheet, if you connect the Vref (IN+) pin to 5V (AVDD) and the  Lo+ (+GND) and the Lo- (-GND) pins to AGND, then the S+ (+OUT) and S- (-OUT) should be centered about 2.5V relative to AGND.  The sensor output sensitivity is 4 to 6mV/V, so the output at full rated pressure should be 20 to 24mV.

    If you actually have a different sensor than the datasheet provided, then I would have to see that specific datasheet to determine how best to connect the sensor to the ADC.  You must make sure that the sensor impedance and the total current used for excitation provides an output voltage within the input range of the ADC.  I'm thinking that the voltage drop created across the sensor is greater than 5V.

    Best regards,

    Bob B