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ADS124S08: Multiple reference usage

GREG SHIPLEY
Intellectual 290 points
Part Number: ADS124S08
Other Parts Discussed in Thread: ADS1248

I am implementing the ADS124s08 in a design that has two strain gauges running on a 5V 0V reference, these voltages will be connected to the REF in pins of the ADC.  I have a third sensor that requires a constant current excitation.  Can I connect it to the current source pins and reference its response to that reference while referencing the other sensors to the voltage reference of REFIN?

  • 0
    TI__Guru 54236 points

    Hi Greg,

    Sorry I am a bit confused on what you want to do here. To be sure, can you draw me a quick block diagram showing how you would like to connect your sensors, and then I will let you know if that is possible with the ADS124S08?

    In general, you can use the 2x external reference inputs for the bridges and you can use the internal voltage reference for the third sensor, as long as you do not intend to do this simultaneously. In other words, take a measurement on Bridge 1 with the first set of Ext VREF Inputs, switch the input/VREF mutiplexers to take a measurement for Bridge 2 with the second set of Ext VREF Inputs, then switch the input/VREF mutiplexers to take a measurement for Sensor 3 with the internal VREF.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Intellectual 290 points

    Perhaps, and most likely, I am misunderstanding how the current excitation pins work.  I was thinking they were the references for the current driven device.  

  • 0 in reply to GREG SHIPLEY
    TI__Guru 54236 points

    Hi Greg,

    The ADS124S08's IDACs are indeed powered by the internal VREF. But the VREF can also be used as the reference voltage for your measurements even if the IDACs are being used.

    Also, please note that there are no dedicated IDAC pins on the ADS124S08, unlike the ADS1248. On the ADS124S08, the IDACs can be routed to any of the 13 analog input channels.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Intellectual 290 points

    never mind, I was thinking the chip had supplies that could be used for constant current applications.  perhaps that was the ADS1248 I was looking at previous.  

    I understand how the references work on this chip.

    On page 97 of the datasheet there is some do's and don'ts.  

    I will be using a 5V and 0V(GND) excitation for my sensors and the ADC analog power side inputs.

    I will also be using the PGA.

    why is there a 2.5V connection to the negative side of the analog channel input for single ended input with PGA?

    I am going to be configured differential and I think I am ok to connect the negative side of the analog channel to the negative side of my bridge and power the bridge off of 5V and 0V and use the same 5V 0V for ADC analog side power and reference inputs.  

    Just making sure I can do what I think I can.

  • 0 in reply to GREG SHIPLEY
    TI__Guru 54236 points

    Hi Greg,

    The ADS124S08 does have 2x constant current sources (IDACs) just like the ADS1248. My point was that the ADS1248 had 2x pins whose only function was for IDAC output, whereas the ADS124S08 IDACs can be routed to any analog input channel. But if you want constant current biasing, for RTDs for example, then the ADS124S08 has this functionality

    The reason the 2.5V is connected to INN when you are using a unipolar 5V supply with the PGA enabled is due input signal limitations of the PGA. If you wanted to measure a unipolar signal range (0-5V), you cannot tie INN to GND in this case because the PGA requires ~150mV headroom on the supplies in order to operate properly. If you connect INN to 2.5V, this is called a pseudo differential measurement, and you are measuring your unipolar input signal (0-5V) against this voltage, instead of GND. This results in a differential input range of +/-2.5V (though you would still need to stay away from the supply rails, so your usable input range is more like +/-2.35V)

    In your case with a differential resistive bridge measurement, the signals you apply to both ADC input pins will always be ~2.5V given a 5V bridge excitation, so this should be fine.

    -Bryan