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ADS1248 Single Ended inputs

Gaetano83
Prodigy 20 points
Other Parts Discussed in Thread: ADS1248

Hello Support,

I have some questions regarding the ADS1248 in order to double check my design before I get something built.

I have to measure 6 single-ended inputs (referenced to GND) having an amplitude ranging from 0V (GND) to 2V. Each of the 6 single-ended inputs is connected to a AINx pin (x=0-5) and AIN6 is connected to GND for reference.

In my setup: I have DVDD = 3.3V and AVDD = 3.3V. I have VREFOUT and VREFCOM connected to a 47uF ceramic capacitor and VREFCOM to GND; I use internal Vref (=2.048V). I would like to set PGA=1 in my application.

I have tied DGND,CLK and AVSS to GND. 

I would like to know if this setup (connections and PGA settings) is OK and what I can do in order to optimize it.

If I want to add a thermocouple input in my design, do I need an additional ADS124x component? Or can I connect it to AIN7 (using suitable conditioning circuits)?

Thank you in advance.

Kind Regards

Gaetano

  • 0
    TI__Guru* 97945 points
    Gaetano,


    Thanks for the post. First, you cannot tie one of the analog inputs to GND. With the input buffering, the GND voltage is just outside the analog input range of the device and needs to be 50mV higher (when the supplies are 0V to 3.3V). It's listed in the datasheet as input common mode range. To get around that, you could generate a bipolar supply and then reference the input to GND, or you could reference the input to REFOUT.

    For thermocouple measurements, many people will take large resistors (say 1Meg) and voltage divide it to get a mid point to attach the thermocouple. This gives it some dc common mode input. However, you can just use the VBIAS option to get a voltage for biasing the thermocouple - that's what it's there for. Note that this voltage was designed for biasing thermocouples and that it shouldn't be loaded to drive any other circuitry.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 20 points

    Dear Joseph,

    Thank you for your answer.

    As I can't generate a bipolar supply in my application and I am using only internal Vref, my only option for referencing the single-ended inputs is to reference them to VREFOUT (pin 9 of ADS128 - 2.048V), according to your reply.

    In order to do this, I have to externally connect one of the analog input pins AINx to VREFOUT (pin 9), because there is no way to internally connect one of Analog Inputs of the PGA (Figure 51, pag 24 of the Datasheet) through registers settings (as I read from the datasheet and from this forum post http://e2e.ti.com/support/data_converters/precision_data_converters/f/73/p/370520/1303163#1303163 ). Then I will measure the difference of VREFOUT with each of my 6 inputs. Is it correct?

    If so, only one analog input will be available for thermocouple measurements, because I am using 6 single ended inputs and 1 for reference.

  • 0 in reply to Gaetano83
    TI__Guru* 97945 points
    Gaetano,


    You've describe the method correctly. Since GND is not a valid input, you need a voltage that is within the common-mode range that is a stable reference point. There isn't anything internal to the device that can be used as a substitute.

    Note that the VBIAS has often been mentioned as an input, but it is not suitable as a reference point because of the noise. It's only usage is as bias point for unbiased thermocouples, where the noise becomes common-mode noise.


    Joseph Wu