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Single Input with ADS1278

Pedro Monje
Prodigy 100 points

Other Parts Discussed in Thread: ADS1278, ADS1274

 

Hello;

 

We have a design in which one ADS1278 (actually they are two working in parallel because we need up to 10 channels with 24-bit resolution, but in this case it does not matter) work with an MCU to acquire data from ten channels. The input of these channels is the output from ten instrumentation amplifiers (because previously the signal levels are very low). That is: in each channel the inputs are the signal (up to 2.5V) and ground. Those were going to be our inputs to the ADC (AINP and AINN respectively), being VREF=2.5V.

 

However, when designing the connections in our HW, we have read (pag. 24 ADS1278 Datasheet) the following (quote):

 

The ADS1274/78 measures each differential input signal VIN = (AINP - AINN) against the common differential reference VREF = (VREFP - VREFN). (…) For optimum performance, the inputs of the ADS1274/78 are intended to be driven differentially. For single-ended applications, one of the inputs (AINP or AINN) can be driven while the other input is fixed (typically to AGND or +2.5V). Fixing the input to 2.5V permits bipolar operation, thereby allowing full use of the entire converter range.

 

We understand that, if instead of setting AINN to ground we set AINN to 2.5V, we will be able to use the entire convert range. Is that right? Can we do that (AINP = Input, AINN = 2.5V)? What could be the best practice?

 

Thanks a lot.

 

Regards,

 

Pedro

  • 0
    TI__Mastermind 23235 points

    Yes, you are correct in your analysis.

    Setting the common mode of the inputs to 2.5V allows the use of the full input range for single-ended inputs since the AINP can swing from 5V to ground, thus achieving a +/-2.5V signal.  If you do this, depending on what your input signal is, you may to level shift the AINP signal.  Make sure that the DC levels match otherwise you may not have a perfectly symmetrical input range.