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ADS130E08: Single-ended inputs and expected conversion codes

Tim Zibrat
Prodigy 30 points
Part Number: ADS130E08

Another engineer and I are in disagreement on how this device works when configured for single-ended inputs.

I have this part with the following configuration:

AVDD = 5V

AVSS = 0V (ground)

DVDD = 3.3V

DGND = 0V (ground)

VREFP = 4.096V

VREFN = 0V (ground)

All INxN inputs are tied to a precision reference of 2.048V to create a common mode voltage

All INxP inputs are single-ended, 0V to 4V signals

So, according to the datasheet, this is a valid configuration and I should be able to measure CM- 1/2 VREF to CM+ 1/2 VREF which would equal:

2.048V - (4.096 / 2) = 0V to 2.048V + ( 4.096V / 2) = 4.096V

The only issue is that I am only generating 1/2 of the ADC conversion values.  In other words, when I input a 0V signal, the ADC reads back 0xC000, I would expect 0x8000.  When I input a 4V signal I am reading back close to 0x4000, I would expect something close to 0x7FFF.  Applying a 2.048V input works fine with an ADC conversion of 0x0.

The "other" engineer says that is the way it is supposed to work, I disagree, I would expect to be able to use the entire range of the ADC.

Who is right?

Thanks,

-tz

  • 0
    TI__Mastermind 22235 points

    Hi Tim,

    thanks a lot for reaching out.
    I hate to disappoint you, but the trophy in this case would go to the “other” engineer :-).

    The ADS130E08 offers a full scale range of +/-VREF (assuming Gain=1).
    That means you could measure voltages on INxP from (CM – VREF) to (CM + VREF), when INxN is connected to CM.
    If you would reduce the reference voltage used for the ADS130E08 to 2.048V, then you would get the result you are expecting.

    Another thing I wanted to point out is the input range limitation of the integrated PGA.
    If you want to stay within the specified input range of the PGA, then your signal on INxP has to stay 200mV above GND.
    If you drive it closer to GND, you may experience increased offset, gain, or INL errors. On this 16-bit device it might not be very noticeable though.

    Regards,
    Joachim Wuerker