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ADS1248: Basic questions about Noise Performance

Takumi Suzuki1
Mastermind 8630 points

Part Number: ADS1248

Hi,

I would like to ask you following questions about "8.1 Noise performance". (datasheet p21)

Q1) Input-refferred noise

Will you tell me the meaning of "input-refferred noise", for sure?

My understanding is the value of "Output Noise(input is shorted to GND)/PGA gain".

Is this correct?

Q2) Relation betrween input-reffered noise and PGA gain

Why is the "input-reffered noise" is smaller when PGA gain is bigger?

For example, 1.1uVrms @Gain=1, 5SPS > 0.68uVrms @Gain=2, 5SPS.

Is this due to the relation between "Input signal range"(dynamic range) and PGA gain?

Q3) Table 1, 3, 5 (datasheet p21 - p23)

The values on the table are "measurement value", not specification, correct?

Do you have the data in case Ta = -10℃ and +90℃ ?

Thank you for your support.

Best Regards,

  • 0
    TI__Intellectual 2885 points
    Hello Takumi,

    1. You are correct. That is the formula for calculating input-referred noise.

    2. The reason for this is because ideally when gain = 1 the input noise is equal to output noise, but as you use a larger gain the signal becomes bigger, thus the signal to noise ratio increases with respect to the PGA gain. If noise is 10mV and the signal amplitude is 1V there is 10% noise, but if the signal is 10V (gain of 10 for example) and the noise is still 10mV there will only be 1% of noise.

    3. I'm not sure what kind of data you're looking for but on page 8 in the data sheet it mentions that the Operating ambient temperature (Ta) is -40C < Ta < 125C. there are several graphs that provide data over that entire temperature range.
  • 0
    TI__Guru** 100095 points

    Takumi-san,


    1. The noise performance is measured with the inputs shorted. I would note that generally this the noise is generally smallest when the ADC is measuring a 0V input. The output noise is measured just as you would any input signal. For the ADS1248, this is ADC data * VREF/PGA gain.

    2. The PGA itself is very low noise, so contributes very little noise to the ADC measurement. It's easiest to consider that the process of measuring the input signal with a delta-sigma ADC has a certain amount of noise as a function of the number of ADC bits. With PGA gain, this noise is spread over the same number of bits, so it appears as a smaller voltage.

    3. Noise Tables 1, 3, and 5 in the datasheet on pages 21 - 23 would be considered typical. I don't have any other data for Ta = -10°C and 90°C, but the noise does not change significantly from temperature.

    Let me know if this answers your questions.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Mastermind 8630 points

    Hi Matt-san, Hoseph-san,

    Thank you so much for your kind reply.

    I think I can understand what you mean.

    Probably I misunderstood one point as below.

    I was thinking that the PGA noise is amplified by PGA.

    That is to say, when noise amplitude is difined as "x", then for example

         PGA = 1 ---> noise = x

         PGA = 2 ----> noise = 2*x 

    The input-reffered noise is not changed, still same between PGA=1 and PGA=2 according to my misunderstanding.

    I should have understood the noise is the same amount at all PGA gain settings.

        PGA = 1 and PGA =2 ---> noise = x, so that input-reffered noise is smaller by larger PGA gain.

    Is my understanding correct?

    Sorry for my poor understanding.

    Best Regards,

  • 0 in reply to Takumi Suzuki1
    TI__Guru** 100095 points
    Takumi-san,


    Yes, your understanding is correct. If you look at the noise tables, improvement on noise often improves as a factor of 2 in lower gains (going from 1 to 2 or from 2 to 4). However, there is less improvement at the higher gains (going from 32 to 64 or from 64 to 128). At that point the noise contribution of the PGA is close to the other noise in the ADC and the improvement in noise from gain is less.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Mastermind 8630 points
    Hi Joseph-san,
    Thank you for your reply, I see!
    Best Regards,