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ADS125H02: Auto-Zero Effective Noise bandwidth

Mark Omo
Prodigy 25 points
Part Number: ADS125H02

How does the Auto-Zero function affect the effective noise bandwidth?  

Section 9.4.2 Auto-Zero Mode describes that the digital filter notches remain in the same locations, but the system has latency equal to the Conversion Latency Time.

If I were using SINC4 at 50SPS, Table 5 shows I would have a bandwidth of 11.4Hz. But in Auto-Zero Mode, I would have a sample rate of 1/(80.43ms) = 12.4Hz. 

Will the noise bandwidth remain unchanged, and will I get noise aliasing from out to 12Hz? If the noise is reduced, I might expect the response to be a convolution of a 2 element boxcar filter response with the SINC filter response. 

The design calculator has a bit for MODE1 labeled CHOP 2 bits wide. Does this refer to the Auto-Zero mode? And some other undocumented mode?, this seems is only used to calculate the Conversiontime in the sheet, not any filter paramaters.

  • 0
    TI__Mastermind 34991 points

    Hi Mark,

    Enabling auto-zero (global chop) mode does not alter the input bandwidth, as this does not change the digital filter frequency response.

    Regarding the design calculator, I believe this is simply an error (a carryover from a similar calculator). The ADS125H02 only has an auto-zero mode, which is a single bit in the MODE1 register.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 25 points

    I am not sure that this makes sense, I captured two different traces of the internal VCOM point and plotted their PSD, and I see distinctly different scaled outputs. 

    It looks like the data rate into the filters is decimated by the Auto-Zero effect resulting in dramatically different frequency responses (same digital filter applied to data 24% of the rate). Perhaps this is obvious and I am just misreading the datasheet. 

    ADC Settings, default other than:

    PGA Gain: 16x

    Sample Rate: 100SPS

    Filter: SINC4

    The AZ-Off line (Orange) matches the SINC4 response from the excel sheet nicely, and the AZ-On trace (Blue) matches the AZ-Off Trace remarkably when scaling the frequency response by 0.2475x and shifting it down 3dB.

  • 0 in reply to Mark Omo
    TI__Mastermind 34991 points

    Hi Mark,

    The global chop feature effectively adds another layer of decimation to the processed data. You have correctly identified that this is approximately a factor of 4, which reduces the sample rate. Referring to the image below, decimation flattens the magnitude of the response and copies it by the decimation factor (this image shows a decimation value of 3, not 4 unfortunately, but the idea is the same). Then, the transform at 1/MT and 2/MT would be discarded, leaving just the response at 0 and 3/MT, which has the same bandwidth as the original signal.

    The Auto-zero mode can reduce the noise by a factor of √2 because the act of averaging two samples is similar to applying a lowpass filter (assuming the noise is Gaussian). But again, this does not change the frequency response or the noise bandwidth since the sample rate is changing at the same time.

    Ultimately, AZ mode does not change the noise bandwidth.

    -Bryan