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ADS1278 spurious signal

Roberto Signori
Prodigy 130 points
Other Parts Discussed in Thread: ADS1278, TMS320F2812

I am using an ads1278 for signal acquisition. The device is connected to TMS320F2812 dsp for signal analysis. The problem is that I have always two spurious signals on each channel, different channel for channel, A/D converter to A/D converter.

The two spurious signal seems 15 to 20 dB higher than the noise. Here attached a pdf of the spurious frequency. The graphs was done with shorted input, directly on the pin of a/d converter.

2438.psd_Dymas1_2.pdf

two spurious signals seems present also in the data sheet of the component.

If there is someone who can help me...

thanks in advance

Roberto

  • 0
    TI__Mastermind 43915 points

    Hi Roberto, 

    The spurious signals that you are seeing on top of the noise floor are called idle tones. These spikes in the noise floor are commonly seen in delta sigma converters and do not affect the signal to noise ratio of the converter. You can think of the idle tone as noise that is read back into the baseband. Effective digital filtering is used to attempt to remove any unwanted frequencies in the baseband, however, tones may still appear.

    The output of the A/D converter is the result of the ratio between the reference voltage and the input voltage. Therefore, any change or effect on the reference voltage will be reflected on the output word. As the converter is powered on and run, there will be a running master clock and SCLK used to properly run the device. At times some of this clock signal may couple back into the reference voltage and would directly effect the ADC end result. To minimize idle tones, we recommend that you have a well filtered reference voltage and a larger cap (maybe 10uF) at the reference input pin. The closer that the cap is to the reference pin on the layout, the better.

    Regards,

    Tony Calabria  

     

  • 0 in reply to Tony Calabria
    Prodigy 130 points

    Hi Tony, thanks for prompt reply.

    Reference seems well filtered. Schematic of reference circuit is derived from ADS1278EVM board with Ref, OP filter and  10uF X7R capacitor very close to reference pin.

    The frequency of the idle tones seem depend directly to the amount of dc present at each input.  Actually i have add a small dc to input (some millivolts) in order to push the idle tones outside from the band of interest. Is there a relationships between dc value and idle tones frequency?

    Regards

    Roberto  

  • 0 in reply to Roberto Signori
    TI__Mastermind 43915 points

    Roberto, 

    In Delta-Sigma converters, as the input is shorted together, the modulator will read back some value very close to 0V (may be some offset internal to the ADC). The modulator may fall into a repetitive reading that may appear as idle tones on the output. As you adjust the DC input to the modulator, you are able to push the idle tones out of the band of interest. This sounds like what you have done in your experiment. 

    Regards,

    Tony Calabria