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Original question:

ADS1292R: Configure ADS1292R for EMG acquisition

ADS1292R: ADS1292 circuit

Harikishore R P
Prodigy 220 points

Part Number: ADS1292R
Other Parts Discussed in Thread: ADS1292,

Hi, I am almost done with my project. Have two questions:

1. When I studied the datasheets of ADS1292 and the evaluation board, I found that in certain circuits (see the figure 1 as an example) two capacitors are connected in parallel with a significant difference in capacitance value. Hence, I designed the schematics (also shown in figure 2) by removing the smaller capacitor as the bigger one allows for some tolerance. Is it the right approach? Or the two parallel capacitors have any purpose? (eliminating the capacitor helps in miniaturization). 

Figure 1:

Figure 1

Figure 2

Figure 2The smaller capacitors are removed.

2. I read (I don't remember the source) that the ADC of ADS1292 has inbuilt digital filters that can eliminate frequencies according to the sampling rate. (For example, If I sample at 1KSPS all frequencies above 500 Hz will be eliminated) Is it okay if I use a 1KSPS for a cut off frequency of 500 Hz since the simple antialiasing filter with a bigger roll-off will allow some frequencies to pass? Currently, I am planning to use 2KSPS for 500 Hz to prevent aliasing. If this is the case, I can sample at 1KSPS. Also, is there a notch filter in the ADC at 50 Hz Fc?

Thank you in advance

  • 0
    TI__Guru 60242 points

    Hi Harikishore,

    I would not recommend removing the lower value capacitors. The smaller capacitors are able to source a small amount of energy much more quickly than the larger capacitor and they are important for ensuring a valid start up routine.

    The delta-sigma modulator does not eliminate frequencies, so much as shifts them higher - out of the sampling band. More detail can be seen in section 8.3.4.3 ADC ΔΣ Modulator of the ADS1292R datasheet. There is not a notch filter at 50Hz as you actually want this noise to be present in the measurement such that it can be cancelled by the RLD. I would not recommend setting such an aggressive cutoff frequency for your anti-aliasing filters as you do not want the measurement to be impacted. (attenuation starts long before the -3dB point) A cutoff frequency around 100KHz would be better. 

  • 0 in reply to Alexander Smith
    Prodigy 220 points

    Dear Alex, thank you for your detailed explanation. If the cut off frequency is set to 100kHz, a sampling rate of 8KSPS will not satisfy the Nyquist Criteria right? The maximum ads can sample is 8KSPS. Correct me if I am wrong please.

    Thank you

  • 0 in reply to Harikishore R P
    TI__Guru 60242 points

    Hi Harikishore,

    Happy to help! 

    I do not entirely understand the question. Nyquist criteria has to do with sampling rate and frequency of the input signal. Where the sampling rate should be at least 2x the input frequency. Anti-aliasing filters are not involved in this and should simply be set to a cutoff frequency that does not affect the input signal - 100kHz should be more than high enough to not effect the input signal.