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Aliasing problem.

Jose de Jesus Colin Robles
Prodigy 200 points
Other Parts Discussed in Thread: OPA4350, LM741, ADS1251

Hi everybody !!! :)

I have a problem with rebuilding the signal in the computer, the Aliasing, so I need to find where is wrong, please just tell, if I've got a sample rate of 2400 KSP/S, then,  I can acquire a 1KHZ signal whitout the aliasing is it not?-.

the data sheet says, prior the ADC, I have to use an OP AMP circuit to ensure the full scale. instead  the OPA4350 I used the lm741 . is that the problem? to ensure a good acquisition  do i have to use the OPA4350?.

Or the problem is Sofware.

ohh I forgot !! the adc is ads1251, in the oscillocope, I can see the data transmition to the microcontroller each falling egde.  

thanks !!! so much have a good day!!

  • 0
    TI__Mastermind 44640 points

    Hi Jose,

    There is a note in the ADS1251 data sheet:

    ...to prevent aliasing of the input signal, the bandwidth of
    the analog-input signal must be band-limited; the bandwidth
    is a function of the system clock frequency. With a system
    clock frequency of 8MHz, the data output rate is 20.8kHz with
    a –3dB frequency of 4.24kHz. The –3dB frequency scales
    with the system clock frequency.


    When you say:

    Jose de Jesus Colin Robles said:
    I've got a sample rate of 2400 KSP/S

    Do you mean that you are applying a 2.4MHz system clock (CLK) or did you mean 2400 SPS data rate?

    When talking about delta-sigma ADCs, there is a difference between "sampling rate" and "data rate". The input signal gets oversampled and filtered (averaged) to produce the output "data rate". For aliasing purposes, you want to take the output data rate as the "effective sampling rate" to determine if you can correctly reconstruct the input signal.

    Also note the digital filter response shown below: (the x-axis is normalized to the ADC's data rate)

    The ADS1251 uses a SINC5 digital filter; therefore, there is not a very wide pass-band. In fact, the –3dB point of the
    filter is 0.2035 times the data output rate. (So assuming you have a data rate of 2400 SPS, the -3dB point is at 488.4 Hz. You may want to use a faster data rate so that your input signal is not attenuated as much by the SINC5 filter.)

    Best Regards,
    Chris