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ADS7949: Operation at 3MSPS with short data frame

Noah Larky
Prodigy 10 points
Part Number: ADS7949

According to the datasheet (with 10/12 bit info removed) "The ADS79479 is 8-bit 2MSPS analog-to-digital converters (ADC). Device operates at 2MSPS sample rate with a standard 16 clock data frame. In addition, it is possible to operate the the ADS7949 (8-bit) at 3MSPS with a short data frame optimized for the number of clocks sufficient for conversion with no drop in performance."

I am a bit confused how this 3MSPS operation works. Is there a way the ADC must be set up for 3MSPS operation? Can it continuously run at 3MSPS indefinitely? In short, what makes the product a 2MSPS ADC instead of a 3MSPS ADC if there is no drop in performance at 3MSPS?

I'm looking for an 8Bit (can't be higher) differential or Pseudo-Differential ADC with Serial SPI output and a high data rate (ideally >2MSPS) that is still low cost, hence this question.

Thanks!

  • 0
    TI__Mastermind 40970 points
    Hi Noah,

    Welcome to TI's E2E Community!

    The ADS794x family requires an acquisition time of 80nS. The conversion time depends on the number of bits. For the ADS7949, the conversion time is 8.5 SCLK periods. When operating SCLK at the 34MHz recommended rate, 8.5 clock cycles is equal to 1/34MHz*8.5=250nS. The total sample time is equal to the acquisition plus conversion, or 80nS+250nS=330nS. You can take /CS high after the 9th rising edge of SCLK and then take it back low 80nS later to achieve a total sample time of 330nS (3Msps). This is described in section '16-Clock Frame' of the datasheet. You do not need to provide 16 clocks in this case, just 9 rising SCLK edges.

    In the above mode of operation, in order to achieve 3Msps, you will only have the minimum 80nS for the acquisition time. This will place a higher demand on the input driving amplifier. If you were just good enough at 2Msps, going to 3Msps could show a degradation in performance, but this would be due to the external amplifier and not the ADC. The 'Application Information' section of the datasheet discusses the input driver in more detail.

    Also, please note that the ADS794x family does have a differential input, but the negative input (AINxN) is limited to within +/-0.2V relative to the GND pin.

    Regards,
    Keith N.
    Precision ADC Applications