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ADS1274: ADS1274 effective input impedance

Idan Ozana
Intellectual 480 points
Part Number: ADS1274

Hi,

In my application I'm using the ADS1274, 24- BIT Analog to Digital Converter. Due to limited PCB space, I'm thinking of giving up the pre amplifiers that drives the input switched capacitor to all four inputs and the reference too. In addition, I want to work with low clk frequency (100kHz), all 4 channels will be active, CLKDIV will set to '1' and with RC input filter (the output rate will be ~39SPS).

In order to do that, I need to have a better understanding about the effective input impedance of etch input and the reference. According to the formula in Figure 69 and with the given parameters, Is it true to say that the effective impedance in my case will be: 14kohm*6.75MHz/Fmod while Fmod=Fclk/40=100kHz/40=2500Hz-> Zeff=14kohm*6.75MHz/2500=37.8Mohm? – Almost no effect as combined with the external input RC filter.

I think I'm wrong about the 6.75MHz in the formula… What is the source of this number? Does this number is somehow related to the 27MHz/ 5.4MHz clock in the datasheet reference? (That not relevant in my case because I'm using a 100kHz clock)

 

Thanks,

Idan

  • 0
    TI__Mastermind 39950 points
    Hello Idan,

    You mentioned Fmod=Fclk/40, so I assume you will operate in Low-Speed mode with CLKDIV set to 1. You are correct, with fclk (pin 27 CLK) running at the minimum input frequency of 100kHz, your input sampling rate (fmod) will be 2500Hz, and Zeff will equal ~38Mohm for the inputs.

    The equation for Zeff shown in Figure 69 depends on Fmod and the input sampling capacitor, shown in Figure 67. Another way to express Zeff ;

    Zeff = 1/(Fmod*Cs), where Cs is the input sampling capacitance.

    The equation in Figure 69 has normalized Zeff to 14kOhm when Fmod is equal to 6.75MHz. In other words, Cs has been expressed as Cs = 1/(Z*F) = 1/(14kOhm*6.75MHz) = 10.6pF. This value for Cs is close to the 9pF shown in Figure 67, but not exact since Figure 67 is a simplified circuit of the input.

    The same approximation has been made for the reference input; Zeff for each channel is equal to 5.2kOhm with Fmod=6.75MHz. For the reference input, the approximate value of the input switched capacitor can be calculated as Cref=1/(Z*F) = 1/(5.2kOhm*6.75MHz) = 28.5pF.

    I hope this helps clear things up!

    Regards,
    Keith N.
    Precision ADC Applications