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ADS1258

Alex Brims
Prodigy 10 points

Other Parts Discussed in Thread: ADS1258

ADS 1258

The question is: In table 8 (Settling time) what is time delay and what is time constant ratio (t/tau).

  • 0
    TI__Mastermind 43915 points

    Hi Alex,

    The ADS1258 has the option to place a buffer amplifier and filter between the MUXout and ADCin pins. This allows you use a single buffer/filter design to for all of your mux inputs and drastically cut down on the components needed for a design. For this reason, you will need to account for additional settling time for your external filter after the mux switches channels. Table 8 is used to help you identify how much time you will need to wait for your external analog filter to settle to the desired point. For example, if you use a simple buffer in unity gain with a RC filter. You can use the RC parameters to solve for tau and then plug into the t/tau equation. Then, adjust the t (time delay) variable until you get the final settling time that you want to achieve.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Prodigy 20 points

    Hi Tony,

    I have a question which is roughly related to this settling time. In Table 7, there is a time delay specified for the auto scan mode. This delay is related to the channel switching and is intended to let the input settle before any conversion takes place. In such case, given the DLY to be 7 (111b), the sampling on the channel is postponed for 384 uS. Does this delay affect the input impedance of the channel which is discussed in Figure 37?

    If it affects the input impedance of the channel, then page 3 which specifies the input impedance to be 65KΩ (which is driven by Figure 37) shall be fixed.

    Regards.

    Kazem.

  • 0 in reply to Kazem Eghtesad
    TI__Mastermind 44780 points

    Hi Kazem,

    Welcome to the TI E2E Forums!

    The differential input impedance is impedance while the ADS1258 is sampling the input. The switch time delay will postpone this sampling; therefore the input impedance will be larger, but only momentarily until sampling begins again.

    This delay helps in the case where a large filtering capacitance between the MUXOUT pins and ADCIN pins must settle after switching multiplexer channels.

    We still recommend adding buffers to the inputs of the ADS1258 for best performance.

    Best Regards,
    Chris