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ADS1225: Datasheet - Input Buffer On time is unclear

Richard Franke
Prodigy 40 points
Part Number: ADS1225


Goal

Understanding the timing criteria required for using the Input Buffer

Issue

The datasheet has the statement "Note that when START is low, the buffer must be disabled to prevent loading of the inputs." (Page 14)

For a standard conversion, the Start input can be on for as little as 17us (Table 3, page 13)

This implies that the buffer would only be enabled for 17us.

Is this sufficient time for the buffer to do its job? If we assume it is, disabling the buffer when I take the start input low, means that for the rest of the conversion time, the buffer is not involved. This seems incorrect?

  • 0
    TI__Guru** 113765 points

    Hi Richard,

    Here is some background.  The START pin when set high places the ADC into converting mode and when set low places the ADC into a low power state (shutdown).  When not using the buffer, the ADC uses a switched capacitor input sampling method and the impedance is based on the movement of charge to replenish the input sampling caps at the modulator sampling frequency.  This is discussed on page 11 of the ADS1225 datasheet.

    When the buffer is enabled, the input impedance is increased as the buffer is placed between the input and the sampling caps.  There is still some bias current drawn as the buffer is chopper stabilized to reduce offset error.  When the START pin is low and the buffer is enabled, there will be a loading of the input by the buffer circuit as the analog is in shutdown mode.

    Now to your question.  If you want to take a single conversion from the shutdown state you need to pulse the START pin for a minimum of 17us.  The required time is necessary for the internal state machine to determine that a conversion is to take place.  The START pin has no direct relationship to the buffer except when the conversion completes and the device goes back to the shutdown state.  The START pin controls the power and start of the conversion.  The START pin does not turn on/off (enable/disable) the buffer.

    So if you want to enable the buffer you should enable the buffer when the START pin goes high, and disable the buffer when the conversion completes.

    Best regards,

    Bob B