ADS124S06: Programmable conversion delay and PGA Startup time

Hi Oba,

Oba said:

1. About programmable conversion delay

My customer just wonder how much delay is appropriate for their system.
I understand it highly depends on the system.
But could you help how to think about it?

The programmable conversion delay only applies to the first conversion when operating in continuous conversion mode (or every conversion in single-shot mode).  The delay is used primarily for analog filter settling when going from a static state to a dynamic state.  Consider that the analog input is static when not selected at the input mux.  When the analog input becomes selected at the input mux, the input changes from a static state (no movement of charge) to a dynamic state (input is sampled) and there can be some analog settling due to filter components at the input.  Depending on the sensor source impedance and the resistor/capacitor values at the input the time required for full settling at the input can vary.  The default setting covers most use cases.  The programmable delay was an added feature so that conversions taken place during the settling time would not need to be collected and discarded by the host processor.

Oba said:

2. PGA Startup time.

The datasheet defines “PGA Start up time :Enabling the PGA in conversion mode” as 190us TYP.
What is this? Is there anything that the user have to think about this item?

Normally this is not a consideration unless you are changing the PGA from disabled to enabled during continuous conversions.  For example, you have set the operating mode to continuous conversions and have either the START pin high or the START command is issued.  This starts the ADC converting on the currently selected mux input channels.  At this point we can use an example that the input channels selected are for a single-ended measurement where the PGA is disabled (powered down).  After several conversion periods you might change channels to a differential input and enable the PGA (power up).  As you have changed selected input channels and changed the PGA configuration there will be a restart of the conversion along with the time selected in the Programmable Conversion Delay register settings.  When using the default setting for the programmable delay the conversion will start approximately 55us after the registers have been written, but the PGA takes about 190us to power up.  This means for this particular use case the conversion result may have error due to the PGA powering up.  The amount of error will greatly depend on the selected data rate.  At slower data rates the error will be much less as the error will tend to average out into the measurement.  At 4ksps, the error can be significant as the power up time for the PGA is very close to the conversion period.

For this particular use case it may be easiest to use the programmable delay of 64*tmod which is greater than the power up timing of the PGA.  So the overall consideration really depends on if the PGA is powered up and down (enabled and disabled) during a measurement cycle and if continuous conversion mode is being used as opposed to single-shot.

Best regards,

Bob B