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ADS1220: ADS1220 duty cycle mode with MUX

richard payne
Genius 4640 points
Part Number: ADS1220

I have two channel pressure bridge sensor and wish to understand more on Duty Cycle. The aim is to capture two readout every 100mSec (10Hz sample rate for both). 

(1) How to MUX two channel in turns while on Duty Cycle Mode.

(2) How duty cycle save power under two channel MUX

(3) Can the pressure sensor be switched off when not measuring?

(4) Confirm that settle time is within one conversion cycle, not two or three due to step change. 

.

 

  • 0
    TI__Guru** 113765 points

    Hi Richard,

    If you are going to switch between mux channels, there is really no advantage to using duty-cycle mode.  In this case you are better off using single-shot mode instead.  When using single-shot mode, you send the START command each time you want to make a conversion.  When changing mux channels, you write the new configuration and then send START again.  Duty-cycle mode takes one conversion (at the normal mode data rate), goes to a lower power state for three conversion periods of the normal mode data rate and then repeats the cycle.  So there is really no great advantage using duty-cycle mode unless you stay on one mux input for a long period of time and use DRDY to wake up your micro.  This is where there is some advantage of using duty-cycle mode for power savings.  However you can basically do the same thing with the micro more efficiently as the micro needs to change the channels anyway, so single-shot mode is the better way to go.

    As to powering on and off the sensor, you can do this as long as the absolute input voltage to the ADS1220 is not exceeded on the analog inputs.  Also, consider the amount of time the sensor output needs to settle as additional time may be required for analog settling.

    The ADS1220 is single-cycle settling, which means every conversion is valid from a digital filter perspective.  However, if the input is changing during a sampling period, the result will reflect this changing behavior.  As the delta-sigma converter is an oversampling ADC, you will see the result of a combination of all the sampling during the conversion period.  In normal or duty-cycle mode the sampling rate of the input is actually 256kHz which is the modulator rate.  The output data rate is the selected data rate in the configuration register, so the actual number of samples needed is relative to the actual sampling of the modulator and the data rate selected.  A step change in the middle of a conversion cycle will reflect both the value before the step change and after.  The same would be true for any analog input settling.

    Best regards,

    Bob B