ADS1256：About switching usage

Hi Ohno-san,

The ADS1256 is a Delta-Sigma ADC which is an oversampling device.  Sampling takes place at the modulator rate, and the digitally filtered output data becomes available at the output data rate selected by register setting.  The digital filter consists of a sinc5 filter followed by an averager.  The rate at which new conversion data becomes available is the Data Rate.   The end of a conversion period where new valid data becomes available is indicated by a high to low transition on the DRDY pin.

When switching between input channels you need to consider the settling delay of the digital filter.  You must also consider any change in configuration.  The t19 time in Figure 19 is only showing the changing of the mux and using the maximum allowable SCLK.  If you were to change data rates between mux channel configurations, you would need to add additional time to the communication cycle for the writing of the register configuration.

See the remainder of my response below.

Best regards,

Bob B

Kenji Ohno1 said:

Part Number: ADS1256

Hi Team,

I want to switch the multiplexer and read the following data.

  Ch.1 (6 significant digits) Data integration time 100ms

  Ch.3 (5 significant digits) Data integration time (T.B.D : ex.50ms)

Measured against analog value 0V.

Acquire Ch1.Ch2 alternately 5 times per second.

Below are the questions.

(1) Since Ch.3 has fewer significant digits than Ch.1, is it okay to shorten the data acquisition time?

      (I understand that there is no problem when I look at Figure.19) [Bob] You can change the data output rates for each channel, but this requires additional time to change the register setting and must be considered when calculating the overall throughput.

(2) Is it necessary to wait for stabilization when Ch.1 and Ch.3 have different input values?

     (For example, when Ch. 1 = 2 V, Ch. 3 = 0.1 V is input) [Bob] You do not need to add additional delay.  When the SYNC command is issued, the digital filter is reset and a new conversion cycle begins at that point for the selected mux input channel.

(3) Is it good to understand that if the measurement is performed while switching between Ch.1 and Ch.3, 

     fluctuations in the value of Ch.3 during integration of Ch.1 will not affect the data of Ch.3 after switching?

     (According to Figure.19, it seems that data is accumulated after switching by Multiplexer.) [Bob] Changes on a channel not being measured will not affect the conversion of the channel being measured as the inputs are isolated by the mux switch.  However, poor PCB layout can cause crosstalk between channels or if the fluctuating channel exceeds the analog input maximum the conversion data could be affected.  This issue is caused by the the system design and not of the ADS1256 device itself.

Best Regards,

Kenji