I have been reviewing the datasheet for the ADS1256 and I could use some clarification on the SPS for sampling multiple channels. Page 21 talks about the settling time for the multiplexor and how you can give the sampling command for the next sample while it is converting for the current sample. It also gives a table of throughput while multiplexing. For instance, with the SPS set at 30K, the cycling throughput for the multiplexor would be 4.3K. Does this mean that the maximum effective sampling rate for multiple channels is 4.3K? If so, how many channels does this represent?
Thank you for the clarification.
Justin
Hi Justin,
The cycle throughput values in Table 14 correspond to the data rate when cycling through channels. The t19 timing spec is the time to get a conversion result for one channel. When cycling through channels, it takes time for the internal mux to change and digital filter to clear out before the next conversion cycle is ready. You can use Figure 19 to get a visual representation as to what the cycle throughput t19 spec actually is.
Regards,
Tony Calabria
I am still having trouble understanding how the timing works between the data rates and switching channels.
An example might help clarify things for me. Lets say we are sampling at 10 Hz. Would we be able sample all 8 channels at this data rate? If not, what settings would we need to configure to get 8 channels sampling at that frequency, what what would the 3db point be for the digital filter for that scenario?
Thanks for the help.
Justin,
So sampling at 10Hz meaning you need a sample every 100ms. If you want to read back all eight channels, you will need to have a data rate faster than 10Hz so that you can read back eight channels in 100ms. You will need to run the device with a throughput of greater than 80Hz to get all the data back from eight channels within 100ms. The digital filter is based off of the data rate that you choose. For example, if you choose a cycle throughput of 98 Hz corresponding to at 100SPS data rate, the 3dB point shown in Table 12 is 44.2Hz.
-Tony
Thanks for going through the example scenario, Tony. Sacrificing the -3dB filter point for multiple channel sampling means this part is out for us.
Hi Tony, I've the same question that Justin, because I don't really understand how to calculate the data rate in Hz for each channel that I want to convert, and the real bit resolution. In my case, I need to read all the 8 channels togheter at 400Hz. This is, every 2.5msec I need to read them all (Is clear that channels are multiplexed and I get them all sequentially so the time between the first bit of the channel 0 and the last bit of the channel 7 must be less than 2.5msec in the SPI port). Which is the SPS I must to configure and how I calculate the resolution in bist of each channel ?
best regards, Esteban
Hi Esteban,
Review page 21 and table 14.These are per individual channel throughput numbers. You need to combine the throughput numbers depending on the amount of channels you are going to cycle through in order to derive the total data rate.