Forum Post: RE: Questions about ADS1258

Hi Bailin,

Welcome to the TI E2E forums!

Please see this excel calculator I put together to see how  the DR, CHOP, and DLY register settings will affect the data rate: (Please visit the site to view this file)

Regarding your questions:

1. On page 18 of ADS1258 data sheets, in Equations (2) and (3), does 11b mean 3 in decimal? Yes!
   
   
2. How do you evaluate CHOP(4.265625+TD)? This is "Chop * (4.26525 + TD)" where CHOP = 0 or 1 and TD is the time delay in number of clock periods (0, 1, 2, 4, 8, 16, 32, 48). 
   
   
3. On page 21, footnote(1) of Table 8, what are t1, t2, ...? How and where can they be obtained? These are RC time constants. If you have multiple RC delays in the input signal path, you can approximate the overall RC delay constant as the root sum of squares of the individual time contants. As you add RC filters to the ADC's input for noise filtering and anti-alaising, keep in mind that you'll have the trade-off between lower filter cutoff frequency vs longer signal settling time.
   
   
4. How long does it take to switch on and switch off CHOP? ...this is turned on and off by writing to the CHOP bit in the CONFIG0 register. The time it takes to turn it on depends on the speed of your SPI clock (SCLK). With Chop on, two readings are made and the data rate is reduced by ~1/2.
   
   
5. If I only use 12 of 16 channels, should I use auto-scan or fixed mode? Auto-scan will cycle through the channels at the fastest rate possible. However, if you do not use the preset inputs, but want to measure AIN4 to AIN12 (for example), then you will need to use fixed-mode.
   
   
6. what is the maximum realistic sampling rate I can get for all 12 channels on the average? Without interrupting the auto-scan cycling you can get up to 23.7 kSPS/12 channels or ~2 kSPS per channel with CHOP off. When CHOP is on, you will reduce the maximum data rate to ~1 k SPS. If you interrupt auto-scan to toggle the chop bit this will reduce the data rate slightly. Perhaps you can read the internal functions less often, so that you only interupt the auto-scan cycling once every 10 cycles.
   
   

Best Regards,
Chris