ADS1178: Query for daisy chain connection

Hello John,

Yes, you should be able to do this, although I have not tried this and am not aware of anyone else using this combination. 

I recommend setting U1 as the high-speed device, and U2 as the low-power.  You will need to use the same clock source for both ADCs and synchronize so the /DRDY pins are synchronized with each other. (When both ADC's have data available, the falling edge of both /DRDY pins should occur at the same time.)  You can then just monitor the /DRDY pin of the higher speed channel U1 and retrieve data from U1 or U1&U2 as needed.

I may be overlooking something here.  If you have a specific combination of speed and channels, please let me know and I can take a closer look if this will work.

Regards,
Keith Nicholas
Precision ADC Applications

Hi Mr. Keith,

Thank you so much for your help.

Currently I use ADS1178 for U2 and ADS1174 for U1. And set U2 as low power mode and set U1 as high speed mode. They connected in daisy chain as below configured. I use 6 channels of U2 and 2 channels of U1, therefore, CH7&8 of U2 are set in power down while CH3~8 of U1 are set in power down. /DRDY of U2 is not connected out, just use /DRDY of U1 for data ready indication. I attach the schematic for your review. 

Thanks

John

模拟量MUX信号ADC.pdf

Hello John,

Everything looks correct on the schematic.  I recommend connecting DIN (pin 12) on the ADS1178 to ground, and not leave this pin floating, as you could get high IOVDD current flow.  I also recommend replacing one of the 0.1uF AVDD bypass capacitors on both ADCs with a 10uF capacitor for best performance.

Also, you will want to set SCLK frequency to a minimum of 6.4MHz (1/2 of fCLK).  You are using format 001, which keeps the channel slots for all channels, including powered down channels, active.  With daisy-chain, you will transfer a total of 12 channels, 16b per channel, and will need to support the maximum data rate of 25ksps.  This requires a minimum SCLK frequency of 12*16*25000=4.8MHz, rounding up to the recommended SCLK frequency of 6.4MHz.

Regards,
Keith