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ADS8556 when using four channels

Atmeg
Intellectual 870 points

Other Parts Discussed in Thread: ADS8556

Hi everyone,

I'm using three ADS8556s in daisy-chain mode with single channel SPI. I want to receive only four channels from each adc. I switch CONVST_A and CONVST_B together, and CONVST_C tied to Vdrive, but all six channels clocked out from each adc, not four, as I expected. Is this a right behavoiur or something wrong in my HW configuration? The last two unused channels tied to ground but returns 7FFE.

Where unused CONVST should be tied and what the difference if it tied to ground instead of Vdrive (I erroneously have already tie this pin to ground on my board)? 

  • 0
    TI__Guru**** 182821 points

    Hi Atmer,

    When using only four channels, you can leave the unused CONVST tied to GND.  You can then restrict your SCLK to provide only the number of clocks needed to output the desired conversion results, which would be 64 in this case.  If you continue to clock data from the device, you should (if CONVST C is grounded) get the data from pair A repeated on the output.  What was the state of the 'C' pair analog inputs when you ran this test?

  • 0 in reply to Tom Hendrick
    Intellectual 870 points

    Hi, Tom

    CONVST_C pin is grounded and two unused inputs are grounded too. I have 3 adc in daisy-chain therefore I must clock it all and can't restrict the clock cycles to 64. Output data are: <ADC1_A1>, <ADC1_A2>, <ADC1_B1>, <ADC1_B2>, <0x7FFE>, <0x7FFE>, <ADC2_A1>, <ADC2_A2>, <ADC2_B1>, <ADC2_B2>, <0x7FFE>, <0x7FFE>, <ADC3_A1>, <ADC3_A2>, <ADC3_B1>, <ADC3_B2>, <0x7FFE>, <0x7FFE>.

    Thanks.

  • 0 in reply to Atmeg
    TI__Mastermind 23235 points

    So from the d/s (p32), in order to put a particular channel in "partial power down", you need to bring the CONVST_x signal low during an ongoing conversion when BUSY is high.  So you probably aren't putting the channel in power down and the data is simply the previous data (or default data since you never performed a conversion).

  • 0 in reply to Greg Hupp
    Intellectual 870 points

    Hi, Greg

    In my board CONVST_C is hard-wired to ground. 0x7FFE seems to be default data. So I understand that no way to obtain only four words instead of six.

    Thanks to all.

  • 0 in reply to Atmeg
    TI__Guru**** 182821 points

    Hi Atmer,

    Do you have a second SPI port available?  If you do, you could take SDOB onto the receiver and loop your SCLK back into this port as well as the SCLK input to the ADS8556 devices in the chain (similar to what is shown in Figure 39) - basically what you would do is use the SPI port on SDOA as a master controller, and treat the second as an SPI slave.  Something like this:

    That would boost your throughput and still give you the opportunity to expand the system to include CHC0 and CHC1 as well down the road.