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ADS8598H: Understand this high-speed ADC converter in serial communication

Peter Bryant
Prodigy 245 points
Part Number: ADS8598H
Other Parts Discussed in Thread: ADS8588H

Good morning,

I wanted to ask some questions about the ADC model ADS8598H about the sampling times https://www.ti.com/lit/ds/symlink/ads8598h.pdf?ts=1701731784615&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FADS8598H . As you can see in the datasheet, it is an ADC with one sampling unit per channel. 8 sampling units for 8 channels. This means that as it can reach 500kSPS, if you use all 8 channels, you would get 4000kSP in 1 second. My question is about the acquisition time. I want to get the data via SPI and this ADC has serial communication so I could use it. I have seen that it has 2 serial outputs DOUTB and DOUTA.

What I want to know, and I find this ADC a bit confusing, is how long it would take to get the results from the ADC over SPI at the maximum SCLK speed (500kSPS) if I use all 8 channels. My doubt is in the sending of these conversions because there are only 2 serial sending channels and not 8. 

Thank you in advance,

Pedro

  • +1
    TI__Mastermind 39075 points

    Hi Pedro,

    When your SCLK is 20MHz frequency and both DOUTA & DOUTB are used in serial mode, total 18x4=72 SCLKs are required to transmit data for 4 ADC channels (the data of the remaining 4 channels are sent to your processor via DOUTB at the same time), total cycle time tcycle = 72xtSCLK+ tCONV+ tD_CNBSY+ tSU_BSYCS + tD_CSDO + tD_CSCN =72x50ns+1.29us+15ns+0ns+12ns+10ns =4.927us, so the max sampling rate is 202.96ksps.

    However, ADS8598H has the same interface design as ADS8588H that can support 50MHz SCLK frequency (20ns minimum SCLK time period) , see the timing table in  ADS8588H datasheet below, so you can increase the SCLK frequency to achieve 500ksps sampling rate on ADS8598H. The datasheet should be updated in the future.

    Regards,

    Dale

  • 0 in reply to Dale Li
    Prodigy 245 points

    Hello Dale,

    I just saw your message, thank you very much!!. I'm going to check out this 16-bit, higher frequency SCLK converter option you recommended. Thank you very much for explaining me how to do the sample calculation with SPI. I only have one additional doubt, in the calculation you have done, you have done it for 4 channels of an output channel (for example DOUTA). Actually, as there are 2 SPI outputs DOUTA and DOUTB, if the master has two SPI input channels, you could get twice as many ksps, right? That is, you would have the 2 x 202.96ksps = 405.92ksps.

    Thanks!

    Have a nice day,

    Pedro

  • 0 in reply to Peter Bryant
    TI__Mastermind 39075 points

    Hi Pedro,

    No, my calculation has considered both DOUTA & DOUTB, the data of the remaining 4 channels are sent to your processor via DOUTB while the data of the first 4 channels are sent to the processor via DOUTA.

    If only DOUT( either DOUTA or DOUTB) is used, total 18x8=144 SCLKs are required to transmit data for total 8 ADC channels, so the max sampling rate will be 101.48ksps

    I just modified my previous response to make it clear.

    Regards,

    Dale

  • 0 in reply to Dale Li
    Prodigy 245 points

    Hello Dale,
    Yes, I understand what you mean. What I meant is that the 202ksps calculation you made is for the two output spi channels in parallel, each with the conversion of 4 input channels (8 in total). But the calculation you made would be the samplings you would get from DOUTA that you would have to add the parallel samplings sent from DOUTB, so it would be 404ksps if I'm not mistaken.

    I mean that in 1 second, the mic will receive 202ksps from DOUTA and 202ksps from DOUTB, receiving 404sps.

    Please, confirm if I am right or if I am missing anything about it. Thank you for all your time Dale.

    BR

  • 0 in reply to Peter Bryant
    TI__Mastermind 39075 points

    Hi Pedro,

    This is a simultaneous sampling ADC, not a ADC with a multiplexer, the time for sending data via DOUTA is the same time requirement for sending the data of the remaining 4 channels via DOUTB, so 202ksps is maximum sampling rate when both DOUTA and DOUTB are used.

    BR,

    Dale