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ADS7957-Q1: Basic question about the relation between max source impedance and throughput rate

Takumi Suzuki1
Mastermind 8570 points
Part Number: ADS7957-Q1

Hi,

I would like to ask you some basic questions about the relation between input source impedance and throughput rate as below.

Q1) Figure 58, datasheet page 40

There is a description "Note A" and there describes that "Rsource <= 250ohm for the 1xREF 12bit setting at 1MSPS" on the latter half of the note.

I suppose that "12bit" should be "10bit" not 12bit from Figure 59(datasheet page 41) data... 

How is my idea?

Q2) Figure 59, datasheet page 41

Is this data the result of measurement data by the description of "9.2.1.2 Detailed Design Procedure", right?

And then, did you adopt 150pF as input capacitor value as Figure 58?

And this data guarantees throughput rate?

For example, in case of 8 bit, when we select Rsource 1000ohm, throughput rate is 1MSPS and you guarantee it.

Q3) Throughput rate - 1

Let me confrim the meaning of "throughput rate" if my understanding is correct, for sure.

For example,

In case of 1MSPS, this means 20 SCLKs/Frame (@20M Hz SCLK) and 20 SCLKs consist of "conversion phase" 16 SCLKs and "Acquisition phase tacq" 6.5 SCLKs.

In case of 500KSPS, this means 40 SCLKs/Frame and this consists of 16 SCLKs (Conversion phase) and 26.5 SCLKs (tacq).    

Is my understanding correct?

Q4) Throughput rate - 2

In case of 1MSPS, the actual throughput rate is 62.5KSPS/ch @16ch.

Is my understanding correct?

Thank you for your support!

Best Regards,

  • 0
    TI__Expert 5545 points

    Hi Takumi San

    Please see my reply to your questions

    1. This can be put in two different way. Rsource <= 250ohm for the 1xREF 10bit setting at 1MSPS" OR Rsource <= 250ohm for the 2xREF 12bit setting at 1MSPS"

    I think there is typing error and need to be fixed. Thanks for pointing this out

    2. Figure.59 and Figure 60 data is measured as per design procedure given in section 9.2.1.2. This data is typically collected on multiple devices and information derived from this table holds true for deriving relation between source impedance and throughput

    3. In case of 1MSPS, this means 20 SCLKs/Frame (@20M Hz SCLK) and 20 SCLKs consist of "conversion phase" 16 SCLKs and "Acquisition phase tacq" 6.5 SCLKs.

    In case of 500KSPS, this means 40 SCLKs/Frame and this consists of 16 SCLKs (Conversion phase) and 26.5 SCLKs (tacq).   

    Your understanding is correct (I assume you are keeping SCLK at 20MHz)

    4. In case of 1MSPS, the actual throughput rate is 62.5KSPS/ch @16ch.

    Your understanding is correct

    Please let me know if you need additional details

    Thanks & Regards

    Abhijeet Godbole

  • 0 in reply to Abhijeet Godbole
    Mastermind 8570 points
    Hi Abhijeet-san,

    Thank you very much for your prompt reply!
    I could understand well.

    Regards,