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ADC128S102: SClk frequency problems

Nicola Martinelli
Prodigy 165 points
Part Number: ADC128S102
Other Parts Discussed in Thread: ADC128S052

Hi all.
I'm observing some strange problems with some ADC128S102.
In my board I have two ADC128S102 and in the same SPI-BUS I have others devices (all devices have indipendent CS pin)
At the beginning (about one year ago), I used the clock frequency f=4MHz because if I increased the frequency I could not read anything. I used this setup for 4 boards.
In the last 5 boards, I had some problems in three of them and so I'm investigating.
I realized that the datasheet recommends 8MHz<f<16MHz.
I checked the signals with the oscilloscope and I saw that the DOUT from the ADC128S102 has a very rise and fall time and this could explain why I can not use a
frequency greater than 4 MHz.
After that I take out the 100ohm resistor connected with the DOUT pin (I replaced it with 0ohmm): the situation is better but I can not use f=8MHz.
It could be that the signal is so slow because of the bus length and because I have too many devices in the same SPI bus. Is there some specification in the datasheet?

Another question is: why the datasheet suggest the use of the 100ohm resistor in the SPI bus (pag.18)?
I saw that in the previous datasheet version there was not this recommendation.

Here attached you can see some pictures (f=3.8MHz):
in the first the initial situation (light blue is DOUT, yellow is the clock)
in the second I have removed the 100ohm resistor
in the third it is possible to see the DIN signal (purple color)

Thanks.

  • 0
    TI__Mastermind 25020 points

    Hello,

    This device is meant to use a clock frequency of 8MHz to 16MHz, as you noted.

    It is still operational at slower frequencies as you have seen, but outside of the recommended clock frequency we cannot state how the device will behave not provide performance specifications.

    It seems that the 100 ohm resistor was slowing down the clock edge too much, and likely causing jitter issues. Series resistors are added to help with reflections on long transmission lines and to help with sharp edge corners. The value used is dependent on the application.

    If a zero ohm works well, then that is fine. Another common value is 49.9 ohms

    Regards

    Cynthia

  • 0 in reply to Cynthia
    Prodigy 165 points

    Hello Cynthia, thanks for the answer.

    Anyway, the problem is that with the zero ohms and f=8MHz the device do not work. I'll try others resistor value.

    Also i saw that by connecting the scope probe (or analyzer probe) in the clock line, the ADC128S102 work good (R=100 ohm, f=4MHz).

    Regards

    Nicola

  • 0 in reply to Nicola Martinelli
    TI__Mastermind 25020 points

    Were you able to find a resistor value that worked well?

    Regards

    Cynthia

  • 0 in reply to Cynthia
    Prodigy 165 points

    Hello Cynthia.

    In the next few days I will try to change the resistance.

    What do you think of the fact that if I connect the scope probe in the clock line the ADC128S102 work good?

    Thanks and regards.

    Nicola

  • 0 in reply to Nicola Martinelli
    Prodigy 165 points

    Hello Cynthia

    I solved the problem reducing the slew rate in the PIC (master).

    In this case the clock signal in the ADC128S102 is without overshoot and undershoot.

    Now the ADC128S102 is working with fsclk=4MHz; in the future I'm thinking to replace it with ADC128S052.

    Here attached you can see the situation with high slew rate and reduced slew rate: the clock signal is yellow and green lines (before and after the R=100 ohm)

    Thanks and regards

    Nicola