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This is a question about ADS1294 SPI SCLK.

Part Number: ADS1294
Other Parts Discussed in Thread: ADS1298

I have a question about SPI SCLK while using ADS1294.

SCLK PERIOD 66.6 ns (MAX 15 MHz) on DATASHEET, but if it actually works, it is fine from 1 to 5 MHz, but data corruption from 6 MHz will occur (using DVD 1.8 V).
On the PCB, the MASTER is connected directly to the series resistance of 33 ohms.
Resolution 24bit, sampling rate 250SPS in use.

1. We would like to ask if the maximum available frequency changes depending on the resolution and sampling rate.
2. Please check if it is right to use at least 61 kHz at resolution 24bit, sampling rate 250SPS.
3. I'd like to know what criteria the Tsclk of datasheet was measured based on.



Thank you.

 

  • Hi DongYeong,

    Thanks for the post. Please see my comments for your questions below.

    1. The maximum SCLK frequency does not depend on the device resolution and sampling rate. 

    2. Please refer to Equation 7 for the minimum SCLK speed calculation. For the condition you specified (DR=250SPS, 24-bit, 4-Channel, CLK=2.048MHz), the minimum SCLK speed is 30kHz. 

    3. The tSCLK parameter in Table 7.6 of the datasheet is specified for TA = -40°C to +85°C and with a load of 20pF || 100kΩ.

    Thanks

    -TC

  • Thank you for your answer.

    I have a question in addition to answer number 1.

    Currently, there is no problem with using SCLK up to 5 MHz, but there is a problem with ID (0x90) not being read from a higher value.

    If SCLK is possible up to 15 MHz without resolution or sampling rate in the above answer, I would like to know why the current problem occurs.

    Are there additional tasks or other factors that are necessary or restricting the use of SCLK?

     

    Thank you.

  • Hi DongYeong,

    It is difficult to tell what the issue is now. Can you probe the SPI bus for the device communication with an oscilloscope and observe the signals?

    If you send multiple-byte commands, you need to meet the command decode timing specification (tSDECODE), as shown in Figure 1. Please refer to Section 9.5.1.2.1 for the detail of SCLK clocking methods.

    Thanks

    -TC

  • Thank you for your answer.

     

    When you check the waveform with the scope, the waveform normally appears up to 4 MHz, but above that, the waveform is distorted and the MISO waveform is different.
    SPI sent to 8bit BURST METHOD.

    Since tSDECODE is 4*tCLK, the maximum available SCLK is thought to be 8(bit) / 4*tCLK = 4MHz (assuming a minimum time interval between the end of the previous 8 bits and the next 8 bits), is this correct?

    And if I want to use SCLK MAX 15 MHz in the previous question, do I use the Continuous Stream Method?

     

    Thank you.

  • Hi DongYeong,

    Since tSDECODE is 4*tCLK, the maximum available SCLK is thought to be 8(bit) / 4*tCLK = 4MHz (assuming a minimum time interval between the end of the previous 8 bits and the next 8 bits), is this correct?

    Yes, this is correct.

    You will not be able to use the Continuous Stream Method if the SCLK is 15 MHz, as you will violate the tSDECODE timing specification. You will need to use the Burst Method for SCLK faster than 4MHz, as you calculated in your previous question.

    There is no relationship between the Continuous Stream or Burst Method with the SPI signal integrity you see. Would you please verify that your hardware design and the controller can operate at an SPI frequency of 15 MHz?

    Thanks

    -TC

  • Thank you for your answer.

    Additionally, I have a question about 9.5.1.4.

    9.5.1.4 shows the DOUT for ADS1298 as a total of 3byte*9 = 27byte. I wonder how DOUT comes out in case of ADS1294.
    Is it correct that 3byte*5 = 15byte comes out? Or is 27byte output the same, but 5-8 channels are not calculated?

    Currently, it is confirmed that it is 15byte based on SCLK.

    Thank you.

  • Hi DongYeong,

    Yes, that is correct. For ADS1294, you will only need to provide 120 SCLK (24-bit STAT + NCH x 24-bit data) or 15-byte to retrieve the data from 4-channel. 

    Thanks

    -TC