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DAC80508: Inquire about sample rate calculation.

Lee SungYun
Intellectual 365 points
Part Number: DAC80508
Other Parts Discussed in Thread: DAC70508, DAC70504, DAC60508

Tool/software:

Hello?

While looking at the DAC product line, I have some questions about the Sample/update rate because there is something ambiguous.
The DAC80508 product is a 16-bit product with a 50 MHz Max. SPI operation method, and is listed as 0.2 MSPS in 'Home/Products/Data converters/Digital-to-analog converters (DACs)'.

Since the DAC70508 (14-bit) products are listed as 1 MSPS, the data has increased by 2 bits, so the speed has become 1/5, so I would like to request a calculation method for comparison.

In addition, please add the 'Sample/update rate' column to 'Home/Products/Data converters/Digital-to-analog converters (DACs)/Precision DACs (≤10 MSPS)'.

S.Y.Lee

  • 0
    TI__Mastermind 33928 points

    Hi Lee,

    The sample rate column is here in the parametric search tool. If it is not there you can adjust what is visible with the columns button:

    Update rate and sample rate can be interpreted differently. Update rate is usually is calculated by maximum SPI speed divided by the number of SPI clocks required to complete the DAC update. Each DAC update for both the DAC80508 and DAC70508 is 24 clock cycles, so the 50MHz/24 cycles is ~2MHz.

    The sample rate usually also includes settling time. For the DAC80508 this is 5us according to the EC table. So that is where the 0.2MSPS update rate is coming from. The 5us settling time is within a +/-2LSB settling band. The DAC70504 has a larger settling band (in terms of voltage) because it has a larger LSB size, so it will have a faster settling time than the DAC80508. 

    Best,

    Katlynne Jones

  • 0 in reply to Katlynne Jones
    Intellectual 365 points

    Thank you for reply.

    I've understand that 50 MHz/24 clock = 2.08 MHz Maximum, DAC80508 (16-bit) is 1/5 us = 200 kSPS, DAC70508 (14-bit) is 1/5 us * 4 = 800 kSPS, DAC60508 (12-bit) is 1/5 us * 8 = 1600 kSPS = 1.6 MSPS.

    However, both 70508 and 60508 are rated at 1 MSPS. Are there any additional restrictions other than the settling time?

    S.Y.Lee

  • +1 in reply to Lee SungYun
    TI__Mastermind 33928 points

    Hi Lee,

    This is not exactly the case.

    Lee SungYun said:
    DAC80508 (16-bit) is 1/5 us = 200 kSPS, DAC70508 (14-bit) is 1/5 us * 4 = 800 kSPS, DAC60508 (12-bit) is 1/5 us * 8 = 1600 kSPS = 1.6 MSPS.

    Settling time is a spec of the output buffer that consists of an overshoot and then a settling band. The +/-2LSB settling is going to be met around the same time for both the DAC70508 and DAC60508 after this first large overshoot. This is why they are shown as the same in the parametric search tool on ti.com. 

    There are times where settling time is less important in terms of update rate. For example, if you are making very small code changes to produce something like a sine wave, the overshoot and settling band will be much smaller as you change from code to code. 

    If you were trying to generate a square wave with zero scale to full scale transitions, then the settling time will start limiting the update rate. 

    Best,

    Katlynne Jones

  • 0 in reply to Katlynne Jones
    Intellectual 365 points

    Thank you for the detailed explanation.

    I think I have definitely learned that it is not possible to know it with just simple calculations.

    S.Y.Lee