LTC6602 input and output

Hello,

I'm planning to use LTC6602 in front of AD7380 ADC for sampling signals in the range of 0-200kHz with a sampling frequency of 1 MHz, 2MHZ or 4 MHz.

I saw that it is possible to use LTC6602 with AC coupled input by properly restoring common mode as done in Fig. 12 of datasheet.

Q1) would it be possible to use the LTC6602 input as a single ended AC coupled signal (obviously restorng the input common mode on both the used and unused input similarly to fig 12)?

Q2) would LTC6602 be able to  drive AD7380 (obviously with the RC filter needed for clock feed through attenuation)?

Thanks in advance for any help

  • 0
    •  Analog Employees 
    on Oct 22, 2020 5:32 AM 1 month ago

    The LTC6602 is a sampled-data filter and must have an aliasing input filter and and output image filter.

    In addition, the LTC6602 output can not drive an ADC input so, use a buffer/driver at the 6602 output

    (the LTC6602 is incorrect when showing an LTC6602 driving directly an ADC).

  • Hello,

    thanks for your reply.

    It's not clear to me why the image filter should have  a 530KHz cut off filter in case LTC6602 clock would be in the range of 70 MHz. What would be the benefit of LTC6602 internal filtering if I should place outside a huge Image filter? IMHO LTC6602 become useless. I thought image issues where related to sampling clock frequency which is high thus relaxing image filtering....

    In any case I have space constraints so would it be possible to place LT6600-2.5 as image filter/buffer between LTC6602 and AD7380?

    Thanks in advance

    Gianluca Chiarini

  • 0
    •  Analog Employees 
    on Oct 22, 2020 6:05 PM 1 month ago in reply to Lucis

    Hello Gianluca.

    Using an LTC6602 with a 200kHz signal bandwidth the clock frequency options are 200kHz x 150 or 200kHz x 300, a 30MHz or 60MHz clock respectively. The LPF1 bit for the 150 and the 300 ratio is zero so, the LTC6602 input sampling is FCLK/3. Using the higher clock, 60MHz, the first image frequencies at the LTC6602 output are in a band 19.8MHz to 20.2MHz and these signal will be sampled by the ADC is the RC filter between the driver and the ADC does not provide sufficient attenuation. If the LTC6602 is tuned to a lowpass frequency lower than 200kHz then use a 900 ratio (LPF1 bit is 1) so the input sampling is equal to FCLK (max FCLK is 90MHz).

    Using an LTC6604-2.5 (a dual, matched LTC6600-2.5) is a great suggestion and I hesitated recommending a more costly solution.

    Best Regards,

    Philip Karantzalis.

  • Hello Philip,

    thanks for your explanation that more or less confirm my thoughts. I selected LT6600 because in my space constraints probably it will be easier to place twice a 4x4mm DFN than one larger 7x4 mm QFN, but this is a detail. I will also use a 3.3V supply and set the AD7380 reference to 3.3V so to have larger swing and avoid dual supply operation (I can't use negative supplies).

    Since I'm squeezing the lemon, and being LT6604 an analog fillter (so I think it doesn't generates additional spurious at some weird frequancy), I would like to avoid the more  passives I can. So I'm guessing if I can omit one of the RC filter between LTC6602 and LT6604 or the one between LT6604 and AD7380 or in any case group them in only one position or at least simplify them.

    What Do you suggest?

    Luca

  • 0
    •  Analog Employees 
    on Oct 23, 2020 4:08 PM 1 month ago in reply to Lucis

    The RC between the 6604 and 7380 is a must for ADC driving. The RC between 6602 and 6604 helps reduce spectrum reduction however remove it is ok.

    Philip.