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ADC34J45: SYSREF / SYNC Input and differential voltage range

user5163865
Genius 3335 points
Part Number: ADC34J45
Other Parts Discussed in Thread: LMK04828, ADC32J45, , DAC37J84, ADC34J24

HI,

To change the ADC / DAC from serial LVDS to JESD204B, our customers are considering a signal chain by combining the following products.


/ LMK04828
/ ADC32J45 or ADC34J45
/ DAC37J84

However, in the ADC32J45 / ADC34J45 data sheet
SYSREF: The input circuit is unknown, and the circuit cannot be examined because the VCM range and amplitude range are not described.
SYNC: The input circuit is unknown, and the circuit cannot be examined because the VCM range and amplitude range are not described.

Question 1)
Customer plans to DC-couple because SYSREF is a pulse signal.
The LVDS output range of the LMK04828 is as follows.

Vodiff: Typ: 395mV * Range not shown
Vocm: Min 1.125V --Max 1.375V (Typ: 1.250V) * Termination 100Ω

Is DC coupling possible? (Termination is 100Ω)
Since the range of Vodiff is unknown, please let me know if there is any problem.



Question 2)
Evaluation board: Why is the ADC34JXXEVM outputting from LMK04828 with LCPECL?

I think it's easier to terminate 100Ω using LVDS.
Please let me know if there is any problem with this method.



Question 3)
Customer wants the SYNC signal to also be DC coupled with the LVDS output of the FPGA.
The LVDS output range from FPGA is as follows.

Vodiff: Min 247mV --Max 454mV (Typ: 350mV) * Termination 100Ω
Vocm: Min 1.000V --Max 1.425V'Typ: 1.250V) * Termination 100Ω

Is DC coupling possible under these conditions? (Termination is 100Ω)

There is an E2E answer in the past at the following URL, but the data sheet has not been revised.

https://e2e.ti.com/support/data-converters/f/data-converters-forum/491033/sync-and-sysref-for-adc34j45?tisearch=e2e-sitesearch&keymatch=ADC34J45#


We are competing with ADI's solution and we need TI's help to D-IN.

It would be helpful to have a detailed example of a signal chain guaranteed by TI.

Best regards,
Hiroshi

  • 0
    TI__Genius 12780 points

    Hi Hiroshi,

    Question 1)

    It is recommended to AC couple SYSREF since SYSREF has an internal DC bias of 0.9V. It is ok that it is a pulse (one-shot) since it should be a signal in MHz range.

    Question 2)

    The EVM is using LVPECL to maximize the signal swing of SYSREF and SYNC in order to satisfy the datasheet requirement. I think that LVDS would be sufficient to drive these inputs, but it has not been tested thoroughly.

    Question 3)

    The SYNC must be DC coupled since an AC coupling cap will adversely affect the rise/fall time of the signal. This can cause JESD initialization problems.

    The signal chain on the EVM has a DC shifting resistor network to acheive +0.9VDC bias for the SYNC inputs. This is proven to work, and would recommend a scheme similar to this.

    As stated previously, LVDS does not have the full 800mV signal swing that is required for SYNC input per the ADC34J24 datasheet (applies to whole family).

    Best Regards,

    Dan