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Can LMH6518 drive ADC08D500 directly?

jlin
Prodigy 30 points
Other Parts Discussed in Thread: ADC08D500, LMH6518, ADC08D1020

We are interested in using the ADC08D500 running at 1GSPS in Interleave mode. The input signal is 1Vp-p max, and we would like to use a variable gain amplifier between the RF input and the ADC.

1) Is the LMH6518 a reasonable choice as a VGA for this application?

2) Can the LMH6518 drive the ADC08D500 directly?

3) With a 1Vp-p max input range, what nominal output of LMH6518 should be set for matching ADC FS input?

  • 0
    TI__Mastermind 44680 points

    Hi jlin

    The LMH6518 could be used to directly drive the ADC08D500, but depending on your complete system needs may not be the best choice.

    Can you please provide some additional information?

    • What is the maximum input frequency?
    • What is the minimum input frequency?
    • Does the RF input signal need to be DC-coupled?
    • Is your input signal Differential or Single Ended?
    • Do you have any specific performance requirements your overall Amplifier+ADC needs to meet? (SNR, SINAD, SFDR, THD, etc.?)
    • When choosing the LMH6518, were you planning to use the Auxiliary Output?

    Best regards,

    Jim B

     

  • 0 in reply to Jim Brinkhurst1
    Prodigy 30 points
    Hi Jim,

    Thank you very much for your quick response. Here are the information you requested:

    1) The circuit is intended for an input signal with 1ns rise time or approximately 350MHz bandwidth.
    2) The RF input signal should be AC-coupled.
    3) The input signal is differential;
    4) The LMH6518 + ADC08D500 is intended to replace a legacy circuit using SA5219D (obsolete) and eight ADC08200s running at 125MHz each with 45 degree phase shifted in each clock. The performance requirements should be same or better than the legacy design.
    5) No plan to use the Auxiliary Output if not needed.

    Best Regards,
    jlin
  • 0 in reply to jlin
    TI__Mastermind 44680 points

    Hi jlin

    I've discussed this with my colleagues in the high speed amplifier group. The LMH6518 is a good fit for this application. The only small concern I have is the maximum input amplitude of 1Vpp in combination with the minimum amplifier gain of -1.16dB will result in a signal which is slightly larger than the ADC full scale range even if the higher FSR setting is used.

    It would be best to incorporate a balanced resistive attenuator circuit to slightly reduce the RF signal amplitude before it is input to the LMH6518. The resistor values can be optimized during your prototype testing and will allow you to use 0 attenuation if that is needed.

    Incorporate DC-blocking capacitors on the amplifier input and output paths. These capacitors can always be replaced with resistors if DC coupling is needed.

    If you can share your schematic design when it is ready we will be happy to give it a quick review before you proceed to layout and manufacturing.

    Best regards,

    Jim B

  • 0 in reply to Jim Brinkhurst1
    TI__Mastermind 44680 points

    Hi jlin

    I have a few other comments related to the ADC operation and usage.

    Since you will be using this converter in DES (Dual Edge Sampling) mode please note that you must return the device to non-DES mode whenever an on-command calibration is required. There are a number of other DES mode specific requirements for this device that should be reviewed in the datasheet to ensure you will have the necessary control of the various mode select pins or register bits. An alternate device, the ADC08D1020 has fewer restrictions on using DES mode, and would enable higher sample rate operation if needed. A board can be designed to support either of these devices if needed. Please see the information in this document for more details.

    /cfs-file/__key/communityserver-discussions-components-files/73/3348.6153.7585.Pin-compatibility-of-1500-vs-1520.pdf

    Best regards,

    Jim B

  • 0 in reply to Jim Brinkhurst1
    Prodigy 30 points
    Hi Jim,

    Thank you.

    We will take a look at the ADC08D1020; and if permitted, we will provide you the schematics design for review.

    Best Regards,
    jlin