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LMH6552: Fully differential TIA for current output DAC buffering?

Sebastian
Prodigy 10 points
Part Number: LMH6552
Other Parts Discussed in Thread: LMH5401, , DAC37J84, THS4567, OPA858, OPA855

Tool/software:

Can I use the LMH6552 (or a similar part like LMH5401) as a fully differential transimpedance amplifier, to convert the differential current output (+/-20mA full scale range) from a high speed DAC to a voltage output line driver, to drive a 100 Ohm differential pair over some distance, let's say 10 inches?

The 100 Ohm differential pair would be a combination of matched PCB traces and twinax cable for the actual board to board connection.

The main reason for this (as opposed to a single ended signal transmission over coax) is that the DAC is located on a physically separate driver PCB, so not near the load point where the signal is needed. Using a differential signal transmission all the way to the load board should help with noise immunity and ground noise rejection.

In the simplest case, the load would be just a 50 Ohm termination with low capacitive load (less than 5pF) and it could be checked with a high quality active oscilloscope probe:

In more detail, the signal at the load would probably be converted from differential to single ended for high CMRR with another amplifier like this:

Does this look like a reasonable setup? Am I making some incorrect assumptions here?

  • 0
    Prodigy 10 points

    In particular, I want to confirm that it would be ok to interface the DAC37J84 with the LMH6552 configured as a transimpedance amplifier as shown above. One benefit is that this uses minimal components and reduces the need for component matching.

    The alternative is to do the DAC I-to-V conversion with load resistors (which need to be matched carefully), and then use regular voltage follower op amps for buffering. In that case, the current output DAC has to drive against a varying voltage across the load resistor - the compliance voltage range of this current output DAC is around +/- 0.6V.

    Theoretically at least, connecting a current output DAC to a TIA is a lot cleaner and has fewer sources of signal error than driving the current output across a load resistor.

  • 0 in reply to Sebastian
    TI__Mastermind 22484 points

    Hi Sebastian,

    Using a fully-differential amplifier (FDA) as a TIA is possible, but will require some consideration for offset currents.

    We have a device, albeit for a lower bandwidth than I presume your needs require, which is a TIA + FDA in the same IC: THS4567.  You can learn about this application by reviewing this device, and a good place to start is a two-stage solution with discrete TIA + FDA (two ICs) to make sure your system approach is 'working'.

    We can use an OPA85x device (such as OPA855 or OPA858) paired with a high-speed FDA for our two-stage approach.

    If you want to start by considering the LMH6552 as a TIA + FDA, one thought is to correctly size the resistor on the non-current input of the FDA.  This will assist with cancellation of offset currents and enable better performance.  I can work on an example for you to share next week.

    Can I confirm the exact use-case is handling the current output of the DAC378J4?  If so it appears you already have found some documentation of such a use-case.

    I am glad to help work on this for you.  Thank you Sebastian.

    Best,

    Alec