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THS4521: As driver for ADC

David Wende
Intellectual 640 points
Part Number: THS4521
Other Parts Discussed in Thread: ADC09QJ800, , THS4541, LMH5401

Hello,

I am considering this part as driver to TI's ADC09QJ800.

In my application the signal will start from close to DC up to about (only) 120MHz.

(Yes - I know this ADC may seem an overkill but there are other factors involved also).

The ADC presents a 50 ohm input impedance to to common mode of 1.1V.

The datasheet of the THS4521 does not mention the output characteristics when driving a load of 50 ohm.

I will have many identical channels, so power dissipation and cost will also factor into the equation.

Could somebody comment on the choice of THS4521 in this application or suggest a better alternative.

2 and 3 Harmonic distortion are not critical for me.

Thanks

  • 0
    Guru 48395 points

    Well David, 

    The THS4521 is no where near fast enough for a 120MHz requirement with low HD,

    The next step up is the THS4541, even that will be running out of HD performance above 100Mhz

    Then you step up to a part like the LMH5401 - which would probably have as good a chance as any 

    You of course have not specified HD targets, nor gain, nor interstage filtering, etc etc. 

  • 0 in reply to Michael Steffes
    Guru 48395 points

    Oh and David, there are some important concepts in this slide deck - before you get too far, you should review this - the concepts are still very applicable, just newer parts available, Key issues and options in driving ADC's.pptx

  • 0 in reply to Michael Steffes
    Intellectual 640 points

    Thanks Michael for the prompt answer and interesting attachment.

    I did state that HD is not critical - but I did forget to state that gain flatness of +/- 2dB out to 100MHz is important.

    The reason being is that in our application, we are really interested in finding the peak frequency component and it's amplitude after applying a FFT on a large sample.

    So a bit of HD is not going to affect these results too strongly.

    Post FDA filter will be a 2-3 pole low-pass from around 120-140 MHz.

    Hard to know what gain we will use, since I have not yet finished design the complete chain.