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ADS4229: ADS4229

AJ Coetzee
Prodigy 50 points

Part Number: ADS4229
Other Parts Discussed in Thread: THS4541, , LMH5401

Concernig Vocm, and suitable fast differential amplifier on input.

I chose the THS4541 differential amplifier, because of its excellent performance. This amplifier has a  lower limit on the Vocm of 0.95V.

The ADS4229 Vocm specification is 0.95V, unless it is clocked faster than 160MHz, in which case it changes to 0.87V, which means that I cannot directly couple the ADC and the differential amplifier.

I need to couple the amplifier and the ADC with a DC offset, because my signal is unipolar and if AC coupled I would lose half of the ADC range, complicating the rest of my design.

Can you propose another differential amplifier that can work with the 0.87V Vocm? Running off 3.3V with a bandwidth of 200MHz and a slew rate above 200V/us, preferably faster.

What would be the effect if I sacrifice the 80mV difference on common mode and just allow for a smaller signal swing? ie I run the amplifier at 0.95V and the ADC at 0.87V.

Any help would be appreciated. If any other solutions exist, please make suggestions. 

  • 0
    TI__Mastermind 45670 points

    Hi AJ,

    I would recommend the LMH5401, will will allow you to move/adjust the output common mode voltage in order to interface with the ADS4229 more cleanly.

    If you use use the THS you suggested above or other amplifier that doesn't meet the ADC's input common mode voltage requirements, then the input signal will not match up to the fullscale of the ADC.

    Will will cause early onset clipping to occur and over-range the ADC. This will ultimately cause noise and distortion to occur.

    Regards,

    Rob

  • 0
    TI__Genius 15321 points

    Hi AJ,

    Here are some plots with common mode voltage for the ADS4229 (pages 20-21 of the datasheet).