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THS4500: Single Ended to Differential Conversion Distortion

Brett Wakley
Prodigy 60 points

Part Number: THS4500

I am getting quite a bit of waveform distortion when using the THS4500 to convert a single ended signal to a differential signal for driving an ADC. I think this is due to my VCM being too low, but wanted to check this.

These are the sort of waveforms I am seeing. Green (CH 2) is the input waveform at 10MHz, blue (CH4) is the output waveform, measured with a differential probe.

image.png

My circuit is as follows. The THS4500 is operating from a single sided +5V supply. The inverting input is biased to 2.25V with a potential divider that has the same impedance as the input resistor on the non-inverting input.
image.png

I am using the Common Mode output from my ADC to drive VCM. It looks like this can sufficiently drive to overcome the 25k input impedance, but it is only 0.9V. Looking at the datasheet I think the limit is 1V from the rails (i.e. between 1 and 4V). It looks like the individual output pins never drop below 1V when I probe them single ended (apart from some ringing), which I think stacks up with the datasheet quoted values of +-3.3V differential output swing with a common mode voltage of 2.5V.

Is my understanding on all this correct?

 

  • 0
    TI__Intellectual 2880 points

    Hi Brett,

    You can't drive the inverting input directly with VMID like that. You'll need a 270ohm gain setting resistor on that leg as well to keep the symmetry. 

    Sounds like you are running the device single-supply 0V --> 5V. And it looks like your input source common-mode would be mid-supply (2.5V). If so, you'll either need a buffer to drive the VMID reference for the FDA or you'll need to change your VMID resistor divider like so to keep the FDA gain of 1V/V.

    If your Vocm is 0.9V and your Input Vcm is 2.5V then its a resistor divider with the gain resistors to get the Vicm of the amplifier. From there you can do KCL with the VMID resistor divider so make sure its set to 2.5V. 

    Hope this is helpful.

    Thanks,

    Evan