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THS4541: Front End design of ADC using FDA

Part Number: THS4541
Other Parts Discussed in Thread: ADC3664,

Dear Team, 

In my project I have to design a front end for ADC3664. 
Here I am mentioning the input specifications. 
Input signal Amplitude:: Bipolar: -1.6 to +1.6V
pulse width: 50ns 

I am planning to use THS4541  for single ended to differential end conversion. 
Here my doubt is, do need to use split supply or can I use single supply  for the above mentioned bipolar signal?

Waiting for your answers

Thank you 

  • Morning SVJS, 

    The FDA will level shift the output and input voltage swings to operate on single supply via the common mode control loop. You only need a single supply on this negative rail input, RROutput device. 

  • Hi Michael, 

    I am not fully clear with the reply you said. 

    Can you please explain, in which case, we need to prefer Split Supply and how it will effect the common mode voltage. 

    The ADC's Vocm in 0.95V, so I need to connect this to THS4541 Vcm. Is it correct?

    I am referring the Evaluation Board of ADC3664 . In that FDA is THS4541 and used -0.23V in Vs- and 3.3V in Vs+. How it is calculated? How it is affecting the input ? If it's Bipolar, do we need to take anything instead of unipolar signal? 

    Thanks in advance for your support.

  • I can't find that text you inserted, but the schematic looks to be a bit in error on input matching. The text does seem to say 0.95V Vcm input to the FDA Vcm which is fine, then 2.8Vpp full scale FDA differential output is then a +/-0.7V on each output for a min. swing to 0.25V on the output pins. We sometimes include the -0.23V supply to get extra headroom, but it does not seem necessary here. 

    To undertand what is happening for the FDA to take a bipolar DC coupled input to operating inside a 3.3V single supply, review this article where the 2nd piece talks about this.

  • Hi Michael, 

    Thank you for your kind support. 

    So, as per above document, I am concluding below points. 

    My input is single ended Bipolar  -1.6 to +1.6V  (Pulse)

    FDA I selected is THS4541.

    So, I am using Vs+ = 3.3V, Vs- = GND, not preferring -0.23V. 
    Vocm is setting as 0.95V, that is directly coming from ADC3664's Vcm. 

    ADC's FS is 3.2V 

    Please confirm these above constraints.


  • Correct except the 3.2V is 3.2Vpp max input differential, then -1dBFS (normal max target) that is 2.8Vpp differential or +/-0.7V on each FDA output pin.