HFA1135 Drive Capability study

The data sheet for the Intersil HFA1135 amplifier specifies a respectable output current delivered into a 100 ohm load resistance.
However, the data sheet also indicates a sensitivity to capacative loads.   Two ATWDs in parallel constitutes a capacative load too large to ignore.

Test Digital Optical Module Settings:
The DOM's internal pulser set at 300 (DAC 12)
The clamp level (DAC13) was set at +200mV with respect to ground (-1.744 with respect to pedestal).
The pedestal was set at 1.944 V, safely within the common mode range of the amplifier..
Room temperature. (Common mode range has a positive temperature coefficient. Performance degrades at -40°C)
Measurements taken with one active oscilloscope probe monitoring the output of the delay line, and the another active oscilloscope probe monitoring the ATWD side of the stage output resistor.


The "^" below the screen snap indicates the choice for Rev 3 of the DOM MB.
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The Effects of Violating the Common-Mode Limit for HFA1135 Amplifier
Vcc=+6.5V, Vee=-3.2V, Vpededestal=2.35V, Analog Ref=2.5V (Operating within Common Mode Range)  Rf=243Ω,  Rg=51.1Ω  (larger than suggested by the data sheet for the given gain)
  Vcc=+5V, Vee=-5V, Vpedestal=2.35V, Analog Ref=2.5V  (Operating at edge of Common Mode Range)   Rf=243Ω,  Rg=51.1Ω   (larger than suggested by the data sheet for the given gain)
10 ohm output resistor screen snap
Not acceptable: Ringing on trailing edge due to capacative loading.
 10 ohm output resistor screen snap
Not acceptable: Ringing on trailing edge due to capacative loading.
Performance with 15 ohm output resistor
Not acceptable: Ringing on trailing edge due to capacative loading.
 Performance with 15 ohm output resistor
Not acceptable: Ringing on trailing edge due to capacative loading.
Performance with 20 ohm output resistor


  • Possibly acceptable.
  • Rise-time better than 20Ω case to the right.
  • Rejected by Azriel
 Performance with 20 ohm output resistor
^ 
  • Operating point near or outside of common mode range at room temperature.
  • Probably acceptable back-porch on trailing edge of pulse.
  • Evidence of distortion (kink) on the leading edge of the pulse.
performance with 25 ohm output resistor
^
  • Leading edge looks distinctly better than for the any case in .column to the right
  • Better rise-time than 20Ω case to the right..
  • Doesn't have the hint of a back porch that the 25Ω version to the right.
  • This pulse shape is favored by Azirel.
 performance with 25 ohm output resistor
 

  • Kink in leading edge
  • Vestigial back porch
performance with 30 ohm output resistor
  • Possibly acceptable
  • Slower rise-time than case above owing to the larger time constant of Rout x Cload
 performance with 30 ohm output resistor
  • Kink in leading edge
  • Distortions in trailing edge