Debugging a op-amp based comparator
Any help would be appreciated.


The THS3491 is not a conventional voltage feedback op-amp but a current feedback op-amp and, because of this, it has a very low input impedance (circa 15 ohms) at the inverting input. It also requires "headroom" on the non-inverting input of greater than 4.3 volts from either supply rail. See page 6 of the data sheet.

The fact that you may have it working with signals that don't share a common ground with the op-amp's power rails is pure luck. You are likely to destroy the device the way you are driving the inverting input (fed from D_bar).

To use this device correctly you should study what current feedback amplifiers require from an input source.

I would be tempted to use a split rail of -5 volts (to give input headroom) and +26 volts then operate the op-amp with a non-inverting gain of about 7 to convert a 3.3 volt 5 MHz to about 24 volts p-p. You will also need a bias voltage to offset the output positively to make an output waveform going from approximately 0 volts to 24 volts.