Electronic – Turning the output of an opamp into a square wave

operational-amplifier

I have a 5Mhz signal which is being amplified by an OPA847 opamp.

In good conditions I get a voltage swing of ~120mV output from the opamp, in bad conditions I get a slightly noisy ~80mV p-p. I am hoping for the final circuit to be single supply, the current test setup uses -5v/+5v.

The scope trace below is from good conditions, modulation can be seen in two places where the phase inverts:

Scope trace in good conditions

My question is quite simple: what is the best way to turn this signal into a square wave for processing by digital logic?

I have considered standard digital logic gates but I don't believe the swing is large enough. I have also considered a comparator with the input signal (biased at Vcc/2) compared to a fixed Vcc/2. I assume this would need some hysteresis but am still not sure if it's the most practical solution.

Best Answer

I'd use a MAX999 comparator. Feed the signal into one pin via a series capacitor and biasing resistor to mid-rail then on the other pin have a resistor feeding a capacitor so that the average level can be followed - it's called a data slicer: -

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With the waveform in the OP's picture looking like it could undulate its average level a fair bit, using a data slicer probably makes sense.

The MAX999 has a 4ns propogation delay and can work from 5V or 3V3 rails. It would directly interface with logic as I have done several times on previous occasions.