Stand alone Hi-speed dual channel Comparator modules from Pulse Research Lab detect sinewave and small signals (as small as 10 mV) and convert them to logic-level outputs. PRL offers models with complementary TTL, NECL, LVPECL, or RS-422 outputs, and two different sets of input thresholds.
All models have an input voltage threshold selectable by toggle switch, with a positive voltage threshold, a negative voltage threshold, and a "zero-crossing" threshold. Zero-crossing is best for converting AC-coupled sinewaves.
When the input signal (blue, in the image below) exceeds the internal voltage threshold (red), the output (green) will transition to a logic High. Otherwise the output will be at a logic Low.
Two sets of voltage thresholds are available:
Standard ±50 mV thresholds, best for converting sensor outputs, weak photodiode outputs, or heavily attenuated pulses.
±400 mV pulses, best for Fast Negative NIM signals (-400 mV), non-standard logic, noisy sensor outputs, or sinewaves that are offset from ground
If the external DC bias has a sufficiently low output impedance, it will over-ride the internally-generated VTH, and the toggle switch setting is a don’t-care.
For an external shunt resistor, the effective threshold voltage, VTHE, will be the result of the resistor-divider network formed by the external shunt resistor and the internal 50 Ohm termination to the selected VTH.
In the following example, the internal VTH is set to +50 mV, and it is pulled down to an effective VTHE of +10 mV via a 12.5 Ω shunt resistor: