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
The input threshold voltage can also be varied independently in each channel by applying an external DC bias voltage or shunt resistor to the D input.
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: