# Switching thresholds for a comparator

Is it possible to use a comparator to detect a negative signal? I have a negative signal that goes from 0V to -1V. I want the comparator to output a logic high when the signal goes below -0.6V and a logic low when the signal goes above -0.4V. Since propagation delay is important in my application I found this comparator to be a good option.

To accomplish this I want to use a negative voltage reference of -0.5V on the Vp pin. Vn is going to be the signal input. In the datasheet on page 3 of 16 is stated that the hysteresis can be set to 120mV with a 80k resistor. A 120mV hysteresis would be fine for me, but I don't know how to connect the Hysteresis pin, can I simply connect the 80k resistor from the hysteresis pin to ground?

If I apply a voltage of -0.5V on Vp and set the hysteresis to 120mV. The switching thresholds would be -0.56V and -0.44V, right?

• @AndrewMorton I do have a negative supply Sep 11, 2021 at 12:30
• Almost any comparator can do that, specially if you have a dual supply. Sep 11, 2021 at 13:12
• What output, what speed, LT1715 may be better choice. Show some schematics, better description. Sep 11, 2021 at 13:30

Your chosen comparator ADCMP603 solves your hysteresis requirement nicely. With a standard comparator you’d have to add feedback from output to + input to get hysteresis.

The ADCMP603 can only accept signals down to -0.5V so you can’t connect your input directly to it.

But you should be able to simply pull up your -1.0 to 0.0V input into a range the comparator can handle with resistors to a postive rail. Assuming that’s 3.3V it would look something like this:

The signal now swings around ground, which is the comparator reference, and there's still enough signal swing to allow hysteresis (set by resistor not shown).

This assumes whatever is generating your -1.0 to 0.0V signal is not perturbed by the 72K pullup to 3.3V. If output impedance is high you'd have to play with resistor values to get a proper swing around ground.

Your chosen part has TTL outputs so doesn't need the pullup resistor on its output that a standard open-collector comparator requires.

• Nice napkin drawing! Sep 13, 2021 at 20:32
• Gotta use 2-ply. Sep 13, 2021 at 20:56

I think you have two options.

simulate this circuit – Schematic created using CircuitLab

Figure 1. (a) Single-ended supply. (b) Dual-rail supply.

a

• R2 and R4 set the threshold at a positive switching value.
• R1 and R3 form a voltage divider pulling the '-' input half way between V+ and IN1.
• R5 is a pull up if the comparator has an open collector / drain output.

b

• The comparator is fed from a dual supply.
• IN1 can vary from V+ to V-.
• R7 and R10 set the switching point.
• The problem is that the output will be pulled to V- which is, most likely, not compatible with your logic. R11 and D2 will clamp a low output at > -0.4 V or so.

You need to figure out if option (b) is suitable for your logic family. If D2's Vf is greater than that of the logic input then the logic input will bear the burden of the clamping current and may cause premature failure.

I did not read the datasheet for your specific comparator so check carefully.