LM393: Slew Rate Requirement

Hello Jim,

A slew rate of 1V/ms would be equivalent to a 3.3ms rise or fall time. All of our comparators can meet that.

The LM393 can meet that, but I caution against using the LM393 at low supply voltages. The specified input range is up to Vcc-2V, so the maximum input voltage would be +1.3V. So make sure the input voltages are between 0V and 1.3V, or use a R-R input device such as the TLV1702.

Hi Paul,

Thanks for the reply.

I found that there is a LM393B. Is this the same? 

Jim

Hi Paul,

    Can I add capacitors on the inputs of comparator, V+ or V-?

previously I was told that it can cause oscillation by adding capacitor on the input of comparator?

Thanks,

Tony

Hi Tony,

It depends on the circuit configuration, resistor values and the size of the capacitor. Why do you want to add capacitors between the inputs?

It is common to add small (~30pF) capacitors to provide EMI/RF protection. We also see larger capacitors (1nF to 1uF) added to "filter" the input signals to prevent chatter.

You do NOT want to add a (large) capacitor to the positive input node *IF* the circuit contains hysteresis feedback (positive feedback). The cap will negate the feedback action by slowing the feedback response (causing "chirping").

Adding a capacitor to the negative input is usually okay, but it will slow down the reaction time (all filters have a delay) if the negative input is the measured signal.

Adding capacitors between the inputs of an op-amp is a nice way to create an oscillator, but usually does not cause an oscillation for a comparator (again, depends on the circuit - you may accidentally create a relaxation oscillator). But that can seriously affect the response time for a comparator, and, depending on the size of the capacitor, can couple signals from the positive input to the negative input, creating other issues.

Reference voltages should be filtered, and we recommend adding hysteresis if the monitored signal is noisy.

Hi Paul,

   one more question,

 I saw oscillations on the output of comparator, can you give me some suggestions to overcome this issue?

I can either add Hysteresis or capacitors on the input of comparator, which way is better?

someone told me if I use fast slew rate comparator, I can solve this issue without additional changes, is it right?

Thanks,

Tony

Hi Paul,

  Our input signal slew rate is about 300mV/ms. The chatter is on the middle of output signal going from high to low? 

can this problem being solved by adding Hysteresis?

Thanks,

Tony

Hi Tony,

Yes. As the inputs come closer together (almost equal), they will trigger on internal noise until the difference overcomes the internal offset voltage.. A little hysteresis should fix that.

But also make sure that the supplies are properly bypassed, both at the supply pins (0.1uF ceramic) and also at the supply point of the pull-up resistor (if located elsewhere).. Also make sure the inputs are routed away from the output trace and that the ground pin is returned to a solid ground.

Do you have a schematic you can share?

Hi Tony,

problem solved? If not, can you show a schematic of your circuit?

Kai