# Op-Amp Comparator Testing

I am having this op-amp Comparator.

My circuit is similar to this circuit present in the datasheet of the IC, page 21.

I have this circuit now in my board which I want to test. I have never tested an op-amp comparator section before.

So, can someone help me and guide over here on what tests should I perform on the above circuit and validate the behaviour of the comparator?

Like, the important parameters of an op-amp (which I know) are like, the slew rate, input voltage, input bias current, output current and feedback voltage.

I don't know what to test in the above circuit and how to validate that the circuit is working fine as expected.

Questions :

1. Don't not mind about the circuit and other circuit specifics. I just want to know what are the important tests that one needs to perform for an op-amp comparator circuit? Can someone list the most important ones?

2. How to measure the slew rate and feedback voltage of an op-amp comparator in the above circuit?

If you want to test the comparator itself, and not the fact that it is configured to have additional hysteresis, there are a few tests you can do. If you want to verify the hysteresis you can apply a slow triangle wave to the input and measure at what voltages the comparator trips high/low at. This is called a (VTC).

Voltage Transfer Characteristic (VTC) plot for hysteresic comparator

1. Probe comparator output and input, put scope into XY mode
2. Apply a low frequency triangle wave.
3. Observe and use cursors to measure the hysteresis.

1. Apply an input to force comparator output to be a constant high/low
2. Measure voltage at output of comparator with no load
3. Attach varying loads and measure voltage at outputs (be sure to not exceed abs max current!)

Rise and fall time characterization with different loads

1. Probe comparator output, and trigger on rising or falling edge.
2. Apply a large signal square wave, DC centered at the trip voltage.
3. Zoom in to the rising edge and measure rise time (10-90%, maybe 20-80%)
4. Zoom in to the falling edge and measure fall time (10-90%, maybe 20-80%)
5. Attach different loads (common is 2kΩ || 1000pF) and repeat

Propagation delay

1. Probe comparator output and input, and trigger on input rising/falling edge.
2. Apply a large signal square wave, DC centered at the trip voltage.
3. Measure the delay from 50% mark on input to the 50% mark on output.
4. Do this for switching low to high and high to low.

Max toggle rate

1. Probe comparator output and input, and trigger on input rising/falling edge.
2. Apply a large signal square wave, DC centered at the trip voltage.
3. Increase the input square wave frequency until the comparator output stops toggling

Make sure you have good bypassing for this one.

As for getting more data, you can repeat these tests at different supply voltages and common mode voltages (common mode being the trip voltage).

For the prop delay, if you have a good function generator/scope, you can measure dispersion of the comparator. I would remove hysteresis for this test. Here you change the input voltage so the input just barely trips the threshold. If your trip voltage is set at 2.5V, you apply a square wave of something like 2V - 2.51V (500mV underdrive, 10mV overdrive). Measure prop delay, and increase the overdrive to 20mV. Repeat. When the prop delay is plotted against the overdrive voltage, there should be a y = 1/x looking curve.

• Thank you for the answer. 1. Could you please tell me what you mean by, "Apply a large signal square wave, DC centered at the trip voltage" ? - Like, could you just provide some sample values or the voltage value specifications for the Op-amp testing for just understanding purposes? 2. And are these two tests enough? Any other tests regarding the current (input/output) for the Op-amp characterization? Aug 3 '20 at 8:48
• If your common mode is 2.5V, applying a square wave from 1.5V to 3.5V is a suitable test for large signal. Small signal stuff would be around the millivolt range. Op-amps have this spec distinction as well. A datasheet likely has a scope capture showing the step response of large and small signal inputs. Common mode being the trip voltage, aka the voltage at which the op-amp chances output levels. I will update my answer with a few other relevant tests in a few hours. Aug 3 '20 at 17:08
• Thank you very much for the answer. Aug 5 '20 at 2:45

Feed in 1volt PeakPkeat at 10,000Hertz. If nothing changes on th eoutput, then alter the DC offset or the amplitude

And monitor with an oscilloscope.

Yu may need to adjust the DC_offset of the sin input.

If you have a dual-channel oscilloscope, then monitor both the hysterysis voltage (Vin+) and the "comparator" output.

• What is this test? Aug 2 '20 at 7:28