Edit: MC33202 image redone with a higher V/div.
I am testing op-amps I've bought from ebay and Mouser with the classic square wave circuit:
The op-amps I tested are:
- TL082 - ebay
- MC33202 - Mouser
- LM2904 - Mouser
- TL072 - ebay
I've set up the circuit for a nominal 50 KHz frequency, R1 = 8.2K, C = 1 nF. The supply voltage was set to 9V.
The other op-amp in the package was configured as an identical oscillator (same R1 and C values, separate V+/2 rail.)
Here are the results:
TL082:
MC33202:
(old image:
)
LM2904:
TL072:
Observations:
The TL082 output is very clean. The TL082 has a high slew rate, and we're getting that here. The frequency is even higher than the programmed 50KHz.
The MC33202 output is clearly slew-rate limited. The datasheet specifies a typical slew rate of 1V/us, and that's about what we're seeing.
The LM2904 output is even more slew-rate limited (datasheet lists 0.3V/us typical), and so we get more of a triangle wave with a reduced frequency of 20 KHz. There is also some distortion on the downwards slope and bottom of the wave form.
The TL072 output is confusing, because according to the datasheet it should have a "high" slew rate.
Questions:
It looks like the TL082 chips are good, and also the MC33202s seem to be performing within spec.
I would like to understand why the LM2904 wave form has the kinks in it and the "inversion" at the bottom.
And what can be said of the "TL072" chips from ebay? Did I get sold some bogus chips?