page 14 summing amplifier-2

The circuit is an operational amplifier used as an inverting differentiator. This circuit can be found in multitude of references such as books, magazines, web pages, etc. Rf = Vout · deltaT / (Cdif · deltaV) For ±10 V output swings: Rf = 10 V · 500 µs / (0.1 µF · 10 V) Rf = 5 kΩ This results in a noisy output, the op-amp is operating in linear region. To get a cleaner square wave output, the op-amp output should be set to switch to the maximum (positive) and minimum (negative) voltage levels. For the operational amplifier used in the circuit: Positive voltage swing VOMP = 12 V, Negative voltage swing VOMN = -12 V. Rf = 12 V · 500 µs / (0.1 µF · 10 V) Rf = 6 kΩ The closest standard value is 5.97 kΩ, this results to a greatly improved output waveform but what is needed is slightly higher than 6 kΩ. A standard value of 6.04 kΩ gives better result. Increasing the resistance gives better margin that the output will switch to maximum and minimum voltage levels but as the resistance is increased (for example, using the more common 6.2 kΩ) output switching delay begins to manifest.