We tried to implement a differential amplifier circuit in our digital oscilloscope. The point of this device was an attempt to input a negative voltage for waves, such as sinusoids, that have voltages from -Vp to +Vp, (our range, specifically, was -2 to +2 volts,) because the ADC does not read in negative voltages. This way, by setting one of our inputs to the amplifier to -2 volts, we could add -2 to +2 volts to the other input, which would effectively provide us a range of 0 to 4 volts. Unfortunately, we had a problem with the LM358 operational amplifier in the laboratory; it seemed to be adding some kind of load impedance that we could not detect.

For instance, the -2 volt input to the amplifier was produced using a voltage divider, yet once this voltage was inputted to the amplifier it dropped all of a sudden to -1.63 volts. Also, though there was a differential voltage on the output, but it was nowhere near its expected value. An example was where the inputted voltage was around -2 volts, and another voltage was 12 volts, and the difference on the output was about 4 volts. Again, this was probably due to some loading impedance. As already mentioned, we decided to implement a simpler solution to the voltage input, by simply manually adding the DC offset. However, since a deal of effort was put into building the differential amplifier, we felt it was necessary to mention this attempt.