Home | <Research on existing front-end solutions | Stage 2 Offset Compensation and Op-amp Input Stage>
This stage takes input from a connected probe and prevents the oscilloscope from putting load on the device under test. If the input impedance of the oscilloscope is low, the amplitude, shape and other characteristics of the measured signal would be changed. This would distort the results of the measurement. A high impedance input will also serve as a simple protection against high voltages.
As described in , a capacitive voltage divider improves the high frequency response of the input stage of the front-end.
This impedance or capacitance input stage is also called a compensated attenuator.
The input signal, in connection with the OpenLab front-end, is attenuated by the factor of 2. This will improve the general voltage input range of the OpenLab oscilloscope. As an additional protection against voltages above the rated oscilloscope specifications, two diodes (D6) clamp the voltage if leaving the allowed range between +5V and -5V .
Figure 1 shows the segment of the front-end schematic which includes the first stage.
To fulfill the standard input-coupling function of a modern oscilloscope, the signal path can be switched between DC and AC coupling (SW1). In DC coupling-mode the measured signal is directly connected to the impedance/capacitance divider. This allows the user of the oscilloscope to measure AC and DC components of the signal. Switching to AC coupling, the signal is fed through a capacitor. This capacitor filters only the DC components of the signal. This ensures that the oscilloscope only displays the AC part of the signal.
Figure 2 shows the output of stage 1 in relation to a test input signal. It is a sinusoidal signal with a frequency of 1kHz and a amplitude of 5V. Stage 1 is directly connected to the simulated oscilloscope probe.
The waveform drawn in red represents the input of stage 1. The output is shown by the blue signal. Because of the characteristics of the simulated probe, the test signal gets attenuated by a factor of 10. This exactly simulates the behavior of a real 10:1 oscilloscope probe.
KALSI, H.: Electronic Instrumentation, vol. 2. Tata McGraw-Hill Publishing Company, 1995
STEFAN SALEWSKI: Digitales Speicher-Oszilloskop (DSO) [Online] Homepage of Stefan Salewski