Using the Configure Chan Characteristics function, you can configure the less common vertical parameters.

Input Impedance

The input impedance parameter allows you to switch between 50 Ω and 1 MΩ input impedance, depending on your digitizer. Refer to Features Supported by Device to find out the input impedance your digitizer supports. The allowed vertical ranges may vary depending on the input impedance. If you measure a signal from a 50 Ω output function generator, set the digitizer for 50 Ω input to match your source. Impedance matching becomes much more important with higher frequency signals to avoid reflections of the signal that may distort your measurements. Alternatively, if you probe a circuit, it is best to use the high impedance setting on the digitizer to avoid changing the characteristics of the circuit.

To protect the 50 Ω resistor, a thermal sensing circuit opens the input if the power dissipation is too high. If this occurs, a warning is returned from all Read, Fetch, and Status functions. The circuit tries to reset itself when Read, Fetch, or Status functions are called, except on SMC-based devices, which require you to reset the circuit by initiating a new acquisition. Some digitizers also support the detection of an ADC overload. ADC overload detection works the same as the 50 Ω overload detection circuit described previously.

Max Input Frequency

The max input frequency parameter sets the –3 dB cutoff frequency for a hardware analog filter. Setting this parameter to zero uses the default bandwidth of the digitizer. The filter attenuates signals greater than the cutoff frequency, which is useful for minimizing high-frequency noise when sampling at lower rates. For example, if you sample at 100 MS/s, you can resolve frequencies up to 50 MHz according to the Nyquist theorem. Any noise in the signal above 50 MHz, such as harmonics of the input signal, are aliased onto a frequency below 50 MHz. The solution is to filter this noise before the signal is digitized. Refer to Features Supported by Device to find out the valid –3 dB bandwidth your digitizer supports. This hardware filter is not available in all digitizers.