The digital input buffer includes the lowpass filter plus AND gate as seen in Figure 10-1. The combination can be used as a pulse shortener allowing a regular function generator to achieve 1-2 ns pulses on IN+. Most function generators can only output pulses as low as 10 ns, therefore the ability to create a 1-2 ns pulse can be achieved on the EVM using the AND gate input (J3). The AND gate compares the input (J3) to a RC delayed version and when both inputs are high the output will go high. The input pulse width or falling edge from the function generator can then be fine tuned in order to see the desired pulse width on IN+. When first starting to fine tune the function generator input to see the pulse on IN+, start with 100 ns then reduce the pulse width down by the nanosecond or smaller to make 1-2 ns visible on the IN+ test point. To bypass the pulse shortener populate R3 with a 0-Ω resistor and remove R9 and R10 to disconnect the AND gate input and output respectively.

Figure 10-1 Pulse Shortener Yielding 1-ns to 2-ns Pulse on IN+