Digital signal level conversion When powered by a single power supply, digital systems often need to convert a pulse train of different polarity into a positive or negative pulse output. This application note introduces three simple circuits that can easily and reliably convert digital signal levels. The MAX913 comparator is used in the design.
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The circuit shown in Figure 1 uses a positive power supply, which can convert a negative pulse train into a positive pulse output. The comparator (MAX913) shown in the figure can provide both in-phase and inverting outputs (if the system requires only one output polarity, a single output comparator can be selected). The inverting input voltage range of the comparator is between 1.8V and 3.0V. Select R1 = R2 to set the comparator non-inverting input voltage at 2.5V. The output of the comparator is the positive pulse train shown in the figure. 
Figure 1. The circuit uses a positive power supply, accepts negative pulse input and produces two complementary bipolar outputs.
The circuit shown in Figure 2 uses a negative power supply, which can convert a positive pulse train into a negative pulse output. The inverting input voltage range of the comparator is between -1.8V and -3V. Select R1 = R2 to set the comparator non-inverting input voltage at -2.5V. The complementary output of the comparator provides a negative pulse train. 
Figure 2. The circuit uses a negative power supply to accept positive pulse input and produce two complementary bipolar outputs.
Figures 3 and 4 use the comparator as a buffer to provide a circuit interface for a system with an input signal opposite to the polarity of the system power supply. The circuit in Figure 3 enables the positive power supply system to accept negative pulse signals; in Figure 4, the input signal is positive and the system power supply is negative. Both circuits use NPN transistors to shift the output level of the comparator by VBE (R5 + R4) / R5 ≈ 4.5V (for single-phase output, a single output comparator can be selected). 
Figure 3. This circuit converts the negative pulse output to a positive pulse output, and can work with the negative power supply comparator and the positive system power supply. 
Figure 4. This circuit converts a positive pulse output to a negative pulse output and can work with a positive power supply comparator and a negative system power supply.