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.
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.
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