Digital signal level conversion - power circuit - circuit diagram - Huaqiang Electronic Network

Digital signal level shifting

In single-supply operation, digital systems often need to convert a pulse train of different polarity into a positive or negative pulse output. This application note describes three simple circuits that make it easy and reliable to convert digital signal levels. The MAX913 comparator is used in the design.

The circuit shown in Figure 1 is powered by a positive supply and is capable of converting a negative pulse train into a positive pulse output. The comparator shown in the figure (MAX913) can provide both in-phase and inverting outputs (if the system requires only one output polarity, a single-output comparator can be selected). The comparator inverting input voltage range is between 1.8V and 3.0V. Select R1 = R2 to set the comparator non-inverting input voltage to 2.5V. The output of the comparator is the positive pulse train shown in the figure.

Figure 1. The circuit is powered by a positive supply and accepts a negative pulse input and produces two complementary bipolar outputs.

The circuit shown in Figure 2 is powered by a negative supply and is capable of converting a positive pulse train into a negative pulse output. The comparator's inverting input voltage range is between -1.8V and -3V. With R1 = R2 selected, the comparator's non-inverting input voltage can be set to -2.5V. The complementary output of the comparator provides a negative pulse train.

Figure 2. The circuit is powered by a negative supply and accepts a positive pulse input and produces two complementary bipolar outputs.

Figures 3 and 4 use the comparator as a buffer to provide a circuit interface for systems where the input signal is of opposite polarity to the system power supply. The circuit of Figure 3 enables the positive power system to accept a negative pulse signal; in Figure 4, the input signal is positive and the system power supply is negative. Both circuits use an NPN transistor to shift the output level of the comparator by VBE (R5 + R4) / R5 ≈ 4.5V (for single-phase outputs, a single-output comparator can be selected).

Figure 3. This circuit converts the negative pulse output to a positive pulse output that works with the negative supply comparator and the positive system supply.

Figure 4. This circuit converts the positive pulse output to a negative pulse output that works with the positive supply comparator and the negative system supply.