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

Digital Signal Level Shifting

In single-supply operation, digital systems frequently require converting a pulse train of varying polarity into either a positive or negative pulse output. This application note explores three straightforward circuits that facilitate the easy and reliable conversion of digital signal levels. The MAX913 comparator is utilized in the design.

The circuit illustrated in Figure 1 operates on a positive supply and can transform a negative pulse train into a positive pulse output. The comparator shown in the diagram (MAX913) offers both in-phase and inverting outputs (if the system only needs one output polarity, a single-output comparator can be chosen). The comparator’s inverting input voltage range spans from 1.8V to 3.0V. By selecting R1 = R2, the comparator's non-inverting input voltage can be set to 2.5V. The comparator’s output generates the positive pulse train shown in the diagram.

Figure 1. This circuit, powered by a positive supply, accepts a negative pulse input and generates two complementary bipolar outputs.

The circuit depicted in Figure 2 functions on a negative supply and can convert a positive pulse train into a negative pulse output. The comparator's inverting input voltage range lies between -1.8V and -3V. When R1 = R2 is 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. This circuit, powered by a negative supply, accepts a positive pulse input and generates two complementary bipolar outputs.

Figs. 3 and 4 utilize the comparator as a buffer to offer a circuit interface for systems where the input signal polarity is opposite to the system power supply. The circuit in Fig. 3 allows the positive power system to accept a negative pulse signal; in Fig. 4, the input signal is positive, and the system power supply is negative. Both circuits employ 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 transforms 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 transforms the positive pulse output to a negative pulse output that works with the positive supply comparator and the negative system supply.