Comparator#Level shifter

As a level converter and the level converter (English level shifter ) is known in the electronics, a discrete or integrated electronic circuit, that the signal levels - an information source to adjust the input signal level of an information sink - usually voltage signals. Level converter can be applied in both the analogue and in digital technology.

Furthermore interface driver or interface converter can also be seen as a level converter.

• 3.1 Unidirectional level converter 3.1.1 voltage division, the voltage limiting
• 3.1.2 level shifter transistor

• 3.2.1 level shifter transistor

Need

Must electronic modules communicate with each other, this is usually implemented via an electrical connection. Since not all the modules operate with the same voltage levels, it is necessary for communication of these assemblies to each other to adapt the signal level of the information signals. This adjustment can be done depending on the requirement by an active or passive electronic circuit.

For special applications it may be required that the electronic modules are to be communicated with each other to separate galvanically. Again, level shifters are used, which also carry the electrical isolation of the information signals in addition to the adjustment of the voltage ranges. This can happen depending on the requirement by means of optocouplers or pulse transformers.

Analog technology

For analog circuits signal level of the transmitter are usually adjusted by amplifier to the receiver. For example, the signal of a microphone is adapted by a microphone preamplifier to the input signal level range of the AUX input of an amplifier.

However, there are applications where a simple increase or decrease of the information signal is not sufficient, and the signal must also be shifted in the voltage range. For example, if an audio signal with a typical voltage range from -100 mV to 100 mV is digitized, it is not only necessary to amplify this signal, but also to shift by adding an offset voltage, for which the offer of the subtracting circuit.

Example of dimensioning

Analog- to-digital converter can measure usually no negative voltages, and therefore the audio signal to be measured has to be moved in the positive voltage range. Can now handle a voltage range of 0 V to 5 V of the analog -to-digital converter, the audio signal must be amplified and superimposed with an offset voltage of 2.5 volts (-100 mV to 100 mV) by a factor of 25. In order to achieve this level shifter, a subtracting amplifier, the inverting input of the amplifier is used as the signal path and the non-inverting input and the offset input. The output signal of the level converter is then inverted before Although, however, the subtracting amplifier can be adjusted to the reference voltage available for forming the offset. Furthermore, the inversion of the digitized signal is basically very simple.

The output voltage of the subtracting function of the input voltage Uinin ( inverting input ) is given by

Thus, a resistance ratio Rf must for a gain of -25: R of 25:1 can be selected.

Now, the input voltage range from -100 mV to 100 mV is mapped as an output voltage range of 2.5V to 2.5V. To further add to now the necessary offset voltage of 2.5 V, the resistors Ra and Rb must be selected such that (assuming 5 V ) at a given reference voltage is a resultant offset voltage of 2.5 V at the output of the subtracting.

The output voltage of subtracting amplifier depending on the input voltage Vref ( non-inverting input ) is given by

By substituting the previously calculated values ​​of the present reference voltage ( 5 V) and the required offset voltage (2.5 V), a resistance ratio Ra obtained: Rb of 51

Digital technology

In digital circuits, it is very common that different components (eg, microcontroller and LCD display ) can be operated with different supply voltages. To now allow the communication of these modules, the logic level must be adjusted to each other. In a digital communication, the connection of the modules, depending on the bus system takes place bi-directional or unidirectional. Depending on the requirements and the level converter must support this.

Unidirectional level converter

Voltage division voltage limiting

Adapt its simplest form, digital level is a voltage divider dar. Hiebei the voltage of the transmitter is divided by resistors and the reduced voltage provided to the recipient are available.

Another possibility to adjust a digital signal is a combination of serial and parallel zener diode dar. resistance through the zener diode, the signal level is maintained at a defined potential and of the cross-flow limited by the resistor.

However, the simplicity of this level converter has numerous disadvantages. Since this level converter is passive, it is thus only possible to reduce signal levels. It can thus transmit only one source of information with higher signal levels of an information sink with lower signal level information, but not vice versa.

Another disadvantage of this simple level shifter is in the power consumption. At a logic one, a constant current continuously flows, which causes losses.

Since the output current of this level converter is very small and each input of a digital circuit has an input capacity, this results in a low-pass behavior. Thus, the maximum transmission frequency is severely limited.

Level shifter transistor

To avoid some of the said problems, a transistor may be used in combination with a pull-up resistor. Here, the input level can be converted to any desired output level by the pull-up resistor is connected to a voltage source having a voltage in the level of the desired output voltage.

A disadvantage of such a level converter is now flowing in the logic zero cross current. Further, this circuit inverts the signal, thereby it may be necessary to invert the logic signal before or after the level conversion.

Bi-directional level shifter

Level shifter transistor

A way to build a simple bi-directional level shifter, is to use a MOSFET with two pull-up resistors, of which each one is connected to the desired operating voltage, respectively.

Is now applied to the input Ua a logical zero (0 V) to the source terminal of the N -channel MOSFET is connected to ground potential. Thus, a positive gate-source voltage, whereby the transistor turns on, the output Ub also draws on mass and rests logical zero.

Ua is a logic one ( the left side power supply potential, in this example 3.3 V) is applied, the source terminal is at the same potential as the gate terminal so that the transistor turns off. The output Ub is then pulled through the pull-up resistors R2 to the supply voltage potential of the right side (in the example 5 V) and where now logically exists one.

Considering now the level converter from the other direction, resulting in a logic one at the entrance Ub a similar scenario. The source terminal of the transistor is now pulled through the pull-up resistors R1 to the supply voltage potential of the left side (in this example 3.3 V ), whereby the source and gate are at the same potential, and the transistor is blocked. At the same time is through this resistor supply voltage potential at the output Ua, so that this output is set to logic one.

Will now be a logical zero at the input Ub applied (0 V), the transistor due to the gate-source voltage of missing blocks remain. Since a discrete MOSFET, production reasons, but always identifies a parallel body diode, this now begins to conduct and accuses the output Vout also at approximately ground potential ( minus the forward voltage of the diode). This now results in a gate-source voltage, the transistor begins to conduct and pulls the output Vout finally to ground, which is now applied to a logic zero to Ua.

Condition for the correct operation of the circuit is a MOSFET, the threshold voltage is lower than the supply voltage of the side A so that the transistor can switch through at all. It is further to be noted that the power supply side A must be smaller than that of the side B, otherwise the higher supply voltage flows through the body diode of the transistor to the side with lower supply voltage.

Other level converter

Level converter is available for almost all applications as integrated electronic circuit. In bidirectional level shifters, the transfer direction is usually chosen by a separate control line. A clear advantage of a level converter as an integrated circuit is the cut-off frequency, which is usually significantly higher than that of a discrete circuit.

Level converter for digital signals with any output voltage can be set up in a very simple way using a comparator. However, the maximum transmission rate is severely limited by the cutoff frequency of the comparator here.

Basically, each level shifter can also be performed as an isolated variant. Analog level converter with galvanic isolation are very similar to an isolation amplifier, albeit an isolation amplifier only amplifies the signal levels and adjusts not directly ( shifts ).

Interface driver

Interface driver can be regarded as a special form of a level converter. In addition to the actual adjustment of the signal level interface driver may have also integrated complete power conditioning circuits to generate the necessary voltages for the output level itself can. Depending on the application have interface driver corresponding outputs, such as output pairs to generate differential signals or power outputs for the operation of current interfaces.