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As the name implies, an encoder line driver sources or “drives” current down the line (signal cable). Unlike the open collector outputs, line driver chips actively force the output high or low, therefore being able to both sink and source current from the load. The main advantage of the line driver is its ability to push higher current through the cable, enabling longer cable runs.
Example of a Typical Encoder Line Driver Circuit
Selecting an appropriate incremental encoder line driver starts with the requirements of the system:
The longer the cable run, the higher the impedance, which degrades signal strength. Longer cable runs are also vulnerable to electronic noise. Choose a line driver compatible with the distance involved. At the same time, avoid over specifying. This mistake doesn’t just squander money, it can also introduce signal problems.
Determine the voltage requirements of the control/readout device. Confirm that the power rails on the machine can supply the required input voltage to the encoder, as well.
The most common output formats are high transistor logic (HTL) and transistor-to-transistor logic (TTL). Each corresponds to characteristic output voltages as follows:
HTL is typically used for push-pull output drivers. Also known as a mirrored-voltage output, it causes the encoder to generate output with the same voltage as the input. If you supply an HTL encoder with a 12 V input, it will deliver a 12 V output. If you supply it with a 5 V input, it will generate a 5 V output.
TTL format uses RS-422 signaling, making it a good fit for noisy environments. The TTL output circuit generates a locked 5 V output signal. TTL outputs can be set to take inputs from 4.5 to 5 V, or 10 to 30 V. This gives users the flexibility of taking a range of input voltages but still generating the output voltage required by the controller or readout device.
The drawback to the TTL encoder format is that it has a strict minimum threshold for input voltage. If the voltage in is 4 V rather than 4.5 V, for example, the encoder will be in an undervoltage condition. The HTL format offers much more flexibility for systems with variable input voltage.
Encoder line drivers can be configured to generate TTL- or HTL-compatible logic levels with a minor hardware change at the board level. The modification results in output that matches the standard voltage levels.
Probably one of the most widely recognized encoder line drivers is the 7272. This is a standard line driver offered across industry. It offers a good compromise among current capability, useable voltage range, and chip cost. 7272 chips have current output capability of 40 to 50mA, corresponding to cable runs of 15 to 20 m. It is available with TTL format for a locked 5 V output or HTL format for flexible output.
User should be careful to properly specify the pull-up resistor so that the encoder does not go into an overcurrent condition. Above a certain temperature, the chip goes into protective mode, which can lead to “dropping out” or stopping output. Below temperature threshold, the chip will reset but repeated faults will eventually lead to unreliability or even premature failure.
Another very prominent output choice is the 4469 encoder line driver. Unlike the 7272, the 4469 has much higher current-driving capability, typically in the 100mA range, depending upon manufacturer. This is a definite advantage in ap¬plications with longer cable runs, usually between 100 and 300 feet. However, this higher current capability does come with a drawback. Higher current means greater heat dissipation, which restricts voltage range. A typical mirrored-voltage output is limited to 15VDC, but input voltage can be increased to 24 volts if a regulated 5 volt output is chosen.
As far as cost is concerned, the 4469 encoder line driver is comparable to the 7272. The 4469 does not have the automatic protective function for overcurrent. It can be driven as hard as possible without worry of a trip-out, but this can have an obvious adverse effect on its useable life.
The 7272 provides a wide voltage range but only limited cable runs. The 4469 can accommodate much longer runs but only with voltage limitations. For applications that require both characteristics, choose a high-power MOSFET like the ic-DL encoder line driver. These high-power MOSFETs offer the best of both worlds: wide voltage range and high current capability to support long cable runs.
The MOSFETs have less voltage drop on the output so the output signals are closer to VCC and ground than that generated by either the 7272 or 4469 chips. The result is an output signal with a wider voltage differential and a faster rise time than the other line drivers. The trade-off for faster switching is that the signal will have some overshoot and ringing. Whether this becomes a problem depends on the other elements of the system like the drives. The impedance of a longer cable run acts as a natural filter. The trade-off is a certain amount of signal degradation but the MOSFET generates higher output current, so they provide better overall performance.
Another benefit of the ic-DL is its adaptive circuitry. This line driver will adapt its output impedance to match load requirements and the type of cable being used. This helps ensure a clean, crisp signal back at the controller regardless of the type of cable, length, and control device. Like the 7272, the ic-DL also features a protective function against over-temperature.
A true “universal voltage” of 5-30VDC can be applied with the ic-DL chip, but this comes at a price. The typical component cost of this kind of chip can be up to five times that of the 7272 or 4469.
The increased voltage/current capacity also enables the ic-DL encoder line driver to support cable runs well in excess of 500m.
See all incremental encoders with High Power MOSFET line driver options here
The 7273 is an open-collector encoder line driver, a simple, economical solution for wide range of applications. It is a sinking type output driver, making it unsuitable for longer cable runs or high-noise environments. For the right application and paired with an appropriately sized pull-up resistor, a 7273 line driver can be a very good solution.
Defined by the TIA standard, the RS-422 is a transceiver that can be used with encoders as a differential line driver. The chip offers TTL-style output with a five-volt regulated output. It is frequently used in legacy systems as a simple, economical solution. RS-422 physical media also can be used with absolute encoders to connect up to 10 devices.