Factories have noisy environments that can make transmitting data over long distances a nightmare ---- unless the noise can be eliminated at the receiver. High-speed optocouplers that can handle data rates of up to 15 Mbaud can isolate high frequency signals typically found in the factory data transmission. These optocouplers, operating in conjunction with line drivers and receivers, can provide insurance to avoid downtime from unexpected errors due to noise. This is especially important in a factory that is frequently reconfigured for manufacturing different products and where motors and relays are part of the operation. This article will provide background on optocouplers and discuss their use in applications.
Figure 1: A typical factory has several motors, relays, sensors, and switches operating in the same area where high-speed data needs to be transmitted and received.
Isolating the noise
Local area data communication systems in a factory environment can be exposed to high levels of noise. The digital interfaces, motor control and sensor signals are all areas where isolation can provide noise reduction. Figure 1 shows the various inputs, outputs and data paths that need isolation in a typical factory situation.
These interfering signals appear between the input and common or ground of the sensor data receiver. This type of noise is termed common mode noise. Grounding and shielding are all part of good engineering practices to reduce the influence of common mode noise. When these noise reduction techniques are not adequate, the designer improves the noise rejection by physically separating the common mode noise source from the interfered circuit. Such physical separation reduces the common mode coupling capacitance, which appears between the input and output of the interface being isolated. Examples of galvanic separation techniques that offer this physical separation, as well as minimal common mode coupling capacitance, are interfaces that use fiber optics and optocouplers.
Another source of electrical noise in a factory, caused when electrical motors, relays, and solenoids are switched on and off, can generate turnoff voltage transients or spikes. The most common way of describing these types of interfering signals is to specify their peak signal and rate of signal change, or dv/dt. Numerous ANSI and IEEE standards exist that describe the kinds of interfering signals, peak voltages and rates of rise and fall dv/dts) found in various industrial environments. The ability of an interface to reject these interfering transients describes the system's common mode transient rejection (CMTR).
High-speed optocouplers vs. current noise solutions High-speed optocouplers include a specification describing their common mode transient rejection capabilities. Logic compatible couplers, such as those from Fairchild's Optologic family, have output in either a logic high or logic low state. The noise immunity or CMTR capability of these couplers are defined as that peak voltage and rate of rise that will cause a change in the optocoupler's logic output. The following equation defines CMTR:
An Optologic device has a gate that can withstand a wide range of peak transient voltages and rates of rise. It will be immune to 250V peak transients, with a rate of rise in excess of 40kV/us. It will also be immune to peak transient voltages that approach the insulation breakdown (>7KV) of the optocoupler if the rate of rise is less than 1KV/us. The part has a typical specification of 15kV/us, with a Vcm of 50V peak.
When common noise problems occur, or when manufacturing engineers want to avoid problems before they occur, especially in situations where the manufacturing area is frequently reconfigured, optoisolators provide an ideal solution. Optoisolators, working in conjunction with line drivers and receivers, break the galvanic connection between the grounds that could be conducting electromagnetic interference (EMI) or AC noise to attenuate common mode noise. They also have DC transparency. Including high-speed optically isolated gates in the factory data communications design greatly improves the noise immunity.
Optocouplers in the network
In the past the use of optically coupled isolators used simple phototransistor optocouplers and noise rejection was not an issue. For these lowspeed devices, the noise was outside of their bandwidth and no specifications for noise immunity appeared on their data sheets. But today as the data rate increases, a broader bandwidth system is required. For high-speed optocouplers the noise is in band--it has the same spectral content as the data communications signal. In this case, the optocoupler's common mode transient immunity (in kV/us) is a measure of its effectiveness in attenuating noise in a high-speed data communication network.
Optocouplers, such as the Optologic family of devices, have been designed with high-speed logic inputs and outputs and are well suited to connect into a data communications network. These optocouplers provide a fully integrated optically isolated logic-to-logic system. For instance, Fairchild's 74OL6000/01 optocoupler has an input section with an amplifier that provides the interface between driving LSTTL gate and the LED emitter (Figure 2). The output section consists of a multistage high-speed amplifier and either a totem pole or open collector output.
Figure 2: Block diagram of an Optologic optocoupler.
The input amplifier IC switches a temperature compensated current source driving a high-speed 850 nm AlGaAs LED emitter. This integration technique eliminates current transfer ratio (CTR) degradation over time and temperature. The emitter is optically coupled to an integrated photodetector/high-gain, highspeed output amplifier IC. The two input chips and the output chip are assembled in a 6- pin, DIP high-insulation voltage plastic package. The package provides a withstand test voltage of 5300 VRMS (1 minute). In addition, the packaging puts the emitter and detector are in the same plane.
This approach has advantages compared to the over/under configuration of optocouplers where the emitter is on top of the detector in the package. The coplanar packaging has lower input/output capacitance and, as a result, improved common mode noise performance. The Optologic family's 15kV/us commonmode noise rejection is ensured through the use of an optically transparent noise shield. This Faraday shield is represented by the dotted line in Figure 2.
The TTL compatible output versions of this product (74OL6000/01) have a totem pole with a fan-out of 10. The CMOS compatible output versions (74OL6010/11) have an open collector Schottky clamped transistor that interfaces to any CMOS logic between 4.5 and 15 volts. The open collector versions may also be used to drive power MOSFETs or transistors up to 15 volts. The output transistor will drive 10 standard TTL loads with a VOL of 0.4 V, and its safe operating range allows it to sink up to 60 mA peak. The active pull-up will source an IOH in excess of 10 mA with a VOH greater than 2.4 V. In addition, the Optologic coupler family typically offers propagation delays of 60ns and can support 15 Mbaud data communication. They can also handle DC, and so are well suited for the signals found in an industrial control environment.
A bi-direction optocoupler network
A bi-directional, optically coupled 1 Mbaud network is shown in Figure 2. This kind of approach is ideal for situations where there are very high levels of noise and voltage differences. In a factory, examples of this network include an electric arc furnace or welding process or an 800V, 3-phase motor. These are not the highest speed applications but isolation is definitely required in addition to DC transparency.
The full duplex, point-to-point communications system is implemented with twisted pair shielded cable. Given the high impedance of this type of cable, it is possible for the optocoupler gates to drive the line directly. In Figure 3, Fairchild's 74OL6000 buffer and 74OL6001 inverter are used in a push-pull mode to differentially drive the line. The receiving end of the line is simply terminated in Zo. Bridging this termination is a differential line receiver that is connected to the 74OL6000 Optologic gate. Power for the line receiver and the Optologic gate is derived from two insulated shields of the twisted pair cable. This system offers a data rate is excess of 1 Mbaud NRZ at a distance of 600 feet.
Figure 3: Full duplex differential optically isolated transmitter and receiver with shielded twisted pair.
A 15Mbaud optically isolated network
The system shown in Figure 4 illustrates an optically isolated transmitter and simplex multi-drop receiver system for speeds up to 15 Mbaud. In this unidirectional system, Optologic input amplifiers offer the feature of very high input impedance that permits their use as bridged line receivers. This application uses a 1000-foot aerial suspension 75- ohm CATV coax cable with data taps at 250-foot intervals. The cable includes a third insulated conductor that is used as the VCC supply source for the input amplifier of the optocoupler gates connected to the transmission line. This third conductor permits one simple isolated supply to power all the optocouplers connected to the communications cable.
In Figure 4, the 74OL6001s optocouplers function as bridged receivers, and as many as 30 receivers could be placed along the line with minimal signal degradation. The communication cable is terminated with a single 75-ohm load at the far end of the line. The VCCI (pins 1 and 3) and VCCO (pins 6 and 4) should be bypassed with a 0.1uF capacitor. When driving low impedance transmission lines such as the 75-ohm coax, a buffer is required to drive the line.
Figure 4: Optical isolation in a multi-drop factory network.
The signal quality "Eye Pattern" at the output of a 74LS04 logic gate at one of the multidrop locations in Figure 4 is shown in Figure 5. The input (upper) and output (lower) traces both have vertical scale of 2V/division. The horizontal scale is 20ns/division for the 100ns pulse width. The measurements were made with a 10MBaud NRZ Pseudo-Random Sequence (PRS). The data quality is well preserved with only a 20 percent eye closure, the difference between the two edges, (20 ns per pulse width of 100ns) being observed at the receiver located 250 feet from the transmitter. This level of performance is well within the requirements of most data transmission networks.
Figure 5: Eye patterns from the optocoupler in a high-speed data network.