The RS-422 specification requires certain differential line receivers to be able to detect small differential voltages over a broad common-mode range. Within the recommended common-mode operating range of −7 to +7 V, these devices are guaranteed to detect input voltage differentials of less than 200 mV.
Yet the data sheets for such receivers show that their input stages can tolerate differential input voltages up to ±25 V and common-mode input voltages up to ±15 V without damage. This allows these devices to find an alternative usage in a simple analog zero-crossing detector circuit.
This circuit illustrates the simplest configuration (Fig. 1). In this design, two receivers in the quad-receiver package are connected to detect the zero crossings of a ±10-V analog input signal. Both receivers compare the input signal with ground. One detector senses when the input signal is more positive than ground. The other detects when the input signal is more negative than ground.
Features of this circuit include simplicity, speed, and the fact that it eliminates any requirement for analog power supplies. It has sufficient deadband for noise rejection as well as adequate precision and repeatability for detection applications (i.e., detecting the sign of an analog speed-control signal for motors).
Testing shows, however, that the two differential receiver channels do not switch at exactly 0.00 V. With a typical DS3486 device, the positive detector switches at about 0.015 V. The negative detector switches at approximately -0.010 V. This results in a deadband of 25 mV around 0 V.
The RS-422 specs require only that switching occurs somewhere within a ±200-mV region around 0 V. For this reason, chip designers have considerable latitude in implementing their circuit. Therefore, be aware that RS-422 receiver ICs from different manufacturers will have individual thresholds and hysteresis behavior. While too little hysteresis allows oscillation to occur, too much hysteresis requires more elaborate zero-detection logic. Using this circuit for comparison testing, we found that the detection thresholds varied a few millivolts from one DS3486 channel to the next.
Through the use of additional resistors, the detection thresholds can be adjusted to balance the detection region around 0 V (Fig. 2). With the resistor from pin 1 to ground, the positive detector threshold is made more positive. The resistor from the input to pin 7 forces the negative detector threshold to become more negative. Typical values are shown for ±25-mV detection using National's DS3486 parts. Trimpots can also be used to provide in-circuit adjustment.