Abstract:
The delay and distortion characteristics of an optical detector can be improved with a comparator having tailored offsets and hysteresis. The comparator is controlled by the output of a transimpedance amplifier, coupled in part through a delay mechanism. The delay mechanism provides a dynamic reference level to one terminal of the comparator.

Description:
BACKGROUND OF THE INVENTION 
     In an optocoupler, a driver stage takes an input signal and converts it to an optical signal. The optical signal is then sensed by a detector stage which converts it back to an electrical signal. Ideally, the two stages (driver and detector) introduce little or no delay or distortion into the signal being coupled. 
     A detector stage offering high switching speed with minimal pulse width distortion and isolation would be highly desirable for an optocoupler. Such a detector can be fashioned with a detector responsive to changes in the output of a photo diode. A delay circuit retains the value of the detector output immediately preceding a change in the diode&#39;s output to provide a dynamic reference level against which the instantaneous output of the detector is compared. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a conceptual schematic block diagram of a detector circuit; 
     FIGS. 2 and 3 are schematic diagrams of optical detector circuits; 
     FIG. 4 is a schematic diagram of a delay circuit for the detector circuit of FIG. 3; and 
     FIGS. 5 and 6 are waveform diagrams for the circuits of FIGS. 2 and 3, respectively. 
    
    
     DESCRIPTION OF THE INVENTION 
     An optical detector can be modeled as shown in FIG. 1. A photo diode  10  senses light and generates current i, which is converted to a voltage by a current-to-voltage converter  20 , producing an output v. When the photo diode  10  is excited by light, a ΔV detector  30  senses a change in the output of the current-to-voltage converter  20  and the ΔV detector  30  generates an output signal, indicating the presence of an input signal, i.e., the light impinging on the photo diode  10 . The ΔV detector  30  detects a change in the output voltage of the current-to-voltage converter  20  by retaining the value of voltage prior to a transition in the output of the photo diode  10  and comparing that against the output value following the transition. 
     A circuit that achieves the foregoing is shown in FIG. 2. A photo diode  100 , having an optional shield  102  for common-mode isolation, generates a current when excited by light. This current may vary from 4-40 μA, depending on the diode and the intensity of the light impinging thereon, and is sensed by the negative input of a transimpedance amplifier  110 . A transimpedance amplifier uses conventional operational amplifier technology and provides the desired linearity and dynamic range. A feedback resistor R f  couples the output of the amplifier  110  to the negative input to define the transimpedance gain, and the positive input is grounded. 
     The negative input and the output of the amplifier  110  are connected to the inputs of non-inverting unity-gain buffers  120  and  130 , respectively, to provide isolation. The outputs of the buffers  120  and  130  are connected to opposite ends of a voltage divider consisting of R 1  and R 2 . Although in FIG. 2 these resistors have been assigned values of 16K and 8K ohms, respectively, other values in the same or a similar ratio could have been selected. One end of the voltage divider (the output of buffer  130 ) is connected to the positive input of a comparator  140  while the negative input of the comparator  140  is connected to the junction of the resistors R 1  and R 2  of the divider. The negative input of the comparator  140  is also connected to a capacitor. In conjunction with the voltage divider and the capacitor, the output of the buffer  120  provides a dynamic reference level to the comparator&#39;s negative input. 
     Initially, no light impinges on the photo diode  100  and thus no current is generated, and the output of the transimpedance amplifier  110  is sitting at its low point. Since no current is flowing, both buffers see the same input, generate the same output, and the inputs to the comparator  140  are likewise equal and sitting at a low voltage (“off” state). In the circuit of FIG. 2, this voltage is approximately 2 volts. The output of the comparator  140  is in the low state due to offsets and hysteresis built into the design of the comparator  140 . 
     When light strikes the photo diode  100 , the diode generates a current and the output of the transimpedance amplifier  110  will rise quickly, owing to the gain dictated by the feedback resistor R f . The voltage at the input to the amplifier  110  will remain low, however, because of the amplifier&#39;s low input impedance. As a result of the amplifier&#39;s output voltage appearing across the voltage divider, the voltage at the negative input of the comparator  140  will also attempt to rise to a proportional value dictated by the values of resistors R 1  and R 2 . However, at the same time, the capacitor will delay any change in the voltage at the negative input to the comparator  140 . Because of this relative relationship between the voltages at the inputs to the comparator and the delaying action of the capacitor, the circuit provides a dynamic reference level. Further, the divider provides a noise margin during the comparator&#39;s on-state. 
     Referring to FIG. 5, the output of the photo diode  100  is shown along with the voltages at the inputs and the output of the comparator  140 . The voltage at the positive input achieves a higher value as shown, while the voltage at the negative input lags, but ultimately reaches a value of v[R 1 /(R 1 +R 2 )]. However, as soon as the voltage at the positive input exceeds that at the negative input by the threshold of the comparator  140 , the output of the comparator  140  swings high. The turn-on threshold in the comparator  140  provides a noise margin during the off-state. 
     When the photo diode  100  stops receiving light, the output of the transimpedance amplifier  110  will drop back to its low value (zero-current voltage) and the voltage at the positive input of the comparator  140  will similarly drop. Since the negative input is tied to a capacitor, the voltage there will now exceed the voltage at the positive input for a period determined by the value of C and R 1  in parallel with R 2 , forcing the comparator  140  to switch and its output thus goes low. 
     In a variation of the circuit of FIG. 2, the shield  102  can be driven by a buffer amplifier to reduce the effect of the shield&#39;s inherent capacitance. Instead of tying the shield to ground, the negative input to the transimpedance amplifier  110  is connected to the input of a buffer amplifier, the output of which drives the shield  102 . 
     Another detector circuit is shown in FIG.  3 . There, a photo diode  200  (and an optional shield  202 ) drives the negative input of a transimpedance amplifier  210  with its customary feedback resistor R f  coupling the output back to the negative input as in the previous circuit. The output of the transimpedance amplifier  210  is provided to the positive input of a comparator  220  directly and to the comparator&#39;s negative input through a delay module  230 . One example of a suitable delay is the RC delay line shown in FIG.  4 . The resistors of the delay line may have a value of 3 KΩ while the capacitors may have a value of 10-15 fF (femtofarads; 10 15− farads), although of course other values could be selected to suit the application. 
     The behavior of the circuit of FIG. 3 will be explained with reference to the waveforms of FIG.  6 . Initially, when the photo diode  200  is not producing any current, the output of the transimpedance amplifier  210  is low, both inputs of the comparator  220  are similarly low, and therefore its output is low due to offsets and hysteresis built into the comparator  220 . When the photo diode  200  is excited, the output of the transimpedance amplifier  210  swings high, forcing the positive input of the comparator  220  high, but the delay module  230  dampens the corresponding rise of the voltage at the negative input. When the difference between the voltages at the two inputs exceeds the turn-on threshold of the comparator  220 , the comparator  220  switches, producing a positive output. In accordance with the time constant of the delay module  230 , the voltage at the negative terminal continues to rise, albeit more slowly, and, ultimately, the voltages at the two inputs are equal. 
     When light is no longer exciting the photo diode  200 , current flow ceases and the output of the transimpedance amplifier  210  drops. Again, given the delay line, the voltage at the negative input remains high until it decays, allowing a difference to build quickly and forcing the comparator  220  to shift low. 
     Similar to the detector of FIG. 2, the circuit of FIG. 3 provides a dynamic reference level. However, since there is no voltage divider but rather a delay line, there is no voltage drop. To minimize pulse-width distortion, the comparator  220  has different turn-on and turn-off thresholds. Built-in hysteresis having different values for turn-on and turn-off can be tailored to the photo diode current rise and fall times that are typically unequal. For example, if the rise time (on time) is half that of the fall time (off time), then equal delays can be achieved by having the comparator&#39;s turn-on hysteresis twice as large as the turn-off hysteresis. 
     The foregoing circuits may be fabricated as integrated circuits or as discrete components. To further improve isolation, the amplifiers, buffers, comparator, and related components can be connected to an isolated supply and ground, while any interfacing circuitry on the output would be connected to an external supply and ground. In that regard, the comparator in either circuit can drive an output circuit such as a tristate or some other suitable output device. 
     While values have been provided in the drawings or specified in the text for some of the components, voltages, and currents, it should be recognized that other values could be selected to suit the application.