HCNR200 HCNR201数据手册(3)

Figure 21. 4 to 20 mA HCNR200 receiver circuit.

LOOPVin

ILOOP

Design Equations:

(ILOOP/Vin)=K3(R5+R3)/(R5R1)K3 = K2/K1 = Constant ≈ 1

Note:

The two OP-AMPS shown are two separate LM158 IC’s, and NOT dual channels in a single package, otherwise, the LOOP side and input side will not be properly isolated; The 5V1 Zener should be properly selected to ensure that it conducts at 187μA;

Figure 22. 4 to 20 mA HCNR200 transmitter circuit.

模拟光耦

Theory of Operation

Figure 1 illustrates how the HCNR200/201 high-linearity optocouplerisconfi gured. The basic optocoupler con-sists of an LED and two photodiodes. The LED and one of the photodiodes (PD1) is on the input leadframe and the other photodiode (PD2) is on the output leadframe. The package of the optocoupler is constructed so that each photo diode receives approxi mately the same amount of light from the LED.

An external feedback amplifi er can be used with PD1 to monitor the light output of the LED and automatically adjust the LED current to compensate for any non-linear-ities or changes in light output of the LED. The feedback amplifi er acts to stabilize and linearize the light output of the LED. The output photodiode then converts the stable, linear light output of the LED into a current, which can then be converted back into a voltage by another amplifi er.

Figure 12a illustrates the basic circuit topology for implement ing a simple isolation amplifi er using the HCNR200/201 optocoupler. Besides the optocoupler, two external op-amps and two resistors are required.

This simple circuit is actually a bit too simple to function properly in an actual circuit, but it is quite useful for ex-plaining how the basic isolation amplifi er circuit works (a few more components and a circuit change are required

to make a practical circuit, like the one shown in Figure 12b).The operation of the basic circuit may not be immedi-ately obvious just from inspecting Figure 12a, particu-larly the input part of the circuit. Stated briefl y, amplifi er

A1 adjusts the LED current (Iin PD1 (IF

), and therefore the current PD1), to maintain its “+” input terminal at 0 V. For example, increasing the input voltage would tend to in-crease the voltage of the “+” input terminal of A1 above 0 V. A1 amplifi es that increase, causing IF to increase, as well as II will pull the “+” terminal of the op-amp back toward PD1. Because of the way that PD1 is connected, ground. A1 will continue to increase IPD1nal is back at 0 V. Assuming that A1 is a perfect op-amp, F until its “+” termi-no current fl ows into the inputs of A1; therefore, all of the current fl owing through R1 will fl ow through PD1. Since the “+” input of A1 is at 0 V, the current through R1, and thereforeIPD1 as well, is equal to VIN/R1.Essentially, amplifi er A1 adjusts IF so that

IPD1 = VIN/R1.

Notice that Ithe value of R1 and is independent of the light output PD1 depends ONLY on the input voltage and

characteris tics of the LED. As the light output of the LED changes with temperature, ampli fi er A1 adjusts IF to compensate and maintain a constant current in PD1. Also notice that IPD1 is exactly proportional to VIN, giving a very linear relationship between the input voltage and the photodiode current.

The relationship between the input optical power and the output current of a photodiode is very linear. There-fore, by stabiliz ing and linearizing Ithe LED is also stabilized and linearized. And since light PD1, the light output of from the LED falls on both of the photodiodes, IPD2 will be stabilized as well.

The physical construction of the package determines the relative amounts of light that fall on the two photodiodes and, therefore, the ratio of the photodiode currents. This

results in very stable operation over time and tempera-ture. The photodiode current ratio can be expressed as a constant, K, where

K = IPD2/IPD1.Amplifi er A2 and resistor R2 form a trans-resistance am-plifi er that converts IPD2 back into a voltage, VOUT, where

VOUT = IPD2*bining the above three equations yields an overall expression relating the output voltage to the input volt-age,

VOUT/VIN = K*(R2/R1).

Therefore the relationship between VIN and Vstant, linear, and independent of the light output

OUT is con-characteris tics of the LED. The gain of the basic isola tion amplifi er circuit can be adjusted simply by adjusting the ratio of R2 to R1. The parameter K (called K cations) can be thought of as the gain of the 3 in the electri-cal specifioptocoupler and is specifi ed in the data sheet.Remember, the circuit in Figure 12a is simplifi ed in order to explain the basic circuit opera tion. A practical circuit, more like Figure 12b, will require a few additional compo-nents to stabilize the input part of the circuit, to limit the LED current, or to optimize circuit performance. Example applica tion circuits will be discussed later in the data sheet.

模拟光耦

Circuit Design Flexibility

Circuit design with the HCNR200/201 is very fl exible because the LED and both photodiodes are acces sible to the designer. This allows the designer to make perf-ormance trade-off s that would otherwise be diffi cult to make with commercially avail able isolation amplifi ers (e.g., band width vs. accuracy vs. cost). Analog isola tion circuits can be designed for applications that have either unipolar (e.g., 0-10 V) or bipolar (e.g., ±10 V) signals, with positive or negative input or output voltages. Several simplifi ed circuit topologies illustrating the design fl ex-ibility of the HCNR200/201 are discussed below.The circuit in Figure 12a is confi gured to be non-invert-ing with positive input and output voltages. By simply changing the polarity of one or both of the photodiodes, the LED, or the op-amp inputs, it is possible t …… 此处隐藏：6289字，全部文档内容请下载后查看。喜欢就下载吧 ……