线性光耦4-20mAdriver

设计DA，主要有以下方案：

1.XTR115

此方案的特点：

z外接元件少，器件由被驱动的电路电源供电，供电范围大，还能够提供一个参考电源和一个供电电源（3.7mA供电能力）；

z如果将ontop 的8AI模块改造一下，将aduc812的两个DA直接作为XTR115的Vin, 从图上可以看出，Vin-应该接aduc812的地，而相对Vloop的地来说，Vin-是浮动的，随负载的大小，驱动电流的大小而变化。

z如果aduc直接由xtr115供电，一是怕供电能力不足，另一方面由于模块的i2c接口的地总是要与aduc的地接在一起的，所以还是存在整个模块组的对Vloop地的电位不断变化；

z如果aduc的两路da都用上，分别接两个xtr115，aduc的地电位如何接？两个xtr115的Vin-的地电位都是浮动的，还不同！

z同样的道理，即使一个aduc只用其一路da, 那多个ontop DA模块组成模块组还是有通过i2c将不同的Vin-共地的问题

z综上所述xtr115适于作为传感器的驱动，不适由作为da的驱动，当然如果i2c、或aduc隔离后与之接还是很好的。在pwm方案中，脉冲经光电隔离后，给xtr115是一种较好的方案。

2.Aduc的DA直接经OP后变成0-20mA或4-20mA,d原理图如下：

图20是current source电路，图21是current sink。 current source可以为被驱动电路提供一个对地的电流，对测量、或驱动阀门来说，有时是必须的选择；

图20的特点是：

z负载由OP的电源供电。

z如果负载的地电位与OP地电位一样，为了保证Q1/Q2工作，负载的压降就要小于5V, 如果电流输出20mA, 负载必须<250欧姆，如果在负载端想用一个电阻将电流变成许多阀门驱动所需的0-10V将不可能；

z如果负载的地电位与OP地电位不一样（好象不可能），为了保证Q1/Q2工作，负载的地电位+负载电阻的压降（电阻＊电流）要小于5V, 如果出现某种情况，负载的地电位较高，上述电路也不能工作。

图21的特点是：

z负载由另外的电源供电，另外电源地必须与OP地连接一致。

z由于是OC输出，负载的允许压降只受负载供电压电压的限制，因此负载电阻可以很大；

z由于负载接在供电电源正极与三极管的C极，负载的两端对地电位是变化的，对测量（比如ONTOP AI 模块）来说，不好测量（想象一下，直接用同个模块上的AI测这个DA！，没法接吧）；有可能某些调节阀门要求负载的一端接地，这也是个问题（这还要研究一下）；

3.线性光耦

Essentially, amplifier A1 adjusts I F ,so that

I PD1 = V IN/R1.

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 temperature. The photodiode current ratio can be expressed as a constant, K, where

K = I PD2/I PD1.

Amplifier A2 and resistor R2 form a trans-resistance amplifier that converts I PD2 back into a voltage, V OUT, where

V OUT = I PD2*R2.

Combining the above three equations yields an overall expression relating the output voltage to the input voltage,

V OUT/V IN = K*(R2/R1).

Therefore the relationship between V IN and V OUT is constant, linear, and independent of the light output characteristics of the LED. The gain of the basic isolation amplifier circuit can be adjusted simply by adjusting the ratio of R2 to R1. The parameter K (called K3 in the electrical specifications) can be thought of as the gain of the optocoupler and is specified in the data sheet.

As a final example of circuit design flexibility, the simplified schematics in Figure 15 illustrate how to implement 4-20 mA analog current-loop transmitter and receiver circuits using the HCNR200/201 optocoupler. An important feature of these circuits is that the loop side of the circuit is powered entirely by the loop current, eliminating the need for an isolated power supply.

The input and output circuits in Figure 15a are the same as the negative input and positive output circuits shown in Figures 13c and 13b, except for the addition of R3 and zener diode D1 on the input side of the circuit. D1 regulates

the supply voltage for the input amplifier, while R3 forms a current divider with R1 to scale the loop current down from 20 mA to an appropriate level for the input circuit (<50 μA).

As in the simpler circuits, the input amplifier adjusts the LED current so that both of its input terminals are at the same voltage. The loop current is then divided between R1 and R3. I PD1 is equal to the current in R1 and is given by the following equation:

I PD1 = I LOOP*R3/(R1+R3).

Combining the above equation with the equations used for Figure 12a yields an overall expression relating the output voltage to the loop current,

V OUT/I LOOP = K*(R2*R3)/(R1+R3).

Again, you can see that the relationship is constant, linear, and independent of the characteristics of the LED.

The 4-20 mA transmitter circuit in Figure 15b is a little different from the previous circuits, particularly the output circuit. The output circuit does not directly generate an output voltage which is sensed by R2, it instead uses Q1 to generate an output current which flows through R3. This output current generates a voltage across R3, which is then sensed by R2. An analysis similar to the one above yields the following expression relating output current to input voltage:

I LOOP/V IN = K*(R2+R3)/(R1*R3).

The preceding circuits were presented to illustrate the flexibility in designing analog isolation circuits using the

HCNR200/201. The next section presents several complete schematics to illustrate practical applications of the

HCNR200/201.