Linear optocouplers are used for analog signal isolation, far better than you think!

Optical isolation is a very common form of signal isolation. Commonly used optocoupler devices and their peripheral circuits. Due to the simplicity of the optocoupler circuit, it is often used in digital isolation circuits or data transmission circuits, such as the 20mA current loop of the UART protocol. For analog signals, optocouplers limit its application in analog signal isolation due to poor input and output line shapes and large changes with temperature.

1. Introduction of linear optocoupler

Optical isolation is a very common form of signal isolation. Commonly used optocoupler devices and their peripheral circuits. Due to the simplicity of the optocoupler circuit, it is often used in digital isolation circuits or data transmission circuits, such as the 20mA current loop of the UART protocol. For analog signals, optocouplers limit its application in analog signal isolation due to poor input and output line shapes and large changes with temperature.

Transformer isolation is the most common choice for high frequency AC analog signals, but not for DC signals. Some manufacturers provide isolation amplifiers as a solution for analog signal isolation, such as ADI’s AD202, which can provide 0.025% linearity from DC to a frequency of several K, but this isolation device first performs voltage-frequency conversion inside. The AC signal is isolated by transformer, and then frequency-voltage conversion is performed to obtain the isolation effect. The internal circuit of the integrated isolation amplifier is complex, bulky, and expensive, so it is not suitable for large-scale applications.

A good option for analog signal isolation is to use a linear optocoupler. The isolation principle of the linear optocoupler is the same as that of the ordinary optocoupler, but the single-shot and single-receive mode of the ordinary optocoupler is slightly changed, and an optical receiving circuit for feedback is added for feedback. In this way, although the two light-receiving circuits are nonlinear, the nonlinear characteristics of the two light-receiving circuits are the same. In this way, the nonlinearity of the straight-through path can be offset by the nonlinearity of the feedback path, so as to achieve linearity. purpose of isolation.

There are several optional chips for linear optocouplers on the market, such as HCNR200/201 from Agilent, TIL300 from TOAS, a subsidiary of TI, and LOC111 from CLARE. Here we take HCNR200/201 as an example.

2. Chip introduction and principle description

The internal block diagram of HCNR200/201 is shown below

Linear optocouplers are used for analog signal isolation, far better than you think!

Among them, 1 and 2 are used as the input of the isolated signal, 3 and 4 are used for feedback, and 5 and 6 are used for output. The current between pins 1 and 2 is denoted as IF, and the current between pins 3 and 4 and between pins 5 and 6 are denoted as IPD1 and IPD2 respectively.The input signal undergoes voltage-current conversion, and the change of voltage is reflected in the current IF. IPD1 and IPD2 basically have a linear relationship with IF, and the linear coefficients are recorded as K1 and K2 respectively, namely

Linear optocouplers are used for analog signal isolation, far better than you think!

K1 and K2 are generally small (0.50% for HCNR200) ​​and vary widely with temperature (between 0.25% and 0.75% for HCNR200), but the chip is designed so that K1 and K2 are equal. As can be seen later, in a reasonable peripheral circuit design, the ratio of the two is K3 that really affects the output/input ratio. Linear optocouplers are using this feature to achieve satisfactory linearity.

The internal structure of HCNR200 and HCNR201 is exactly the same, the difference lies in some indicators. Compared to HCNR200, HCNR201 provides higher linearity.

Some metrics for isolation with HCNR200/201 are as follows:

* Linearity: HCNR200: 0.25%, HCNR201: 0.05%;

* Linear coefficient K3: HCNR200: 15%, HCNR201: 5%;

* Temperature coefficient: -65ppm/oC;

* Isolation voltage: 1414V;

* Signal bandwidth: DC to greater than 1MHz.

It can be seen from the above that, like ordinary optocouplers, the real isolation of linear optocouplers is current. To truly isolate voltage, auxiliary circuits such as operational amplifiers need to be added at the output and output. The typical circuit of HCNR200/201 is analyzed below, and how to realize feedback and current-voltage, voltage-current conversion in the circuit is deduced and explained.

3. Typical circuit analysis

A variety of practical circuits are given in the manual of Agilent’s HCNR200/201, one of which is more typical as shown in the figure below:

Assume that the input terminal voltage is Vin, the output terminal voltage is Vout, and the two current transfer coefficients guaranteed by the optocoupler are K1 and K2 respectively. Obviously, the relationship between , and , depends on the relationship between .

Put forward the circuit of the pre-stage op amp, as shown in the following figure:

Linear optocouplers are used for analog signal isolation, far better than you think!

Let the voltage at the negative end of the op amp be , and the voltage at the output end of the op amp is

Linear optocouplers are used for analog signal isolation, far better than you think!

Vo=Voo-GVi (1)

Among them is the output voltage when the differential mode of the input of the op amp is 0, and G is the gain of the op amp, which is generally relatively large.

Ignoring the input current of the negative terminal of the op amp, it can be considered that the current passing through R1 is IP1, and according to the Ohm’s law of R1:

Linear optocouplers are used for analog signal isolation, far better than you think!

The current through R3 is IF, according to Ohm’s law:

Linear optocouplers are used for analog signal isolation, far better than you think!

Among them, it is the voltage of pin 2 of the optocoupler. Considering that the voltage when the LED is turned on is basically unchanged, it is treated as a constant here.

According to the characteristics of the optocoupler, namely

K1=IP1/IF (4)

Substitute the expression of sum into the above formula, we can get:

Linear optocouplers are used for analog signal isolation, far better than you think!

The above formula can be obtained by transforming:

Linear optocouplers are used for analog signal isolation, far better than you think!

Considering that G is very large, the following approximation can be made:

Linear optocouplers are used for analog signal isolation, far better than you think!

In this way, the output is related to the input voltage as follows:

Linear optocouplers are used for analog signal isolation, far better than you think!

It can be seen that in the above circuit, the output is proportional to the input, and the proportional coefficient is only determined by K3 and R1, R2. Generally, R1=R2 is selected to achieve the purpose of only isolation and no amplification.

4. Auxiliary circuit and parameter determination

The above derivation assumes that all circuits work in the linear range. To do this, it is necessary to select the op amp reasonably and determine the resistance value of the resistor.

4.1 Op amp selection

The op amp can be powered by a single power supply or a positive and negative power supply, and the example given above is a single power supply. In order to enable the input range from 0 to VCC, the operational amplifier needs to be able to work with full swing. In addition, the operational speed and slew rate of the operational amplifier will not affect the performance of the entire circuit. TI’s LMV321 single operational amplifier circuit can meet the above requirements and can be used as a peripheral circuit of HCNR200/201.

4.2 Determination of resistance value

The selection of the resistor needs to consider the linear range of the op amp and the maximum operating current IFmax of the linear optocoupler. When K1 is known, IFmax determines the maximum value of IPD1, IPD1max. In this way, since the minimum range of Vo can be 0, in this case, considering that the large IFmax is beneficial to the transmission of energy, the maximum value is generally taken.

In addition, since the op amp working in the deep negative feedback state satisfies the virtual short characteristic, therefore, considering the limitation of IPD1, the determination of R2 can be determined according to the required magnification. For example, if no amplification is required, just set R2=R1 That’s it.

In addition, because the optocoupler will generate some high-frequency noise, a capacitor is usually connected in parallel at R2 to form a low-pass filter. The value of the specific capacitor is determined by the input frequency and the noise frequency.

4.3 Parameter determination example

Suppose it is determined that Vcc=5V, the input is between 0-4V, the output is equal to the input, and the LMV321 op amp chip and the above circuit are used. The process of parameter determination is given below.

* Determine IFmax: about 25mA recommended for device operation in the HCNR200/201 manual;

* Determine R3: R3=5V/25mA=200;

* Determine R1:;

* Determine R2: R2=R1=32K.

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