老外的两个4-20ma光耦隔离输出电路_4-20ma隔离电路

图在二楼…楼下还有光隔离输入

第一个。用一个普通四光耦的线性部分
Optically Isolated 4- To 20-mA Current-Loop Transmitter Is Accurate, Inexpensive
Sep 25, 2008 W. Stephen Woodward | Electronic Design

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Galvanically (that is, optically or electromagnetically) isolated 4- to 20-mA current loops offer robust noise immunity and tolerate long cable runs. These advantages, combined with simple unshielded two-wire cabling, make this mature signaling standard popular for transmitting analog data in noisy industrial and process control environments. Unfortunately, the conversion of an analog voltage output to an isolated current-loop signal is relatively complicated. In addition to the actual signal isolator components, multiple floating power supplies are typically required.

The current-loop transmitter in the figure employs an unusual trick to inexpensively implement an optically isolated 4- to 20-mA transmitter: operation of the quad-channel LED/transistor optoisolator (an NEC PS2501-4) in a linear mode. Normally, this would be a dubious idea because LED/transistor optoisolator response is typically very nonlinear and temperature-dependent, making it incompatible with accurate transmission of precision analog data.

Working around this limitation and achieving adequate analog accuracy from this class of component requires meeting two goals:

• Nonlinearity and temperature-coefficient compensation through feedback matching of reference elements in a multichannel microcircuit so that nonlinearities will cancel.
• Scrupulous duplication and tracking of operating points (voltage and current bias) of the reference elements.

The first is achieved by matching LED/transistor pair U2c to U2a, b, and d. U2c is part of the feedback loop of op-amp U1, causing the LED drive current to be controlled so that I3 = IIN. Because all four LEDs in U2 are serially connected, the other three optical pairs receive an identical LED drive current, causing their phototransistors to conduct the same collector currents. That is:

I1 = I2 = I3 = I4 = IIN, which is 0 to 5.33 mA

However, this equality depends not only on the physical matching of the four U2 channels, but also on the match of their bias voltages, which is the second design goal. This goal is achieved through the equality of the 1.25-V set point of U1 to the 1.25-V internal reference of regulator U4.

Remaining circuit details include calibration trims for minimum (4 mA) and full-scale (20 mA) output currents and the option of operation with an external loop-voltage supply (passive mode) or with the dc-dc converter, U3 (active mode). ISUM is:

3(IIN + IMIN) = (0 to 16 mA) + 4 mA = 4 to 20 mA

第二个　用双光耦作PWM控制LM３１７

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Simple Current-Loop Transmitter Converts PWM To 4-to-20-mA Output
Nov 20, 2000 W. Stephen Woodward | Electronic Design

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Over long cable runs, an isolated 4-to-20-mA current loop offers robust noise immunity and tolerance. This characteristic makes it popular for analog data transmission in noisy industrial and process control environments. Unfortunately, the conversion of a digital output to an isolated current-loop signal is a relatively complicated proposition. In addition to the actual signal isolator components, paraphernalia such as multiple floating power supplies and DACs are typically required.

A simpler scheme is used for this circuit (Fig. 1). It’s based on pulse-width modulation to implement a cheap but accurate current-loop transmitter. A dual-channel isolator (E1-E2-Q1-Q2) provides galvanic isolation of the PWM input. It uses this input to chop the 1.25-V reference voltage of the LT317AT regulator in response to the 1- to 2-kHz PWM input applied to the E1/E2 LED pair. Phototransistors Q1 and Q2 switch R4 between the top and bottom ends of the R1/R2 current-set resistance.

The pulse-width duty factor of the PWM input varies between 0% and 100%. At the same time, the dc component of the isolated analog waveform sourced to R4 varies from 0.25 to 0.0 V relative to A1-pin 3. A1’s low-pass, gain-of-four filter extracts this dc portion and applies it to the adjust pin of VR1. This allows the voltage between the VR1 adjust and A1-pin 3 to fluctuate from 1.0 to 0.0 V. As a result, the voltage across the R1/R3 resistance varies from 0.25 V (at a duty factor of 0) to 1.25 V (at a duty factor of unity). Consequently, a 4- to 20-mA current is drawn from the out terminal of VR1 and circulated in the output loop.

The full-scale 20-mA output is trimmed via R1. The impedance of the low-pass filter network is quite high (≈ 3 MΩ). This limits the phototransistor saturation offset to ≈ 1 mV. Meanwhile, the picoamp-level bias current of A1 prevents these big resistors surrounding its summing point from causing significant errors. Relatively long (tens of milliseconds) filter time-constants are needed for adequate ripple reduction. The high-feedback network impedances allow this to be achieved with only modestly sized capacitors.

Performance of the resulting current loop is respectable. The 1% guaranteed tolerances of the critical components (VR1, R1, and R2) yield typical accuracies greater than 8 bits. Resolution, differential linearity, and monotonicity are equal to that of the PWM digital waveform—that is, except in cases where the duty factor values are smaller than 0.01% or larger than 99.99%. At these extremes, the control signal may spend less time in the zero or one state than is required by the isolator to completely switch from one rail to the other. This results in a modest increase in differential nonlinearity for this very restricted range of values.

With risetimes less than 100 ms, the settling time to 0.1% is less than 500 ms. The output ripple is less than 0.01% of full scale. If the dc-dc converter is omitted, passive transmitter operation displays a voltage compliance spanning 7 to 45 V. In other words, it ranges from the minimum voltage drop necessary for proper operation to the maximum operating voltage of VR1.

The dependence of the output current upon loop voltage is less than 50 ppm/V over the whole range. This is due to the excellent line regulation behavior of VR1. Adding the dc-dc converter enables active transmitter operation with an isolated output compliance of 24 V. All circuit power comes from a single groundreferred 5-V rail.

A number of methods can be used to generate the PWM input. In microcontroller-based applications (e.g., Motorola 68HC12), the onchip counter/timer hardware can provide a convenient source. Figure 2 illustrates another possibility. This idea uses a constantfrequency PWM multivibrator that was derived from an earlier IFD (“A New Stable RC Pulse Generator,” Feb. 8, 1999, p. 104-106). That circuit is adapted to digital control via the Xicor X9440 dual “smart analog” comparator/potentiometer combination. The convenience of the X9440’s strapless-address SPI interface partially makes up for its somewhat limited (6-bit) resolution.