[Technology sharing] two-wire smart meter signal isolation solution _ Jin shengyang mornsun

Abstract: two-wire smart instruments are widely used in a single instrument with simple application and low cost. However, two-wire circuits need to transmit both energy and high-precision signals, which makes signal isolation very difficult. This article introduces the basic working methods of two-wire circuits. Based on this, the shortcomings of the existing passive current signal isolator in the two-wire signal isolation circuit are analyzed, the signal isolation scheme based on TS107L-F-2 two-wire isolation converter with higher transmission precision and its distribution expansion scheme are introduced. Finally, the HART signal isolation transmission scheme of TS107HL-F-2 in two-wire circuit is introduced in view of the communication requirements of intelligent instruments.

Keywords: intelligent instruments, two-wire system, signal isolation, HART communication, signal conditioning module?

 

1. Basic two-wire Signal Transmission Mode

Figure 1 Typical two-wire Application diagram

The typical circuit 1 of two-wire signal transmission mode is shown in. The signal detection end consists of a sensor and a two-wire converter. The signal receiving end consists of two-wire circuit power supply VL and current detection circuit. Generally, the current detection circuit converts the current signal into a voltage signal by using the Precision Resistor RL and then transmits it to the control system through the adconverter.

The function of the two-wire converter is to convert the voltage Vin of the two-wire circuit into the power supply voltage VCC of the sensor, and control the load current is of the two-wire circuit Vin according to the sensor input signal sin. Here is the 4-to-4 of two-wire circuit transmission ~ The current signal is 20mA.

In a two-wire circuit, there is only one voltage source VL, and the load is Vin and VRL respectively. Because the current detection resistance flows through the current signal is, the voltage VRL is determined by the is when the resistance remains unchanged. According to kikhof's voltage Law, the voltage Vin of the two-wire interface changes with VRL and satisfies the following equation (1 ):

Vin = VL-VRL (Pressure Drop regardless of line impedance) (1)

Because is determined by the circuit signal, Vin determines the input energy of the two-wire converter, and thus determines the maximum output energy of the two-wire converter VCC.
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[Technology sharing] two-wire smart meter signal isolation solution _ Jin shengyang mornsun