The initial circuit configuration for a measuring instrument that receives voltage signals (analog signals or pulse signals) from sensors and other sources can be categorized into "symmetrical input" and "asymmetrical input."
A symmetrical input is a three-terminal input circuit also called a "balanced input" because the impedance between each of the two input terminals and the common terminal is equal and balanced. This circuit is designed to receive signals that are out of phase with respect to the common terminal and have equal amplitude, such as the line driver signal of a rotary encoder or the RS422 signal. When this circuit is used to detect the instantaneous voltage difference between two input signals, it is called a "differential input" and is commonly used in analog signal processors.
Differential circuit
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In addition to analog signals, it is also used for pulse transmission. In the diagram above, VA and VB are output in opposite phases and are called line driver outputs, and the differential input on the receiving side is called a line receiver. The AMP amplifies the difference between VA and VB and does not amplify Vn (common-mode noise), so the effects of noise can be suppressed.
An asymmetric input is a three-terminal input circuit in which the impedance between each of the two input terminals and the common terminal is different.
A differential input circuit with one terminal connected to the common terminal, resulting in a two-terminal input circuit with one input terminal and a common terminal, is called an "unbalanced input" or "single-ended" circuit. In general measuring instruments, BNC connectors are used, and the 3C-2V coaxial cable used with them has two wires: a signal wire and a shielded wire that serves as both the signal and common wire. Furthermore, in the case of "isolated" signals described later, a three-wire cable with a signal wire, a common wire, and a shield is used, and this circuit is frequently utilized.
Single Ending
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It is simple and widely used.
In amplifiers and similar devices, the common lines of the input circuit and the common lines of the output circuit are shared, but they are isolated from the outer casing, resulting in an "input with an isolated common." A circuit configuration that is isolated from the outer casing, power supply, and all output terminals is called a floating input, and separate power supplies are provided for the input circuit and the internal circuit, with the signals isolated using photocouplers or similar devices to create a floating input.
The same applies to output circuits; try rereading the above example, replacing the word "input" with "output."
To reduce the influence of external electric or magnetic fields, elements and circuits are sometimes shielded by enclosing them. This is done by enclosing them in a ferromagnetic material ("magnetic shielding") or by enclosing them in a conductor and grounding them ("electrostatic shielding").
A "measuring grounding terminal" is a terminal that is connected to the measurement circuit or shielding conductor and grounded when taking measurements. Short-circuiting the common terminal and the grounding terminal with a shorting bar is an example of this.
The "protective earthing terminal" is provided for safety purposes and is an earthing terminal connected to the conductive part of the equipment for connecting to an external earthing system.
The impedance seen from the input terminal of a device towards the device itself is called "input impedance," and it is usually represented by the equivalent value of a parallel-connected resistor (or a resistor and a capacitor).
Input impedance (equivalent circuit)
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When an input/output circuit is represented by an equivalent circuit consisting of a signal source and a resistor, Zo is called the output impedance and Zi is called the input impedance.
If the capacitance C is equal to the signal frequency f, then its resistance value changes with frequency as 1/(2πfC). Catalogs often specify the frequency, for example, an input impedance of 30kΩ (20kHz).
The impedance seen from the output terminal of a device towards the device itself is called the "output impedance," and it is equivalently represented by a circuit in which this output impedance is connected in series with a signal source to produce the output. Since a signal source can only supply a finite amount of power, if the load resistance exceeds the allowable range, it will not be able to output the correct signal (voltage or current), which can cause malfunction. Therefore, in order to obtain the output as specified without being affected by the output impedance, the "load resistance" is specified as 10kΩ or more.
When connecting multiple devices, the load resistance of the output device becomes the input impedance of the receiving device. Make sure that the load resistance is within the allowable limits.
(Excerpt from the email newsletter issued on September 24, 2002)
