3.2. High Voltage Measurement: Be Watchful of Heat Emissions from Input Circuits and the Voltage Coefficient
This section consists of a primary input circuit for dividing the input voltage into easily-processed low voltages, and a range switching circuit for normalization. The input accuracy of the powermeter is greatly influenced by the characteristics of the primary input amp. Also, when the voltage circuit is connected to the measurement circuit, the voltage input resistance acts as a load with respect to the power supply. Therefore, it can be favorable to have as large of a resistance value as possible.
3.3. Current Input Circuit: Shunt Resistance
The current input circuit converts current input into easily-processed signals. The shunt resistor can be used depending on the measured current values and objectives.
· Shunt Resistor
Voltage is detected on both sides of the shunt resistor when current is passed through it. Measurement with resistors is more technically established than other methods, and highly accurate measurements can be taken because there is wide array of resistors to choose from. However, resistors present the problem of drift resulting from heat released when current flows through them. To suppress heat emissions, the resistance value must be reduced. However, when the resistance is low, it becomes difficult to maintain flatness-of-field in the frequency characteristics because the inductance component inside the resistor becomes relatively large. Also, since the voltage on the output side becomes miniscule, attention must be given to the thermal electromotive force inside the resistor. The thermal electromotive force arises due to the junction inside a resistor of one type of metal with a conductor of another type of metal. To obtain highly accurate measurements, selection of the material for the resistor and conductor is critical.
