Current Measurement

Editor : ElectRobot

 To measure the current, connect the normal current meter in series with the sample. The internal structure of a classic current measuring instrument displays the value of the current and the rotational motion according the amount of current running through the magnet and rotary machines. (Figure 1). It can be regarded as a current meter using the physical laws of electromagnetic induction. Following the development of IC-based semiconductor technology, such as OP-AMP, the compact current meter has replaced the classic current meter. In combination with a digital processing technique, the measurement result can determine the values, which are conveniently expressed as numbers. A wide array of products is derived from this simple model, ranging from an easily portable, light-weight model to a high-end model with high-precision performance.

(Figure 1) The classic current meter (ref. : Google image search)

 The current measurement method consists of a semiconductor IC which is applied to many areas of common current measuring from a few Pico amperes to many Pico amperes. The internal structure depends on the current range measured.

 1) High-Side current measurement

 A semiconductor IC, especially when it is the current measurement circuit of the OP-AMP, has either a High-Side and Low-Side setting. This distinction is based on the location of the current measurement as connected to the measuring circuit. The High-Side current measurement method connects the place where the current first flows to the position of the measurement circuit. The sample of Figure 2 depicts the measuring device and the power supply device as connected to measure the current in the circuit structure.

(Figure 2) High-Side current measurement structure

The current measuring device has inside a current measurement resistance, called a shunt. If the Instrumentation OP-AMP is connected at the both ends of the current measuring resistance, the device can be used as a basic meter. The Instrumentation OP-AMP outputs a voltage difference between both ends of the resistor and, which can be converted to a digital display using the ADC and the processor (Figure 3).

(Figure 3) The internal circuit structure of the current meter

The voltage value is measured according the Ohm’s law divided by the shunt resistor value. The flowed current value can be calculated with this voltage value. In this structure, the accuracy of the current value is dependent on the shunt resistor quality and performance of Instrumentation OP-AMP. When selecting the shunt resistor, establish the maximum current measurement range to determine the voltage value and the power consumption value applied to both ends of the shunt. In general, estimate the value of the voltage both ends of the shunt within several tens of mV range. Instrumentation OP-AMP needs to be selected after review of the maximum common-mode voltage to be measured, offset voltage levels, temperature drift and differential amplification ratio in priority. Depending on the development of semiconductor techniques, variations of Instrumentation OP-AMP supplies TI, Analog and Maxim IC etc. are available.

 2) Low-Side current measurement

 Low-Side current measurement is formed as a contrasting structure to the High-Side current measurement. It is configured as a circuit structure position to be measured after flowing current. Refer to Figure 4.

(Figure 4) Low-Side current measurement structure

Other than their differing measurement positions, the internal circuit structure of the Low-Side and the High-Side current measurement circuit structures are identical. The two current measuring methods, as described above, have their respective advantages and act as appropriate measurement methods depending on the application.

For the High-Side, the internal circuit structure is protected against the flow of a current overload protection and short-circuit situations because it can be measured before entering the current flow to the load. On the other hand, the common-mode voltage is the same level as the supply voltage, and the Instrumentation OP-AMP must be used to make high common-mode voltage specifications.

In the case of the Low-Side, nearly zero conditions follow the common-mode voltage, and the device offers a variety of the cheaper series of the Instrumentation OP-AMP (or the regular OP-AMP can be used). However, the configurations difficult to connect to a load that cannot protect the load to ground. Figure 5 explains these strengths and weaknesses.

(Figure 5) Comparison strengths and weaknesses of High-Side and Low-Side current measurement

There is one way more to introduce the Low-Side current measuring method. It is a ZRA (Zero Resistance Ammeter) structure that is different in structure from the previously-described shunt and Instrumentation OP-AMP structure. It is usually applied to the Electrometer and Cyclic Voltametery measuring circuit and is similar in design with the inverting circuit which uses the circuit operating characteristics of the OP-AMP. (Figure 6)

(Figure 6) Zero Resistance Ammeter

The ideal OP-AMP conditions are the following: 1) the input current flowing into infinite input resistance is zero, 2) the voltage of the two input terminals has a characteristic which operates to be equal. Therefore, the output voltage of the ZRA circuit is two times that of the flowing current, as shown in the feedback resistor Rf in Figure 6. The measured voltage can also be obtained by using a flowing current value of Ohm’s law. ZRA current measurement method is used in many places for measuring microscopic currents below uA. Instrument configuration is possible to configure a multi-stage feedback resistor to measure the hyperfine level of the fA current and the class A; however, the instrument configuration is insufficient for a circuit region with A in the “ten’s” range. The disadvantages of the surge input OP-AMP and the static shock damage is avoided easily with proper attention to their use.

In addition, the current method of measuring the different structures exists, but future innovations using most of the electronic circuit with the current measurement will be introduced. It may utilize only the basic law of Ohm for the instrument for current measurement. In the way described previously, the various instruments of McScience’s are designed and manufactured using the High-Side current measurement method. M600 Lifetime Tester, K101 PV Meter, Q101 Battery Parameter Tester are the typical measuring instrument. In addition to the ZRA, there are other products, such as Cyclic Voltametery and Photocurrent Measurement Unit.