Abstract: This paper describes the low-end, high-galvanometer circuit structure and their applications. Keywords: current sense limiting current measurement technology has a very wide range of applications, many systems need to detect inflow and outflow of current size. For example, the current protection / current monitoring equipment ,4-20mA current loop system, the programmable current source, Linear / switch mode power supply, and the need to control inflow and outflow of current ratio of the charger or battery gauge. Since many applications are portable, so the current detection circuit must also have a small size, low power consumption.
High / low current-circuit
Low end of the current-sense resistor in series circuit to ground (Figure 1), and high current-sense resistor circuit in series to the high voltage side (Figure 2). Both methods have features: low-current-mode circuit in the ground to add extra resistance, high current-mode will have to deal with a large common-mode signal.
Figure 1 shows the lower end of the current-amplifier to ground level as the reference level, a sense resistor connected to the inverting terminal. The input signal range of the common mode signal: (GNDRSENSE * ILOAD). Despite the relatively simple low-end current-circuit, but there are several low-end fault condition is not detected by the current-circuit, which will load in a dangerous situation, using high-galvanometer circuit can solve these problems.
High-current-circuit directly connected to the supply side, to detect any failure of the follow-up circuit and take the appropriate measures of protection, especially suitable for automatic control applications, because these application circuit is often used as a reference in case.

Current-traditional high-end circuit
The traditional high-end / low end of the current-mode realization of a variety of programs, most based on discrete or semi-discrete components circuits. Galvanometer circuit usually end with a precision op amp and a number of precision resistors and capacitors, the most commonly used high-end current-differential op amp circuit to do gain amplification and signal level to the reference to the shift from high-end (Figure 3) :
VO = IRS * RS; R1 = R2 = R3 = R4
The program has been widely used in the actual system, but there are three main drawback of the circuit:
1) The input resistance is relatively low, equal to R1;
2) an input resistance generally have larger error value;
3) require a higher degree of resistance of the match to ensure an acceptable CMRR. Any change in resistance will produce 1% of the CMRR reduced to 46dB; 0.1% change in the CMRR to 66dB, 0.01% change in the CMRR to 86dB. Current testing requires a higher-end measurement techniques, which promote the development of high-end integrated circuit galvanometer. The low current detection technology seems to be no corresponding progress.

Differential amplifier with integrated to achieve high-current detection
Differential current sense amplifier circuit for high-end more user-friendly, as recently introduced a number of types of integrated circuit solutions. Within integrated circuit includes a precision op amp and matching good resistance, CMRR up to about 105dB. MAX4198/99 is one such product, its CMRR is 110dB, the gain error is better than 0.01%, and the use of 8-pin mMAX small size package.
Galvanometer circuit dedicated high-end completion of the internal high-current detection contains all the functional units can be up to 32V common mode voltage detection of high-current, and to provide contrast in proportion to ground level as the reference point of the current output. Need to do on the current precise measurement and control applications such as power management and battery charging control, are suitable for such programs.
MAXIM high-end amplifier in the current-sense resistor used in the high-end and place the power to be detected between the power input circuit, a sense resistor on the high end does not give additional ground loop impedance, the technology improves the performance of the circuit and simplifies the cloth version requirements.
MAXIM launched a series of bi-directional or unidirectional current sensing IC, some bi-directional current sensing IC built-in sense resistor, can detect the inflow or outflow of the current size of the circuit was seized by a pin display indicating the current direction of polarity. Adjustable gain current sense IC, fixed gain (+20 V / V, +50 V / V, or +100 V / V), including single and double current detection comparator chip or a fixed gain current sense IC, are packaged in small volume, such as SOT23, can meet the size requirements of demanding applications. Figure 4 is composed MAX4173 high-side current detection circuit.

Graph output voltage sense resistor relationship is:
Vo = RGD * (Iload * Rsense) / RG1) * b where b is the mirror current coefficient
The above equation can be further simplified as: Vo = Gain * Rsense * Iload; Gain = b * RGD/RG1
Gain were: 20 (MAX4173T), 50 (MAX4173F), 100 (MAX4173H).
Can be seen through the above formula, CMRR by the galvanometer circuit integrated in the decision process (typical 90dB), no longer affected by external resistors.

An integrated circuit galvanometer has the following advantages:
1, the device's consistency
2, excellent temperature drift characteristics
3, small size
4, low-power
5, easy to use
Select a sense resistor Notes
RSENSE sense resistor should be selected based on following principles:
1, the voltage loss: the sense resistor voltage caused by resistance to the General Assembly had IR values ​​decreased. In order to reduce voltage loss should be used in small resistance sense resistor.
2, accuracy: the larger sense resistor can get a higher low current measurement accuracy. This is because the voltage on the sense resistor the greater the op amp's offset voltage and input bias current of relatively smaller.
3, efficiency and power: When the current is large, RSENSE I2R power on can not ignore. In considering the sense resistor and power, you need to pay attention to this point. If you allow heat sense resistor, the resistor can be larger.
4, inductor: If ISENSE contains a large number of high frequency components, the sense resistor's inductance to be very small. Maximum inductance wire wound resistor, metal film resistors better.
5, the cost: If the right price is too high current-sense resistor, you can use an alternative program (Figure 5). It uses the printed circuit board as the sense resistor line. PCB copper as "resistance" is not accurate, circuit requires a full-scale current adjustment potentiometer value. In addition, the copper wire drift higher (about 0.4% / ℃), working under a wide temperature range, the system needs to consider this point.

Adjustable linear current source
Adjustable linear current source (Figure 6) is the use of high-current detector consisting of a typical application circuit, IC1 will be converted into the corresponding R1 current proportional voltage signal to control the voltage regulator chip IC2 to generate a stable output current, D / A converter to provide digital control of IOUT. To achieve 12 BIT accuracy (60mA per LSB) of the requirements, you can use the parallel interface or serial interface MAX530 MAX531. 10 BIT Accuracy (250mA per LSB), you can use the parallel interface or serial interface MAX503 MAX504.