ICs & Semiconductors
Accurate application monitoring
Darren Wenn considers current sensing techniques and outlines the advantages and disadvantages of three typical high-side sensing configurations
C
urrent sensing is a fundamental requirement in virtually every item of equipment that features electronic
control or supervision of its operation. More sophisticated monitoring brings many benefits; longer battery life in hand-held devices and the more efficient and quieter operation of equipment. Precision control depends on precision measurements; and accurate monitoring of current flow is an essential function. Current is almost always measured
indirectly, often inferred from the voltage (V = I.R) developed across a resistor placed in the current path. Current sense resistors are low-cost, can offer high measurement accuracy from very low to medium current levels, and are applicable to both AC and DC circuits: their disadvantages include adding an additional resistance into the measured circuit path, which may increase source output resistance and result in undesirable loading effect; and the associated power loss (P = I2.R). Therefore, current sensing resistors are rarely used beyond low and medium current sensing applications.
These disadvantages can be minimised by using low-value sensing resistors. However, the voltage drop across the sensing resistor may then become low enough to be comparable to the input offset voltage of subsequent analogue conditioning circuit, which would compromise measurement accuracy. If the measured current has a significant high- frequency component, the sense resistor's inherent inductance must be low, or the reactive voltage developed across it will degrade measurement accuracy. Other important parameters in the performance of a sense resistor include resistance tolerance, temperature coefficient, thermal
EMF, temperature rating, and power rating – which should be high enough to handle short duration and transient peaks.
Low- and high-side sensing Two basic techniques can be categorised as low-side current sensing and high-side current sensing: each has advantages and disadvantages.
As shown in Figure 1, low-side current sensing connects the sensing resistor between the load and ground. Normally, the sensed voltage signal (VSEN = ISEN RSEN) is so small that it needs to be amplified by subsequent op amp circuits (e.g., non-inverting amplifier) to get the measurable output voltage (VOUT). This configuration provides low input common- mode voltage, with ground-referenced input and output, together with simplicity and low cost: but is susceptible to ground path disturbance, and the ground level seen by the load (application circuit) is lifted from system ground since RSEN adds undesirable resistance to the ground path. Other disadvantages are that any high load current caused by accidental short (to true ground) goes undetected; and the need for low VDD parts.
In a single-supply configuration, the most important aspect of low-side current sensing is that the common-mode input voltage range (VCM) of the op amp must include ground. Select low-side current sensing where short circuit detection is not required, and ground disturbances can be tolerated.
As shown in Figure 2, high-side current sensing connects the sensing resistor between the power supply and load, eliminating ground disturbance effects, allowing the application to be directly connected to ground, and enabling
detection of short-circuit currents. However, the measurement arrangement must be able to handle very high and dynamic common-mode input voltages, leading to complexity and increased costs, and the need for high VDD parts.
In a single-supply configuration, the VCM range of the difference amplifier must be wide enough to withstand high common-mode input voltages, and the difference amplifier must be able to reject dynamic common-mode input voltages.
High-side sensing implementation High-side current sensing is typically selected in applications where ground disturbance cannot be tolerated, and short circuit detection is required, such as motor monitoring and control, overcurrent protection and supervising circuits, automotive safety systems, and battery current monitoring. Figure 3 shows the first of several high- side configurations; a single op amp difference amplifier that consists of the MCP6H01 op amp and four external resistors, to amplify the small voltage drop across the sensing resistor by the gain R2/R1, while rejecting the common-mode input voltage. The difference amplifier’s common-mode
rejection ratio (CMRRDIFF) is primarily determined by resistor mismatches (R1, R2, R1*, R2*), not by the op amp's CMRR; tight tolerance resistors will add cost. For R2/R1 = 1, resistor tolerances of 0.1%, yield a worst case DC CMRRDIFF of 54 dB: if 1% resistors are used the figure will be only 34 dB.
RSEN should be much less than R1 and
R2 in order to minimise resistive loading effects. The difference amplifier’s input impedances, seen from V1and V2, are unbalanced. Note that the resistive loading
Figure 3. Single op amp difference amplifier
effect and the unbalanced input impedances will degrade the CMRRDIFF. The reference voltage (VREF) allows the amplifier’s output to be shifted to some higher voltage, with respect to ground. VREF must be supplied by a low- impedance source, to avoid making CMRRDIFF worse.
In addition, as shown in Figure 3, the input voltages (V1, V2) can be represented by common-mode input voltage (VCM) and difference-mode input voltage (VDM): V1 = VCM + VDM/2 and V2 = VCM +
VDM/2
VOUT = (V1 – V2) G + VREF = VDM G + VREF, where G = R2/R1 In order to prevent VOUT from saturating supply rails, it must be kept within the allowed VOUT range between VOL to VOH. The VCM range of the difference amplifier has been increased due to the resistor dividers made by R1, R2, R1* and R2*. In brief, this places limits on the difference amplifier's VDM and VCM specifications. In summary, difference amplifiers offer
reasonable common-mode rejection ratio, wide common-mode input voltage range, low power consumption, low cost and simplicity: their drawbacks include resistive loading effects, unbalanced input impedances and the fact that adjusting the difference amplifier’s gain requires changing more than one resistor value.
Figure 1. Low-side current sensing 28 September 2011
Figure 2. High-side current sensing Components in Electronics
Figure 4. Three op amp instrumentation amplifier
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