Current-Sense Amp Offers Four-Quadrant Operation

Feb 1, 2008 12:00 PM
By Alfredo H. Saab, Applications Engineering Manager, and Tina Alikahi, Applications Engineer, Maxim

Four-Quadrant Operation

One solution is to design a high-side current-sensing circuit using discrete op amps. Naturally, this approach has its challenges, too. One difficulty is the fact that most modern high-performance op amps are low-voltage devices, with a total supply voltage of 5 V or less. This fact complicates the circuit design when the common-mode range (CMR) is high.

The CMR of a high-side current-sense amplifier has three values. The absolute maximum CMR voltage is the value that, if exceeded, will destroy the amplifier. The operating CMR is the range in which the amplifier will still process the sense-resistor drop. And finally, the useful CMR is the range that the current-sense resistor can swing through while the amplifier still delivers an output within the error limits.

If a single op amp, standard instrumentation amplifier configuration is used, the operational amplifier input-voltage range (usually smaller than the output-voltage swing) is the factor that limits the operating CMR. This factor forces the use of a very small fractional gain, making drift and noise problems worse.

With a two op amp instrumentation amplifier topology (where both are configured as inverters), the op amp inputs remain at the reference point — usually 0 V. So, the CMR is limited only by the operational amplifier output swing, allowing a higher CMR for a given supply voltage.

For all these amplifier types, the accuracy at the high end of the useful CMR is set mostly by the CMRR. A better CMRR will introduce a smaller uncertainty and, therefore, better accuracy for a given common-mode voltage. The CMRR is a function of the tolerance of the resistors used. As the limits of the CMR are extended higher, the resistor tolerance required for a given accuracy becomes unbearably small. Such precision resistors may not be available, and even when they are, the cost of such resistors may be unbearably high.

The circuit in Fig. 2 alleviates these problems. First, the circuit amplifies the sense-resistor voltage. Then, it applies that signal to the amplifier that does the level translation from the common-mode level of the current-carrying line to the ground-referred level. In this way, the ratio of the desired signal to the spurious (or uncertainties) is increased by the gain of the first amplifier. This approach effectively increases the accuracy of the measurement by the same factor.

Since the first amplifier reference point needs to move with the common-mode level, it must be powered by an isolated power source. The circuit in Fig. 2 can operate under common-mode voltages up to ±75 V and has an input range (voltage burden) of ±80 mV for an output of ±2.5 V.

The V_OUT/V_DIFF gain is 30, which is accurate within 0.1%. The output-offset voltage at room temperature is less than 100 µV. The current burden is 5 MΩ or 10 µA total when V_CM is 50 V. The absolute maximum voltage (nonoperational) for V_CM is limited by the breakdown voltage of R1, R2 or T1, whichever breaks first.

Amplifiers A1 and A2 make the first amplifier, powered by an isolated power supply with the MAX845 high-frequency power-supply IC driving a low interwinding capacitance transformer T1. A3 and A4 form the wide-CMR, high-CMRR instrumentation amplifier.

Fig. 3 shows the amplifier's dc CMRR behavior over temperature, which is critical to maintaining accuracy. The graph in this figure allows designers to determine the amount of error introduced by the common-mode voltage into the output voltage at each temperature.

For the amplifier described here, this error is adjusted to zero at room temperature (by means of the 500-Ω potentiometer). The effect is attributed to changes in the ratios R1/R3 and R2/R4 across the temperature range, and is about 2.5-mV change at the output from -10°C to 50°C.

Fig. 4 shows the same information as Fig. 3, but as a function of frequency. It presents the CMRR versus frequency needed to calculate the uncertainty when V_CM is an ac voltage, or contains ac components.

Fig. 5 is the gain as a function of frequency for the sensed voltage signal. This plot shows the range of frequencies (starting from dc) over which the amplifier is useful, within a given acceptable error.

The frequency performance is important because it also involves the phase shift behavior, which matters when the current measurement is used to calculate power. In such calculations, there are two considerations. First, there is the frequency dependency of the absolute amplitude of the current in the power formulas. But then, too, the cosine of the phase angle weighs in as the so-called “power factor” in the power calculations.

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