Improving High-Side Current Measurements

Aug 1, 2008 12:00 PM
By Maurizio Gavardoni, Product Definer, Maxim Integrated Products, Sunnyvale, Calif.

An IC that combines a high-side current-sense amplifier with an analog voltage multiplier can easily measure the power dissipated in a load. One multiplier input connects to the load voltage and the other to an internal analog of the load current (i.e., a proportional voltage produced by the internal current-sense amplifier). The multiplier output (V_L × I_L) is then a voltage proportional to load power.

The internal multiplier also can enable extra accuracy in high-side current measurements, for applications in which the current signal is digitized by an analog-to-digital converter (ADC). Whether the ADC's voltage reference is internal or external to the ADC, the accuracy of the digitized load-current measurement depends strongly on the accuracy and stability of that reference.

To minimize this dependency on voltage-reference accuracy, connect the multiplier's external input to the reference voltage through a resistive divider (Fig. 1). The current measurement is then ratiometric. Any error or drift in the reference voltage has a proportional effect on the ADC's input, and thereby achieves a first-order cancellation of full-scale error caused by the reference voltage.

The circuit shown can measure battery charge and discharge currents in a wide range of applications. It works equally well with a voltage reference internal to the ADC, driving the R1-R2 divider.

The IC's multiplier output (P_OUT) feeds a 16-bit ADC whose input voltage range is 0 V to V_REF. V_REF, provided here by an external voltage regulator, should be between 1.2 V and 3.8 V (3.8 V in this case). The multiplier input must be limited to a range of 0 V to 1 V, which is accomplished by dividing the 3.8-V reference voltage with the R1-R2 resistor divider.

Assuming R2 = 1 kΩ and R1 = 2.8 kΩ, then V_IN = 1 V. The IC has a gain of 25 between V_SENSE and I_OUT, and a sense-voltage range (V_SENSE) of 0 V to 150 mV, which produces (at both P_OUT and I_OUT) an output range of 0 V to 3.75 V.

Thus, the use of P_OUT (instead of I_OUT) confers an advantage: the signal fed to the ADC, which is proportional to current in the load, is scaled by V_REF. The following equation relates the P_OUT/V_REF ratio to I_LOAD, R_SENSE, and the values of R1 and R2:

P_OUT/V_REF = I_LOAD × R_SENSE × 25 × V_REF × R2/(R1+R2)/ V_REF = I_LOAD × R_SENSE × 25 × R2/(R1+R2).

Note that the ratio of ADC input to ADC full scale (P_OUT/V_REF) does not depend on the accuracy of V_REF.

Overall accuracy of the current measurement depends on many factors: resistor tolerance, amplifier gain error, voltage offset and bias current, reference voltage accuracy, ADC errors and drift versus temperature for all the above. This circuit improves accuracy by eliminating only one of these causes: the reference voltage inaccuracy. V_REF is affected by at least three sources of error: initial dc error as a percentage of the nominal value, V_REF changes with load and V_REF changes with temperature.

A graph of the multiplier input (IN) versus temperature, with V_CC = 5 V and V_SENSE constant at 100 mV, shows the affect of temperature on the reference voltage (Fig. 2). To see the advantage of the ratiometric output at P_OUT, compare the P_OUT/V_IN ratio and its linear ideal with the I_OUT/V_IN ratio and its linear ideal, as they vary with temperature (Fig. 3). Note that the ratiometric P_OUT output (top) does not deviate from the ideal.

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