Analog multiplier improves high-side current sensing measurement accuracy

Combining an analog multiplier with a high-side current-sense amplifier enables the measurement of battery charge and discharge currents in laptop computers or other portable instruments. This article discusses how to improve the current measurement accuracy by adding the ADC reference voltage to one input of the analog multiplier.

High-side current sense amplifiers are used extensively in applications that require very high reliability and accuracy. In laptop computers, they are used to monitor the charge and discharge current of the battery. They can also be used to monitor the current of USB ports and other voltages. In order to control system heating and power loss, the output power of these voltages must be reduced. In portable consumer products, high-side current sense amplifiers are used to monitor the charge and discharge current of lithium batteries. In automotive applications, such amplifiers can not only monitor battery current, but also be used for motor control and GPS antenna detection. In communication base stations, such amplifiers are also used to monitor the current of power amplifiers.

In many applications, a high-side current sensing amplifier can be directly connected to an ADC. Some ADCs use an external reference voltage to determine the full-scale input range, and their output accuracy depends largely on the accuracy of the reference voltage. This article describes how to use an analog multiplier with an integrated high-side current sensing amplifier to detect the charge and discharge current of the battery in most applications. This design effectively improves the detection accuracy by adding the ADC reference voltage to the input of the analog multiplier.

High-Side and Low-Side Current Sensing Techniques

High-side and low-side current sensing are two common current measurement methods. High-side sensing is to place a current sensing resistor between the power supply (such as a battery) and the load; low-side sensing is to connect a current sensing resistor in series with the ground loop. This method has two disadvantages compared to high-side sensing: first, if the load is accidentally short-circuited, the low-side current sensing amplifier will be bypassed and cannot detect the short-circuit state; second, due to the introduction of unexpected impedance in the ground loop, the ground plane is split.

Figure 1 High-side current sensing (MAX4211)

High-side current sensing also has a disadvantage: the current-sense amplifier must support high common-mode voltage inputs, the amplitude of which depends on the specific voltage source. High-side sensing is mainly used in current-sense amplifiers, while low-side sensing can use a simple operational amplifier as long as the amplifier can handle the common-mode input referenced to ground.

Measuring Power Using a High-Side Current-Sense Amplifier

Figure 1 illustrates how to measure the power delivered to a load (defined as the product of the load current and voltage) using a high-side current-sense amplifier with an integrated analog multiplier. The high-side current sense provides a voltage output proportional to the load current, which is applied to the analog multiplier, whose other input is the load voltage. The multiplier outputs a voltage proportional to the load power.

The analog multiplier here provides more than just power measurement. It can also be used for other purposes. If its external input is not connected to the load voltage, it can also be connected to the reference voltage of the ADC. In this case, the multiplier will no longer measure power, but rather relate the output voltage of the current sense amplifier to the reference voltage of the ADC.

Figure 2 illustrates this usage, where a high-side current sense amplifier measures the battery charge current. The voltage output (POUT) is applied to a 16-bit ADC with an input range of 0V to VREF. Here, an external voltage regulator provides VREF, with a voltage range of 1.2 to 3.8V (3.8V in this example). The input range of the multiplier is 0 to 1V, which can be achieved by dividing the 3.8V reference voltage by R1/R2. Assuming R2 = 1kΩ and R1 = 2.8kΩ, VREF = 1V. The gain of the MAX4211 is 25, so the voltage measurement range is: 0 to 150mV, and the output voltage (for POUT and IOUT) ranges from 0 to 3.75V (proportional to the current flowing into the load).

The advantage of using the POUT of the current sense amplifier as the output instead of IOUT is that the signal applied to the ADC (proportional to the load current) can be stepped down by VREF. Using POUT as the output reduces the requirements for the accuracy of the reference voltage, because the digital output of the ADC depends on the ratio of the input voltage to the reference voltage (representing the full-scale value). Because POUT is a function of the reference voltage VREF, the "VREF" ratio eliminates the effect of the reference on the ADC measurement accuracy, and theoretically has nothing to do with the reference voltage and its accuracy. However, if IOUT is connected to the ADC, any error in the reference will affect the output.

Equations (1) and (2) give the ratios of POUT and IOUT to the ADC input/full-scale range, respectively, which explains the above conclusions.

POUT/VREF=ILOAD×RSENSE×25×VREF×R2/(R1+R2)/VREF=ILOAD×RSENSE×25×R2/(R1+R2) Formula (1)

IOUT/VREF=ILOAD×RSENSE×25/VREF Formula (2)

From equation (1), it can be seen that, when outputted from POUT, the ADC accuracy will be independent of the accuracy of VREF; while when outputted from IOUT, an error inversely proportional to VREF will be generated.

Figure 2. Measuring battery charge current using a current-sense amplifier (MAX4211) and an ADC with an external reference.

The overall accuracy of Figure 2 depends on many factors: resistor accuracy, amplifier gain error, voltage offset, bias current, reference voltage accuracy, ADC error, and temperature drift of the above parameters. In addition, Figure 2 shows a solution to improve system accuracy, which shows that one of the error sources (reference voltage error) can be eliminated by using an analog multiplier and a current-sense amplifier. The accuracy of VREF is related to at least three factors: initial error (percent of nominal value), VREF change with load, and VREF change with temperature.

Figure 3 POUT/IOUT vs. VREF curve for the circuit in Figure 2, VSENSE = 125mV

Figure 3 illustrates the second error source. As the VREF load increases, the VREF output drops from 3.8V to 1.2V. POUT will change with VREF in the same way. Figures 4 to 6 show how VREF and the MAX4211 output change with temperature when VCC = 5V and VSENSE is fixed at 100mV. The operating temperature of the circuit in Figure 2 changes from -40°C to +85°C in 20°C steps (-20°C, 0°C, +25°C, +45°C, and +65°C). The curve in Figure 4 shows the results of VREF changing with temperature. Figure 5 shows the curves of IOUT and IOUT/VREF changing with temperature in the circuit in Figure 2. If the IOUT output is used to drive the ADC, IOUT/VREF is proportional to the ratio of the ADC input signal to the full-scale signal.

The ratio of IOUT/VREF changes with temperature and Figure 4 shows that the reference (VREF) changes with temperature.

Figure 4 Curve of VREF changing with temperature in the circuit of Figure 2

Figure 5. IOUT and IOUT/VREF vs. temperature for the circuit in Figure 2, VSENSE = 100mV

Figure 6 POUT and POUT/VREF vs. temperature for the circuit in Figure 2, VSENSE = 100mV

Finally, Figure 6 shows the curves of POUT and POUT/VREF with temperature. As can be seen from Figure 6, POUT/VREF has nothing to do with the change of VREF with temperature (see Figure 4). The downward bend of VREF between 0°C and +45°C is compensated after POUT output. Because VREF does not appear in the POUT/VREF curve, accordingly, the output of the ADC will not be affected by the change of VREF with temperature.

in conclusion

High-side current-sense amplifiers with integrated analog multipliers are often used to measure load power. However, this integrated multiplier can also provide another function. The current-sense amplifier can be connected to an ADC with an internal or external reference. In both cases, the overall measurement accuracy is mainly related to the accuracy of the reference voltage. If the load current is multiplied by the reference voltage VREF and then output to the ADC, the reference voltage error can be eliminated. With this design, the load current measurement accuracy can be improved even when using a low-cost, low-precision reference voltage.