AFS090 datasheet(106/334 Pages) MICROSEMI

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Device Architecture

2- 90

R e visio n 2

Care must be taken when choosing the right resistor for current measurement application. Note that

because of the 10× amplification, the maximum measurable difference between the AV and AC pads is

VAREF / 10. A larger AV-to-AC voltage drop will result in ADC saturation; that is, the digital code put out by

the ADC will stay fixed at the full scale value. Therefore, the user must select the external sense resistor

appropriately. Table 2-38 shows recommended resistor values for different current measurement ranges.

When choosing resistor values for a system, there is a trade-off between measurement accuracy and

power consumption. Choosing a large resistor will increase the voltage drop and hence increase

accuracy of the measurement; however the larger voltage drop dissipates more power (P = I2 × R).

The Current Monitor is a unipolar system, meaning that the differential voltage swing must be from 0 V to

VAREF/10. Therefore, the Current Monitor only supports differential voltage where |VAV-VAC| is greater

than 0 V. This results in the requirement that the potential of the AV pad must be larger than the potential

of the AC pad. This is straightforward for positive voltage systems. For a negative voltage system, it

means that the AV pad must be "more negative" than the AC pad. This is shown in Figure 2-71.

In this case, both the AV pad and the AC pad are configured for negative operations and the output of the

differential amplifier still falls between 0 V and VAREF as required.

Figure 2-70 • Positive Current Monitor

0-12 V

RSENSE

ACx

AVx

CMSTBx

10 X

Current Monitor

VADC to Analog MUX

(refer Table 2-36

for MUX channel

number)

AFS090 Datasheet(PDF) 106 Page - Microsemi Corporation