M1AFS600-PQ208 Datasheet, PDF (107/334 Pages) Microsemi Corporation – Fusion Family of Mixed Signal FPGAs

0-12 V
R_SENSE
I
AVx
ACx
Fusion Family of Mixed Signal FPGAs
CMSTBx
Current Monitor
10 X
VADC ^{to Analog MUX}
(refer Table 2-36
for MUX channel
number)
Figure 2-72 • Positive Current Monitor
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
V_AREF / 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
V_AREF/10. Therefore, the Current Monitor only supports differential voltage where |V_AV-V_AC| 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 2-73.
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 V_AREF as required.
Revision 4
2- 91