Device Architecture
I
R
0-12 V
SENSE
AVx
ACx
CMSTBx
to Analog MUX
(refer Table 2-36
for MUX channel
number)
V
ADC
10 X
Current Monitor
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 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 2-37 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 2-73.
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 VAREF as required.
2-92
Preliminary v1.7
ACTEL [ Actel Corporation ]
