M1AFS250-FGG256I Actel Corporation, M1AFS250-FGG256I Datasheet - Page 108

• Manufacturer Part Number: M1AFS250-FGG256I
• Description: Actel Fusion Family Of Mixed Signal Fpgas
• Manufacturer: Actel Corporation

Device Architecture
2- 92
Figure 2-73 • 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
by the ADC will stay fixed at the full scale value. Therefore, the user must select the external sense
resistor appropriately.
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 = I
The Current Monitor is a unipolar system, meaning that the differential voltage swing must be from 0 V to
V
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
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
AREF
/10. Therefore, the Current Monitor only supports differential voltage where |V
/ 10. A larger AV-to-AC voltage drop will result in ADC saturation; that is, the digital code put out
0-12 V
AVx
Current Monitor
Table 2-38
R
SENSE
shows recommended resistor values for different current measurement
CMSTBx
ACx
I
R e visio n 1
AREF
10 X
as required
.
VADC
for MUX channel
(refer Table 2-36
to Analog MUX
number)
Figure
AV
-V
2-74.
AC
2
| is greater
× R).