afs600 Actel Corporation, afs600 Datasheet - Page 97

• Manufacturer Part Number: afs600
• Description: Actel Fusion Programmable System Chips Mixed-signal Family With Optional Arm Support
• Manufacturer: Actel Corporation

Care must be taken when choosing the right resistor for
current measurement application. Note that because of
the
difference between the AV and AC pads is V
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
different current measurement ranges. When choosing
resistor values for a system, there is a trade-off between
measurement
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
differential voltage where |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
Figure 2-73 • Negative Current Monitor
Terminology
The accuracy of Fusion Current Monitor is ±2 mV
minimum plus 5% of the differential voltage at the
input. The input accuracy can be translated to error at
the ADC output by using
gain of the Current Monitor Circuit, as described in the
"Current Monitor" section on page
N = 8, V
AV and AC, the Error (E
10. Therefore, the Current Monitor only supports
Accuracy
E
2
10×
ADC
× R).
AREF
=
amplification,
shows recommended resistor values for
(
= 2.56 V, zero differential voltage between
2mV
accuracy
+
0.05 V
ADC
AV
EQ
) is equal to 2 LSBs.
the
and
–10.5 V
–
AV
V
0 to
2-3. The 10 V/V gain is the
AC
-V
maximum
AVx
AC
)
power
Current Monitor
×
| is greater than 0 V.
2-81. For 8-bit mode,
(
10V
R
SENSE
) V ⁄
consumption.
×
measurable
AREF
-------------- -
V
CMSTBx
ACx
AREF
2
N
/ 10. A
EQ 2-3
AREF
A d v an c ed v1 . 4
I
/
10 X
be "more negative" than the AC pad. This is shown in
Figure
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
as required
Table 2-37 • Recommended Resistor for Different Current
where
Current Range
> 5 mA – 10 mA
> 10 mA – 20 mA
> 20 mA – 50 mA
> 50 mA – 100 mA
> 100 mA – 200 mA
> 200 mA – 500 mA
> 500 mA – 1 A
> 1 A – 2 A
> 2 A – 4 A
N is the number of bits
V
V
V
2-73.
AREF
AV
AC
is the voltage at AV pad
is the voltage at AC pad
.
V
is the Reference voltage
Range Measurement
ADC
Actel Fusion Programmable System Chips
for MUX channel
(refer Table 2-36
to Analog MUX
number)
Recommended Minimum
Resistor Value (Ohms)
0.025 – 0.05
0.05 – 0.1
0.3 – 0.5
0.1 – 0.2
10 – 20
2.5 – 5
0.5 – 1
5 – 10
1 – 2
2-83
AREF