User's Manual

30 DRAFT P440 Data Sheet / User Guide
4.3.4 Chassis Ground
The P440 is provided with a chassis ground. Each of the six mounting holes is copper plated on the
top, bottom, and inside of the hole. The mounting holes are not covered with silk screen. These
holes are not connected to any ground planes or signals of any sort. The one exception to this rule is
connected to Digital_Ground through the parallel combination of a 0.01uF capacitor and 1.0 MOhm
resistor. The position of this hole is shown in Figure 14.
4.3.5 Fused_Ground and Digital_Ground
Connecting power to the board is relatively straightforward, but there is subtlety associated with the
ground. The subtlety is associated with the difference between Fused_Ground and Digital_ Ground.
As a general rule, it is best to connect to the Fused_Ground and avoid the Digital_Ground. This is
not a concern for developers who will typically interface to the P440 using either the Ethernet or the
USB connectors and power the unit through the USB Power jack. It is also not a concern when the
P440 is integrated into a final product through either the Locking Connector or the User Mezzanine
connector.
It might be an issue if the user intends to connect to either the Ethernet Mezzanine connector or GPIO
pins on the Factory Mezzanine connector. Normally the Fused_Ground is the preferred connection,
but there are some cases in which it might be better to connect to the Digital_Ground. Customers
intending to make use of these connections should contact the factory and discuss the issue in more
detail.
4.3.6 P440 Power Requirements
When operating continuously, a standard P440 requires approximately 2 watts. However, two other
factors need to be considered. First the power consumption of electronics will vary with temperature.
Second, the efficiency of the regulators declines with increasing input voltage. Basically, the
regulators have been designed for optimum efficiency when operated at 5 volts.
Figure 16 indicates how the power consumed from a 5 volt supply changes with temperature for two
different P440s. The temperatures shown were measured by the onboard temperature sensor. Note
that the units require a bit more power when operated as a receiver than as a transmitter. These
results are typical.
Fig. 16: Power Consumption as a function of board temperature for two representative P440s
(red and blue) when operated as a transmitter (left) and as a receiver (right)
Figure 17 indicates how the efficiency of the onboard regulators changes with input voltage. This
data was measured while the P440 was transmitting and the onboard temperature sensor indicated a
temperature of 37°C.
Power Consumption (Watts)
Temp (Deg C)
Power Consumption (Watts)
Temp (Deg C)
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
-40 -20 0 20 40 60 80 100
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
-40 -20 0 20 40 60 80 100