REVISION 08/26/15
5
P9025AC FOD TUNING GUIDE
AN-886
The P9025AC has a 1.8V reference connected by a current mirror to the FOD2 resistor which reflects the current to an internal
ADC. The external FOD2 resistor creates a voltage at the ADC input and the offset is proportional to the resistance to GND at
the FOD2 pin. The ADC input voltage (cannot be measured externally) can be determined using the following equation:
Next the Power received reported value is shifted proportionally to the V
FOD2
voltage. At 0.9V, the offset is 0 mW. At 0 V, the
offset is -300mW and at 1.8V, the offset is +300mW. The Rx reported received power shift can be calculated using the following
equation:
where,
VFOD2 is found using equation 6
The V
FOD2
scaling factor is 333.333 (unit considered as mA)
Table 2: FOD2 Global Offset Example Values based on RFOD2 Resistance
The above table is for reference to validate accurate calculations and check results. Additionally, the following curve may be used
to estimate the appropriate FOD2 resistance to achieve the required additional offset shift:
FOD2 Resistor (k
Ω
)
VFOD2 Voltage (V)
Global Offset (mW)
25
1.8
300.00
35
1.28
-128.57
50
0.90
0.00
100
0.45
-150
250
0.18
-240
350
0.128
-257
OPEN
0
-300.00
