currents for both the driver and power amplifier stages, it is essential that the current
measurements are performed to as high an accuracy as possible.
The output current measurement is made by sampling the voltage developed across two low
value resistors connected in parallel, R23 and R24, each of 0.22Ω. This gives a shunt
resistance of 0.11Ω giving a scaling factor of approximately 110mV/A. The DC voltage is
amplified by A1-A which has a gain of x10, giving an overall scaling factor of approximately
1.1V/A.
Assuming an amplifier current of 2.5A, this gives a voltage into the A-D convertor of 2.75V,
and the raw output from the convertor will be:
2.75 / 5 * 4096
= 2253
The maximum full-scale value of this ammeter is thus 2.5 * 4096 / 2253 or 4.545A, and has a
quantisation step size of 4.545 / 4096 or 1.1mA. This is the intrinsic resolution limit of the
ammeter.
In order to take advantage of this resolution, at a nominal input current of 2.5A, the required
calibration adjustment factor would be 2220. Thus the actual current calculation would be:
display
I
= ADC * Calibration Factor / 2,000,000
As an example, assuming nominal settings:
display
I
= 2253 * 2220 / 2,000,000
= 5,001,660 / 2,000,000
= 2.5008 Amps
Power Measurement
This is the least accurate of the measurements. Since power is proportional to the square of
the voltage, the output from the A-D convertor has to be squared. Any differences in
construction, or differences in components will therefore lead to a squaring of the error terms,
leading to a greater uncertainty in the measurement compared to the voltmeter and ammeter.
In addition, the diodes used to rectify the RF voltage developed in the dual directional coupler
are also non-linear, thus the accuracy of the measurement will degrade significantly at low
power levels.
The forward power scaling factor is approximately 2.05V/10W, which would result in a
convertor output of:
2.05 / 5 * 4096
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