PIC18F44J50-I Datasheet, PDF (404/562 Pages) Microchip Technology – 28/44-Pin, Low-Power, High-Performance USB Microcontrollers

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The CTMU current source may be trimmed with the
trim bits in CTMUICON, using an iterative process to
get an exact desired current. Alternatively, the nominal
value without adjustment may be used; it may be
stored by the software for use in all subsequent
capacitive or time measurements.
To calculate the optimal value for RCAL, the nominal cur-
rent must be chosen. For example, if the A/D Converter
reference voltage is 3.3V, use 70% of full scale, or
2.31V as the desired approximate voltage to be read by
the A/D Converter. If the range of the CTMU current
source is selected to be 0.55 A, the resistor value
needed is calculated as RCAL = 2.31V/0.55 A, for a
value of 4.2 MΩ. Similarly, if the current source is cho-
sen to be 5.5 A, RCAL would be 420,000Ω and
42,000Ω if the current source is set to 55 A.
FIGURE 26-2:
CTMU CURRENT SOURCE
CALIBRATION CIRCUIT
PIC18F46J50 Device
Current Source ^CTMU
A value of 70% of full-scale voltage is chosen to make
sure that the A/D Converter is in a range that is well
above the noise floor. Keep in mind that if an exact cur-
rent is chosen that is to incorporate the trimming bits
from CTMUICON, the resistor value of RCAL may need
to be adjusted accordingly. RCAL may also be adjusted
to allow for available resistor values. RCAL should be of
the highest precision available, keeping in mind the
amount of precision needed for the circuit that the
CTMU will be used to measure. A recommended
minimum would be 0.1% tolerance.
The following examples show one typical method for
performing a CTMU current calibration. Example 26-1
demonstrates how to initialize the A/D Converter and
the CTMU. This routine is typical for applications using
both modules. Example 26-2 demonstrates one
method for the actual calibration routine.
ANx
RCAL
A/D Converter
A/D
MUX
DS39931D-page 404
 2011 Microchip Technology Inc.