Microchip Technology DM183037 Data Sheet
PIC18F97J94 FAMILY
DS30575A-page 514
2012 Microchip Technology Inc.
The CTMU current source may be trimmed with the
ITRIMx bits in CTMUCON1, using an iterative process
to get the exact current desired. Alternatively, the nom-
inal value without adjustment may be used. That value
may be stored by software for use in all subsequent
capacitive or time measurements.
To calculate the optimal value for R
ITRIMx bits in CTMUCON1, using an iterative process
to get the exact current desired. Alternatively, the nom-
inal value without adjustment may be used. That value
may be stored by software for use in all subsequent
capacitive or time measurements.
To calculate the optimal value for R
CAL
, the nominal
current 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
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
R
CAL
= 2.31V/0.55
A, for a value of 4.2 MΩ. Similarly,
if the current source is chosen to be 5.5
A, R
CAL
would
be 420,000Ω, and 42,000Ω if the current source is set
to 55
to 55
A.
FIGURE 26-2:
CTMU CURRENT SOURCE
CALIBRATION CIRCUIT
CALIBRATION CIRCUIT
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. If an exact current is chosen to
incorporate the trimming bits from CTMUCON1, the
resistor value of R
sure that the A/D Converter is in a range that is well
above the noise floor. If an exact current is chosen to
incorporate the trimming bits from CTMUCON1, the
resistor value of R
CAL
may need to be adjusted accord-
ingly. R
CAL
also may be adjusted to allow for available
resistor values. R
CAL
should be of the highest precision
available in light of the precision needed for the circuit
that the CTMU will be measuring. A recommended
minimum would be 0.1% tolerance.
The following examples show a typical method for
performing a CTMU current calibration.
•
that the CTMU will be measuring. A recommended
minimum would be 0.1% tolerance.
The following examples show a typical method for
performing a CTMU current calibration.
•
demonstrates how to initialize the
A/D Converter and the CTMU.
This routine is typical for applications using both
modules.
This routine is typical for applications using both
modules.
•
demonstrates one method for the
actual calibration routine.
This method manually triggers the A/D Converter to
demonstrate the entire step-wise process. It is also
possible to automatically trigger the conversion by
setting the CTMU’s CTTRIG bit (CTMUCON<0>).
This method manually triggers the A/D Converter to
demonstrate the entire step-wise process. It is also
possible to automatically trigger the conversion by
setting the CTMU’s CTTRIG bit (CTMUCON<0>).
A/D Converter
CTMU
ANx
R
CAL
Current Source
A/D
Trigger
MUX
A/D
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