
17. A/D Converter
17.2.7
Power Consumption Reduce Function
When not using the A/D converter, the VCUT bit in the AD0CON1 register can isolate the resistor ladder of the
A/D converter from the reference voltage input pin (VREF). Power consumption can be reduced by shutting off
any current flow into the resistor ladder from the VREF pin.
When using the A/D converter, set the VCUT bit to 1 (VREF connected) before setting the ADST bit in the
AD0CON0 register to 1 (A/D conversion starts).
Do not set bits ADST and VCUT to 1 simultaneously, nor set the VCUT bit to 0 (VREF not connected) during
A/D conversion.
Even if the VCUT bit is set to 0, VREF remains connected to the D/A converter.
17.2.8
Output Impedance of Sensor Equivalent Circuit under A/D Conversion
To take full advantage of the A/D converter performance, Internal capacitor (C) charging shown in
Figure 17.9must be completed for the specified period (T) as sampling time. Output impedance of the sensor equivalent
circuit (R0) is determined by the following equations:
where:
VC = Internal capacitor voltage
R = Internal resistance of the MCU
X = Precision (error) of the A/D converter
Y = Resolution of the A/D converter (1024 in 10-bit mode, and 256 in 8-bit mode)
Figure 17.9 shows a connection example of analog input pin and external sensor equivalent circuit.
The impedance R0 can be obtained if VC changes from 0 to VIN-(0.1/1024) VIN in the time (T), when the
difference between VIN and VC becomes 0.1LSB. (0.1/1024) means that A/D precision drop, due to
insufficient capacitor charge, is held to 0.1LSB at time of A/D conversion in the 10-bit mode. Actual error,
however, is the value of absolute precision added to 0.1LSB.
When
φAD = 10 MHz, T = 0.3 μs in A/D operating mode with the sample and hold function. Output impedance
(R0) for sufficiently charging capacitor (C) in the time (T) is determined by the following equation:
Thus, the allowable output impedance R0 of the sensor equivalent circuit, making the precision (error) 0.1LSB
or less, is approximately 13.9 k
Ω maximum.
VC
VIN 1
e
1
CR0
R
+
()
---------------------------- t
–
=
When t = T,
VC
VIN
X
Y
----VIN
–
VIN 1
X
Y
----
–
==
e
1
CR0
R
+
()
---------------------------- T
–
X
Y
----
=
1
CR0
R
+
()
----------------------------T
–
X
Y
----
ln
=
R0
T
C
X
Y
----
ln
--------------
–
R
–
=
13.9
3
×10
R0
0.3
6
–
×10
1.5
12
–
×10
0.1
1024
-------------
ln
---------------------------------------------------
–
7.8
3
×10
–
=
Using T = 0.3
μs, R = 7.8 kΩ, C = 1.5 pF, X = 0.1, Y = 1024,