M16C62P_06 Datasheet, PDF (264/421 Pages) Renesas Technology Corp – 16-BIT SINGLE-CHIP MICROCOMPUTER M16C FAMILY / M16C/60 SERIES

M16C/62P Group (M16C/62P, M16C/62PT)
18. A/D Converter
18.2.5 18.2.5 Current Consumption Reducing Function
When not using the A/D converter, its resistor ladder and reference voltage input pin (VREF) can be separated
using the VCUT bit in the ADCON1 register. When separated, no current will flow from the VREF pin into the
resistor ladder, helping to reduce the power consumption of the chip.
To use the A/D converter, set the VCUT bit to “1” (Vref connected) and then set the ADST bit in the ADCON0
register to “1” (A/D conversion start). The VCUT and ADST bits cannot be set to “1” at the same time.
Nor can the VCUT bit be set to “0” (Vref unconnected) during A/D conversion.
Note that this does not affect VREF for the D/A converter (irrelevant).
18.2.6 Output Impedance of Sensor under A/D Conversion
To carry out A/D conversion properly, charging the internal capacitor C shown in Figure 18.11 has to be
completed within a specified period of time. T (sampling time) as the specified time. Let output impedance of
sensor equivalent circuit be R0, internal resistance of microcomputer be R, precision (error) of the A/D
converter be X, and the resolution of A/D converter be Y (Y is 1024 in the 10-bit mode, and 256 in the 8-bit
mode).
VC is generally
VC=
⎧
VIN_⎨1
–
–
e
C-----(^{--R-----0-1---+-----R-----})
t_⎫
⎬
⎩
⎭
And when t = T,
VC
=
VIN – ^X---
Y
VIN
=
VIN_⎝⎛1 – ^XY---_⎠⎞
–
e
C-----(^{--R-----01----+-----R----})^-
T
=
X---
Y
– _C-----(--R-----01----+-----R----₎-T=
ln ^X---
Y
Hence, R0= – ---------T---------- – R
C • ln ^X---
Y
Figure 18.11 shows Analog Input Pin and External Sensor Equivalent Circuit. When the difference between
VIN and VC becomes 0.1LSB, we find impedance R0 when voltage between pins VC changes from 0 to VIN-
(0.1/1024) VIN in time T. (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 f(φAD) = 10 MHz, T = 0.3 µs in the A/D conversion mode with sample & hold. Output impedance R0 for
sufficiently charging capacitor C within time T is determined as follows.
T = 0.3 µs, R = 7.8 kΩ, C = 1.5 pF, X = 0.1, and Y = 1024. Hence,
R0= – --------------0---.-3-----^×-----1---0---–---6--------------- – 7.8×103 = 13.9×103
1.5 × 10–12 • ln ---0---.-1----
1024
Thus, the allowable output impedance of the sensor equivalent circuit, making the precision (error) 0.1LSB or
less, is approximately 13.9 kΩ. maximum.
Rev.2.41 Jan 10, 2006 Page 249 of 390
REJ09B0185-0241