M16C26A Datasheet, PDF (229/352 Pages) Renesas Technology Corp – 16-BIT SINGLE-CHIP MICROCOMPUTER M16C FAMILY / M16C/Tiny SERIES

M16C/26A Group (M16C/26A, M16C/26B, M16C/26T)
14. A/D Converter
14.5 Output Impedance of Sensor under A/D Conversion
To carry out A/D conversion properly, charging the internal capacitor C shown in Figure 14.5.1 has to be
completed within a specified period of time. T (sampling time) as the specified time. Let output imped-
ance of sensor equivalent circuit be R0, microcomputer’s internal resistance be R, precision (error) of
the A/D converter be X, and the A/D converter’s resolution be Y (Y is 1024 in the 10-bit mode, and 256
in the 8-bit mode).
1t
VC is generally VC = VIN{1-e ^c(R0+R) }
And when t = T, VC=VIN- ^X VIN=VIN(1- ^X )
Y
Y
1
e ^c(R0+R)
T
=
X
Y
Hence,
1
-
T = ln
C(R0+R)
X
Y
T
R0 = -
C•ln
X
-R
Y
Figure 14.5.1 shows analog input pin and externalsensor equivalent circuit. When the difference be-
tween 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 timer T. (0.1/1024) means that A/D precision drop due to insufficient
capacitor chage 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(XIN) = 10MHz, T=0.3µs in the A/D conversion
mode with sample & hold. Output inpedance R0 for sufficiently charging capacitor C within time T is
determined as follows.
T = 0.3µs, R = 7.8kΩ, C = 1.5pF, X = 0.1, and Y = 1024. Hence,
R0 = -
0.3X10^-6
1.5X10^-12•ln ^0.1
1024
- 7.8 X 10³ ≅ 13.9 X 10³
Thus, the allowable output impedance of the sensor circuit capable of thoroughly driving the A/D con-
verter turns out of be approximately 13.9kΩ.
Sensor equivalent
circuit
R0
VIN
Microcomputer
(1)
R (7.8kΩ)
(1)
C (1.5pF)
Sampling time
VC
Sample-and-hold function enabled:
3
φAD
Sample-and-hold function
disabled:
2
φAD
NOTES:
1. Reference value
Figure 14.5.1 Analog Input Pin and External Sensor Equivalent Circuit
Rev. 2.00 Feb.15, 2007 ^{page 212} of 329
REJ09B0202-0200