M16C30P_07 Datasheet, PDF (195/317 Pages) Renesas Technology Corp – 16-BIT SINGLE-CHIP MICROCOMPUTER M16C FAMILY / M16C/30 SERIES

M16C/30P Group
16. A/D Converter
16.2.5 Output Impedance of Sensor under A/D Conversion
To carry out A/D conversion properly, charging the internal capacitor C shown in Figure 16.8 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, 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).
VC is generally
– ------------1-------------- t
VC=
⎧
VIN_⎨
1
–
e
C(R0 + R)
⎫
⎬
⎩
⎭
And when t = T,
VC
=
VIN –
X---
Y
VIN
=
VIN_⎝⎛1 – ^XY---_⎠⎞
–
e
------------1-------------- T
C(R0 + R) ₌
X---
Y
– _C-----(--R-----01----+-----R----₎-T=
ln ^X---
Y
R0= – ---------T---------- – R
Hence,
C • ln ^X---
Y
Figure 16.8 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 circuit capable of thoroughly driving the A/D converter
turns out to be approximately 13.9 kΩ.
Sensor equivalent
circuit
R0
VIN
Microcomputer
R (7.8kΩ)
C (1.5pF)
Sampling time
VC
Sample-and-hold function enabled:
3
φAD
Sample-and-hold function disabled:
2
φAD
Figure 16.8 Analog Input Pin and External Sensor Equivalent Circuit
Rev.1.22 Mar 29, 2007 Page 179 of 291
REJ09B0179-0122