M16C62_M Datasheet, PDF (457/615 Pages) Renesas Technology Corp – 16-BIT SINGLE-CHIP MICROCOMPUTER M16C FAMILY / M16C/60 SERIES

A-D Converter
Mitsubishi microcomputers
M16C / 62A Group
SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER
2.7.15 Sensor’s Output Impedance under A-D Conversion
To carry out A-D conversion properly, charging the internal capacitor C shown in Figure 2.7.29 has to be
completed within a specified period of time. With T as the specified time, time T is the time that switches
SW2 and SW3 are connected to O in Figure 2.7.28. 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).
t
Vc is generally VC = VIN {1 – e ^{– C (R0 + R)} }
And when t = T,
VC=VIN –
X _{VIN=VIN(1 –}
X
)
Y
Y
Hence, R0 = –
T
e ^{– C (R0 + R)} = ^X
Y
T
X
–
=ln
C (R0 +R)
Y
T
–R
C • ln ^X
Y
With the model shown in Figure 2.7.29 as an example, 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(XIN) = 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 = 3 pF, X = 0.1, and Y = 1024 . Hence,
R0 = –
0.3 X 10^-6
0.1
3.0 X 10 ^–12 • ln
1024
–7.8 X10³ 3.0 X 10³
Thus, the allowable output impedance of the sensor circuit capable of thoroughly driving the A-D con-
verter turns out to be approximately 3.0 kΩ. Tables 2.7.14 and 2.7.15 show output impedance values
based on the LSB values.
Sensor-equivalent circuit
R0
VIN
Microprocessor's inside
R (7.8kΩ)
C (3.0pF)
VC
Figure 2.7.29 A circuit equivalent to the A-D conversion terminal
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