M30622MA-XXXFP RENESAS [Renesas Technology Corp], M30622MA-XXXFP Datasheet - Page 427

• Manufacturer Part Number: M30622MA-XXXFP
• Description: 16-BIT SINGLE-CHIP MICROCOMPUTER M16C FAMILY
• Manufacturer: RENESAS [Renesas Technology Corp]

A-D Converter
412
Figure 2.7.29 A circuit equivalent to the A-D conversion terminal
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).
With the model shown in Figure 2.7.29 as an example, when the difference between V
0.1LSB, we find impedance R0 when voltage between pins V
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(X
impedance R0 for sufficiently charging capacitor C within time T is determined as follows.
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 k . Tables 2.7.14 and 2.7.15 show output impedance values
based on the LSB values.
Vc is generally V
And when t = T,
Hence, R0 = –
T = 0.3 s, R = 7.8 k , C = 3 pF, X = 0.1, and Y = 1024 . Hence,
R0 = –
3.0 X 10
IN
0.3 X 10
) = 10 MHz, T = 0.3 us in the A-D conversion mode with sample & hold. Output
C
Sensor-equivalent circuit
C • ln
–12
= V
V
e
–
• ln
C
–
=V
T
IN
C (R0 +R)
C (R0 + R)
-6
V
{1 – e
IN
Y
X
1024
IN
T
0.1
–
T
– R
Y
X
–
=
=ln
C (R0 + R)
V
–7.8 X10
IN
R
Y
=V
X
0
t
Y
X
IN
(1 –
3
}
3.0 X 10
Y
X
R (7.8k )
C (3.0pF)
Microprocessor's inside
)
C
3
changes from 0 to V
SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER
V
C
IN
IN
Mitsubishi microcomputers
M16C / 62 Group
-(0.1/1024) V
and V
C
becomes
IN
in