m5m51016btp Renesas Electronics Corporation., m5m51016btp Datasheet - Page 247

• Manufacturer Part Number: m5m51016btp
• Description: Renesas 16-bit Single-chip Microcomputer M16c Family / M16c/20 Series
• Manufacturer: Renesas Electronics Corporation.

M30245 Group
Rev.2.00 Oct 16, 2006
REJ09B0340-0200
Figure 2.9.24 A circuit equivalent to the A/D conversion terminal
2.9.13 Sensor’s Output Impedance under A/D Conversion (reference value)
To carry out A/D conversion properly, charging the internal capacitor C shown in Figure 2.9.23 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 2.9.23. 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 2.9.24 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 2.9.12 and 2.9.13 show output impedance values
based on the LSB values.
Vc is generally V
And when t = T,
Hence, R0 = –
T = 0.3 µs, R = 7.8 k , C = 3 pF, X = 0.1, and Y = 1024 . Hence,
R0 = –
3.0 X 10
page 238 of 354
Sensor-equivalent circuit
IN
0.3 X 10
) = 10 MHz, T = 0.3 us in the A/D conversion mode with sample & hold. Output
C
C • ln
–12
= V
V
V
e
–
IN
• ln
C
–
=V
T
IN
C (R0 +R)
C (R0 + R)
-6
{1 – e
IN
X
Y
1024
T
0.1
–
T
– R
R
Y
X
–
0
=
=ln
C (R0 + R)
V
–7.8 X10
IN
Y
=V
X
t
Y
X
IN
(1 –
3
R (7.8k )
C (3.0pF)
Microprocessor's inside
}
3.0 X 10
X
Y
)
C
3
changes from 0 to V
V
C
IN
IN
-(0.1/1024) V
and V
2. A/D Converter
C
becomes
IN
in