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