Datasheet

14. A/D Converter
page 212
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14.5 Output Impedance of Sensor under A/D Conversion
To carry out A/D conversion properly, charging the internal capacitor C shown in Figure 14.5.1 has to be
completed within a specified period of time. T (sampling time) as the specified time. Let output imped-
ance 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 VC = VIN{1-e
c(R0+R)
}
And when t = T, VC=VIN- VIN=VIN(1- )
e
c(R0+R)
=
- T = ln
Hence, R0 = - - R
Figure 14.5.1 shows analog input pin and externalsensor equivalent circuit. When the difference be-
tween 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 timer T. (0.1/1024) means that A/D precision drop due to insufficient
capacitor chage 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) = 10MHz, T=0.3µs in the A/D conversion
mode with sample & hold. Output inpedance R0 for sufficiently charging capacitor C within time T is
determined as follows.
T = 0.3µs, R = 7.8kΩ, C = 1.5pF, X = 0.1, and Y = 1024. Hence,
R0 = - - 7.8 X 10
3
≅ 13.9 X 10
3
Thus, the allowable output impedance of the sensor circuit capable of thoroughly driving the A/D con-
verter turns out of be approximately 13.9kΩ.
Figure 14.5.1 Analog Input Pin and External Sensor Equivalent Circuit
1
1
T
t
C(R0+R)
1
X
Y
X
Y
X
Y
X
Y
T
C•ln
X
Y
1.5X10
-12
•ln
0.1
1024
0.3X10
-6
R0
R (7.8kΩ)
C (1.5pF)
V
IN
VC
Sampling time
Sample-and-hold function enabled:
Sample-and-hold function disabled:
3
φ
AD
Microcomputer
Sensor equivalent
circuit
2
φ
AD
(1)
(1)
NOTES:
1. Reference value