M16C80 Datasheet, PDF (224/358 Pages) Renesas Technology Corp – RENESAS 16-BIT SINGLE-CHIP MICROCOMPUTER M16C FAMILY / M16C/80 SERIES

M16C/80 Group
27. Usage Precaution
With the model shown in Figure 27.1 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 us 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 27.1 and 27.2 show output impedance values based
on the LSB values.
Sensor Equivalent Circuit
R0
VIN
Microcomputer
R (7.8k )
C (3.0pF)
VC
Figure 27.1 Anolog Input Pin and External Sensor Equivalent Circuit
Rev.1.00 Aug. 02, 2005 ^{Page 213} of 329
REJ09B0187-0100