R8CSLASH13 datasheet(146/224 Pages) RENESAS

146 / 224 page

Rev.1.20

Jan 27, 2006

page 135 of 205

REJ09B0111-0120

R8C/13 Group

14.7 Output Impedance of Sensor under A/D Conversion

To carry out A/D conversion properly, charging the internal capacitor C shown in Figure 14.11 has to

be completed within a specified period of time. T (sampling time) as the specified time. 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 VC = VIN {1 – e}

And when t = T,

VC=VIN –

VIN = VIN(1 –

)

–

T = ln

Hence, R0 = –– R

Figure 14.11 shows analog input pin and external sensor equivalent circuit. When the difference

between VIN and VC becomes 0.1 LSB, 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 0.1 LSB at time of A/D conversion in the 10-bit mode. Actual

error however is the value of absolute precision added to 0.1 LSB. When f(XIN) = 10 MHz, T = 0.25 µs

in the A/D conversion mode with sample & hold. Output impedance R0 for sufficiently charging capaci-

tor C within time T is determined as follows.

T = 0.25

µs, R = 2.8 kΩ, C = 1.5 pF, X = 0.1, and Y = 1024 . Hence,

R0 = –– 2.8 X 103

7.3 X 103

Thus, the allowable output impedance of the sensor circuit capable of thoroughly driving the A/D

converter turns out to be approximately 7.3 k

Ω.

C (R0 +R)

C • ln

C (R0 + R)

–

C (R0 + R)

–

6.0 X 10 –12 • ln

1024

0.1

0.25 X 10 –6

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