M30620FCAFP datasheet(457/615 Pages) RENESAS

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A-D Converter

Mitsubishi microcomputers

M16C / 62A Group

SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER

2-140

To carry out A-D conversion properly, charging the internal capacitor C shown in Figure 2.7.29 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 2.7.28. 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 –

)

–

=ln

Hence, R0 = –– R

With the model shown in Figure 2.7.29 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

µs 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 = ––7.8 X103

3.0 X 103

2.7.15 Sensor’s Output Impedance under A-D Conversion

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

Ω. Tables 2.7.14 and 2.7.15 show output impedance values

based on the LSB values.

Figure 2.7.29 A circuit equivalent to the A-D conversion terminal

C (3.0pF)

VIN

Microprocessor's inside

Sensor-equivalent circuit

R (7.8k )

Ω

C (R0 +R)

C (R0 + R)

–

C (R0 + R)

–

C • ln

3.0 X 10 –12 • ln

1024

0.1

0.3 X 10-6

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