M16CSLASH29 datasheet(278/499 Pages) RENESAS

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15. A/D Converter

page 254

Figure 15.29 Analog Input Pin and External Sensor Equivalent Circuit

15.5 Output Impedance of Sensor under A/D Conversion

To carry out A/D conversion properly, charging the internal capacitor C shown in Figure 15.29 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, MCU’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 15.29

shows analog input pin and externalsensor equivalent circuit. When the difference be-

tween 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 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 103 ≅ 13.9 X 103

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.

1 T

C(R0+R)

C•ln

1.5X10-12•ln

0.1

1024

0.3X10-6

R (7.8kΩ)

C (1.5pF)

VIN

Sampling time

Sample-and-hold function enabled:

Sample-and-hold function disabled:

φAD

MCU

Sensor equivalent

circuit

φAD

(1)

NOTE:

1. Reference value

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