R28 datasheet(357/473 Pages) RENESAS

357 / 473 page

R8C/28 Group, R8C/29 Group

18. A/D Converter

Rev.2.10

Sep 26, 2008

Page 338 of 441

REJ09B0279-0210

18.6

Output Impedance of Sensor under A/D Conversion

To carry out A/D conversion properly, charging the internal capacitor C shown in Figure 18.11 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, internal resistance of microcomputer be R, precision (error) of the A/D converter be X,

and the resolution of A/D converter be Y (Y is 1024 in the 10-bit mode, and 256 in the 8-bit mode).

VC is generally

And when t = T,

Hence,

Figure 18.11 shows the Analog Input Pin and External Sensor Equivalent Circuit. 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 = 0.25

µs in the A/D conversion mode without sample and hold. Output impedance R0

for sufficiently charging capacitor C within time T is determined as follows.

T = 0.25

µs, R = 2.8 kΩ, C = 6.0 pF, X = 0.1, and Y = 1024. Hence,

Thus, the allowable output impedance of the sensor equivalent circuit, making the precision (error) 0.1LSB or less,

is approximately 1.7 k

Ω. maximum.

Figure 18.11

Analog Input Pin and External Sensor Equivalent Circuit

R (2.8 k

Ω)

C (6.0 pF)

VIN

MCU

Sensor equivalent

circuit

NOTE:

1. The capacity of the terminal is assumed to be 4.5 pF.

0.25

–

6.0

–

0.1

1024

------------

•

---------------------------------------------------

–2.8

×10

1.7

×10

≈

–

----

•

-------------------

–R

–

CR0

()

--------------------------

–T

----

CR0

()

--------------------------T

–

----

VIN

---- VIN

VIN 1

----

–





–

VIN 1

CR0

()

--------------------------

–







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