M32CSLASH86 datasheet(275/505 Pages) RENESAS

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Page 252

18. A/D Converter

)

(

18.2.7 Power Consumption Reducing Function

When the A/D converter is not used, the VCUT bit in the AD0CON1 register isolates the resistor ladder of

the A/D converter from the reference voltage input pin (VREF). Power consumption is reduced by shutting

off any current flow into the resistor ladder from the VREF pin.

When using the A/D converter, set the VCUT bit to "1" (VREF connection) before setting the ADST bit in

the AD0CON0 register to "1" (A/D conversion started). Do not set the ADST bit and VCUT bit to "1"

simultaneously, nor set the VCUT bit to "0" (no VREF connection) during the A/D conversion. The VCUT

bit does not affect the VREF performance of the D/A converter.

18.2.8 Output Impedance of Sensor Equivalent Circuit under A/D Conversion

For perfect A/D converter performance, complete internal capacitor (C) charging, shown in Figure 18.8,

for the specified period (T) as sampling time. Output Impedance of the sensor equivalent circuit (R0) is

determined by the following equations:

VC = VIN {1 – e

}

When t = T, VC = VIN –

VIN = VIN (1 –

)

–

T= ln

R0 = –

– R

where:

VC = Voltage between pins

R = Internal resistance of the microcomputer

X = Precision (error) of the A/D converter

Y = Resolution of the A/D converter (1024 in 10-bit mode, and 256 in 8-bit mode)

Figure 18.8 shows analog input pin and external sensor equivalent circuit. The impedance (R0) can be

obtained if the voltage between pins (VC) changes from 0 to VIN-(0.1/1024) VIN in the time (T), when the

difference between VIN and VC becomes 0.1LSB.

(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 ØAD = 10 MHz, T = 0.3

µs in the A/D conversion mode with the sample and hold function. Output

impedance (R0) for sufficiently charging capacitor (C) in the time (T) is determined by the following equation:

Using T = 0.3

µs, R = 7.8 kΩ, C = 1.5 pF, X = 0.1, Y = 1024,

R0 = ––7.8 X103 = 13.9 X 103

Thus, the allowable output impedance of the sensor equivalent circuit, making the precision (error)

0.1LSB or less, is approximately 13.9 k

Ω maximum.

C (R0 +R)

C • ln

C (R0 + R)

–

C (R0 + R)

–

1.5 X 10 –12 • ln

1024

0.1

0.3 X 10-6

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