M32C80 Datasheet, PDF (253/352 Pages) Renesas Technology Corp – 16/32-BIT SINGLE-CHIP MICROCOMPUTER M16C FAMILY / M32C/80 SERIES

M32C/80 Group
17. A/D Converter
17.2.7 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.
17.2.8 Output Impedance of Sensor Equivalent Circuit under A/D Conversion
For perfect A/D converter performance, complete internal capacitor (C) charging, shown in Figure 17.8,
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
1
C (R0 + R)
t
}
X
X
When t = T, VC = VIN – _Y VIN = VIN (1 – _Y )
–
e
1
T
C (R0 + R)
=
X
Y
1
X
–
T= ln
C (R0 +R)
Y
T
R0 = –
–R
C • ln ^X
Y
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 17.8 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 = –
0.3 X 10^-6
0.1
1.5 X 10 ^–12 • ln
1024
–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 kΩ maximum.
Rev. 1.00 Nov. 01, 2005 ^{Page 234} of 330
REJ09B0271-0100