AN2688 Freescale Semiconductor / Motorola, AN2688 Datasheet - Page 7

• Manufacturer Part Number: AN2688
• Description: Implementing a 10-Bit Sigma-Delta Analog-to-Digital Converter Using the HC9S08Rx MCU Family Analog Comparator
• Manufacturer: Freescale Semiconductor / Motorola

Application Description
MOTOROLA
Figure 5
MC9S08Rx MCU including the analog comparator feature. Assuming the system
employs two NiCd batteries of 1.2 V and the full input signal range is to be 2.4 V,
R
resistors are not critical, carbon composition resistors or metal film resistors with
5% tolerance can be used. C
modulator sampling frequency and oversampling ratio. The modulator sampling
frequency is the MCU bus clock frequency divided by the number of cycles
required to run the software instructions. As mentioned in
the modulator sampling frequency can be set to 50 kHz adjusting properly the timer
interruption as a function of the 8-MHz bus clock. Therefore, for an oversampling
ratio of 130, the input signal bandwidth is limited to be 190 Hz and the integrator
capacitor should be below 18 nF.
Typically, application systems have two separate capacitors across the power pins.
In this case there should be a bulk electrolytic capacitor, such as a 1-µF tantalum
capacitor, to provide bulk charge storage for the overall system and a 0.1-µF
ceramic bypass capacitor located as close to the MCU power pins as practical to
suppress high-frequency noise.
Implementing a 10-Bit Sigma-Delta Analog-to-Digital Converter
1
Figure 5. First-Order Continuous-Time Σ∆ AD Converter Circuit Diagram
V
and R
In
Using the HC9S08Rx MCU Family Analog Comparator
Freescale Semiconductor, Inc.
For More Information On This Product,
shows the schematic diagram of the 10-bit Σ∆ AD converter using a
2
47 kΩ
must be set to 47 kΩ to prevent integrator saturation. Since these
R
1
1 nF
C
1
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RESET
47 kΩ
R
2
25 pF
C
2
1
RESET
is a ceramic capacitor. Its value depends on the
PTA1
16 MHz
1 MΩ
R
3
64
48
52
ACMP0
51
53
XTAL
C
25 pF
3
9S08Rx
MCU
18
V
V
DD
19
Supply
V
SS
= 2.4 V
Software
100 nF
30
31
32
33
38
39
40
46
13
14
15
C
PTC0
PTC1
PTC2
PTC3
PTC4
PTC5
PTC6
PTC7
PTB0
PTB1
PTB2
4
Application Description
Description,
RESULTS L:H
1 µF
C
5
AN2688/D
7