AN2688 Freescale Semiconductor / Motorola, AN2688 Datasheet - Page 4

• 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

AN2688/D
Implementation with HC9S08Rx MCU Family
4
The HC9S08Rx MCU with the analog comparator function can be configured as a
first-order continuous-time Σ∆ AD converter as depicted in
by adding:
The R
conversion is achieved by driving the resistor R
resistors must be adjusted to allow full range operation avoiding the integrator
saturation. A suitable capacitor value must be adjusted regarding the sampling
frequency and the input signal bandwidth. Once again, the R
low-pass filtering with the option to include an additional anti-alias analog filter.
One internal HC9S08Rx MCU timer accumulates the number of 1’s (or 0’s) at the
output of the comparator to compute the average of the analog input signal. A
second timer controls the number of accumulator counts. Each accumulator
increment is carefully controlled by software and it defines the Σ∆ modulator
sampling frequency and then the oversampling ratio M. To achieve a 10-bit output
word length, the accumulator must sample 1024 times the input signal. Therefore,
the conversion time increases when a more accurate solution is required.
ANALOG INPUT
Implementing a 10-Bit Sigma-Delta Analog-to-Digital Converter
Figure 4. First-Order Continuous-Time Σ∆ AD Converter Implementation
•
•
•
with The HC9S08Rx MCU Including the Analog Comparator Feature
Using the HC9S08Rx MCU Family Analog Comparator
SIGNAL
1
Two external resistors
One capacitor
Properly programming the internal timers to filter and decimate the
comparator digitized output.
Freescale Semiconductor, Inc.
EXTERNAL COMPONENTS
-C
For More Information On This Product,
1
integrator is similar to the Σ∆ modulator in
INTEGRATOR
PASSIVE RC
R
1
Go to: www.freescale.com
C
1
R
2
V
COMPARATOR
BG
OUTPUT
BUFFER
+
–
IMPLEMENTATION BY SOFTWARE
V
DD
2
SAMPLING
with an output buffer. The
CK
Figure
HC9S08Rx MCU
Figure
1
-C
DIGITAL FILTER
3. The 1-bit DA
1
network provides
4. This is done
MOTOROLA
n BITS