AN2688 Freescale Semiconductor / Motorola, AN2688 Datasheet - Page 2

• 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
Σ∆
2
Modulation at a Glance
Figure 1
The input signal is fed to the comparator through the integrator and the quantized
output is fed back and subtracted from the input. This feedback forces the average
value of the quantized signal to track the average input. Any difference between
them accumulates in the integrator and eventually corrects itself. Jointly with the
integrator, the feedback loop attenuates the quantization noise at low frequencies
while shaping the noise power to the high frequency range. Since the signal is
sampled at a frequency greater than the Nyquist rate, high frequency noise can be
removed without affecting the signal band by means of a digital low-pass filter
operating at the output of the Σ∆ modulator.
Figure 2
quantized output oscillates in such a manner that its local average equals the
average input. The digital data coming from the modulator is a single bit stream of
1’s and 0’s and the density of digital 1’s is proportional to the average value of the
input signal. The high frequency data from the modulator is then low-pass filtered
in order to remove any shaped noise and to decimate to the Nyquist rate. This
decimation results in an n-bits binary output format.
Implementing a 10-Bit Sigma-Delta Analog-to-Digital Converter
•
•
•
∆Σ MODULATOR
Using the HC9S08Rx MCU Family Analog Comparator
ANALOG INPUT
An analog integrator
A quantizer or comparator enclosed in a feedback loop via a single-bit
digital-to-analog (DA) converter
A digital low-pass filter.
Freescale Semiconductor, Inc.
For More Information On This Product,
illustrates the basic architecture of a Σ∆ AD converter comprising:
exhibits the response of the modulator to a ramp input signal. Note the
SIGNAL
Go to: www.freescale.com
∑
CONVERTER
Figure 1. Σ∆ AD Converter
1-BIT DA
INTEGRATOR
∫
V
V
REF+
REF–
1-BIT QUANTIZER
CK
STREAM
BIT
AND DECIMATOR
DIGITAL FILTER
MOTOROLA
n BITS