AN2688 Freescale Semiconductor / Motorola, AN2688 Datasheet - Page 10

• 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

Freescale Semiconductor, Inc.
AN2688/D
The Σ∆ modulator evaluation is performed by analyzing the single-bit output signal
spectrum through the Fast Fourier Transform (FFT). Input power is normalized to
the quantization step defined by the comparator supply voltage whereas the
Figure 9
sampling rate is set to 128 kHz with the timer interruption feature.
shows
the output spectrum for a 125 Hz sinusoidal input signal with –8 dB of normalized
power.
NOTE:
The noise shaping concentrates the noise power in the high frequency range of the
spectrum.
The spectrum detail in
Figure 10
demonstrates a SNR of 55 dB for a –8 dB input
power.
shows the measured Σ∆ modulator SNR versus the input signal
Figure 11
Figure
11, the measured dynamic range
amplitude. Extrapolating from the data of
of the AD converter is found to be 65.7 dB and its maximum SNR is 61.2 dB. For
large input signals, the precision of the converter is limited by quantization noise
Figure 9
and harmonic distortion as apparent from the output spectrum in
and
Figure
10. The 10-bit digital output is monotonically linear as expected.
Figure 9. Measured Output Spectrum of the First-Order Continuous-Time Σ∆ Modulator
10
Implementing a 10-Bit Sigma-Delta Analog-to-Digital Converter
MOTOROLA
Using the HC9S08Rx MCU Family Analog Comparator
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