Digital Signal Processing Reference
In-Depth Information
Gain
+
Coarse
4 - bit
ADC
Fine
4 - bit
ADC
+
Vin
DAC
−
4 MSBs
4 LSBs
Fig. 7
A sub-ranging ADC with two 4-bit subconverters
3.2
Oversampled A/D Converters
Oversampled converters use a sampling rate that is much higher than the Nyquist
rate. The oversampling ratio (OSR) is defined as the ratio of the actual sampling
rate and the Nyquist rate. The OSR is typically between 8 and 512. Oversampled
converters can utilize noise shaping, which means shaping the spectrum of the
quantization noise so that most of the noise power is moved outside of the signal
band. The sampling rate is subsequently lowered close to the Nyquist rate by
decimation. This operation involves a so-called decimation filter, which is a lowpass
filter that limits the signal bandwidth to avoid aliasing. The decimation filter also
attenuates the out-of-band quantization noise.
3.2.1
Benefits of Oversampling
frequency. Assuming that no noise shaping is done, the noise is evenly distributed
in the frequency domain from zero to the Nyquist frequency. When oversampling is
used, the portion of the quantization noise that is outside of the signal band can be
filtered out, and the quantization noise power on the signal band is reduced to
q
2
12
(
1
OSR
)
.
2
OSR
σ
=
(15)
In the oversampled case the signal-to-noise ratio is therefore given by
SNR
OSR
=
6
.
02
b
+
1
.
76
+
10 log
(
OSR
)
dB
(16)
where the last term is the benefit achieved by oversampling. Thus, SNR is increased
by 3 dB when the sampling rate is doubled, which corresponds to 0.5 bits increase
in resolution.
