Texas Instruments | How delta-sigma ADCs work, Part 2 | Application notes | Texas Instruments How delta-sigma ADCs work, Part 2 Application notes

Texas Instruments How delta-sigma ADCs work, Part 2 Application notes
Data Acquisition
Texas Instruments Incorporated
How delta-sigma ADCs work, Part 2
By Bonnie Baker
Signal Integrity Engineer
A strong addition to the process-control design environment is the delta-sigma (DS) analog-to-digital converter
(ADC). This device’s claim to fame is its high 24-bit resolution, which provides 224 or about 16 million output
codes. Granted, not all of the lower bits are noise-free,
but it is not unusual for a DS ADC to have 20 noise-free
bits, or about 1 million noise-free output codes. This is at
least four times better than the performance of 16-bit
converters.
Figure 1 shows a block diagram of a DS ADC. As
explained in Part 1 of this article series (see Reference 1),
the modulator of a DS converter shapes the data in such a
way as to allow high reso­lution by reducing low-frequency
noise. Part 1 also pointed out that the undesirable characteristics of the modulator output are high-frequency noise
and a high-speed, 1-bit output rate. Once the signal resides
in the digital domain, a low-pass digital-filter function can
be used to attenuate the high-frequency noise, and a
decimator-filter function can be used to slow down the
output-data rate. This article, Part 2, will consider each
function independently, although real-world designs intertwine them in the same silicon.
The digital-filter function
The digital-filter function implements a low-pass filter by
first sampling the modulator stream of the 1-bit code.
Figure 2 shows a first-order, low-pass averaging filter. An
averaging filter is the most common filter technique used
in DS converters. As can be seen, the digital filter in
Figure 2 is a weighted averaging filter. Almost all DS ADCs
incorporate a class of averaging filters called sinc filters,
named for their frequency response. Many DS devices,
especially audio devices, use other filters in conjunction
with sinc filters as part of a process called two-stage decimation. Low-speed industrial DS ADCs usually use only
the sinc filter.
Figure 1. Block diagram of DS ADC
Sample Rate (fS )
∆Σ
Analog
Input
Modulator
fS /fD = Decimation Ratio (DR)
Data Rate (fD)
Digital
Filter
Digital
Output
Decimator
Filter
Digital/Decimation Filter
Figure 2. First-order, low-pass averaging filter
Input
Delay
Delay
Delay
b1
b2
b3
bi
Σ
Σ
Σ
Output
5
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The output rate of a digital filter is the
Figure 3. Outputs of a digital filter
same as the sampling rate. Figure 3 shows
a digital filter’s outputs. In the time domain
(Figure 3a), the digital filter is responsible
7FFFFF
for the high resolution of the DS converter.
Signal
Notice that the 24-bit code train resembles
the original signal. However, in the frequency
domain (Figure 3b), the digital filter applies
0000000
only a low-pass filter to the signal. In so doing,
it attenuates the modulator’s quantization
fS
noise; but it also reduces the frequency
800000
bandwidth, as any good low-pass filter will.
Quantization
Noise
With the quantization noise reduced, the signal re-emerges in the time domain.
(a) Time domain
(b) Frequency domain
The signal is now a high-resolution, digital
version of the input signal, but it is still too
fast to be useful. The designer could have
This may seem a bit distressing. Previously, there was a
the converter deliver every one of the samples, but it
beautiful sine wave that was well-defined with a large
would be pointless to do so because:
number of samples. Throwing away a large number of
• This converter would require a very fast controller or
those samples leaves a skeleton of the original signal; but,
processor.
remember, most of those samples are not “real.” They can
• While it might appear that there is an abundance of
be thought of as the filter’s work-in-process samples. In
high-quality samples at the high sampling rate of the
fact, according to the Nyquist theorem, the new “skeletal”
modulator, most of them don’t provide any useful inforversion of the signal has exactly the same informational
mation, since a low-pass filter has been applied. In other
content as the previous waveform, but now it is at a
words, the extra samples are interpolations or intermemanageable data rate. Decimating some of the samples
diate results.
has not caused any information to be lost.
The decimator-filter function
Figure 4 conceptually shows the decimation process.
The digital filter’s time-domain output in Figure 3a has
The second function of the digital/decimation filter is the
been brought forward to Figure 4a. Figure 4b shows the
decimator. The word “decimate” was originally used by the
decimator-filter function’s output signal.
Roman army to mean the killing of every tenth man of a
This completes the description of the digital-filter and
group that was guilty of mutiny. In the case of the digital/
decimator-filter
functions in a DS converter.
decimation filter, the “decimation” of the digital filter’s
samples is much more dramatic. In the decimation circuit,
Pulling the DS ADC together
the digital signal’s output rate is reduced by throwing
Part 1 of this series showed the inner workings of the
away or “killing” portions of the output data. The way to
modulator in the time and frequency domains. It also
do this is to discard some of the samples.
showed how the modulator shaped noise into higher
Figure 4. Digital/decimation filter’s output from decimation process
7FFFFF
7FFFFF
0000000
0000000
800000
800000
fS /fD = Decimation Ratio (DR)
(b) Output-data rate (fD)
(a) Input sampling rate (fS)
6
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frequencies because of an oversampling sysFigure 5. Increased DR provides a lower-noise, slower
tem with negative feedback. As previously
output signal
stated in the present article, the digital/
decimation filter reduces high-frequency
noise and passes the input signal to the outSignal
Signal
put of the converter at a reduced data rate.
The combination of these two components
provides a high-resolution ADC.
The meaningful variables in this system
are the modulator’s sampling rate (fS ) and
the digital/decimation filter’s output-data
fS
fD
fS
fD
rate (fD). The ratio between these two variQuantization
Quantization
ables is defined as the decimation ratio
Noise
Noise
(DR). The decimation ratio is equal to the
fS /fD = Decimation Ratio (DR)
number of modulator samples per data output. Decimation ratio values range anywhere
(a) High DR decreases noise
(b) Low DR increases noise
from 4 in the Texas Instruments (TI)
ADS1605 ADC to a maximum of 32,768 for
TI’s ADS1256 ADC.
Consider the output spectrum of the DS
noise that was shaped by the modulator stage and reduces
modulator in Figure 5. The modulator samples at a frethe data-output rate of the device to a usable frequency.
quency of fS and, in doing so, shapes the quantization
noise into higher frequencies. Many DS converters permit
There is a strong relationship between the output-data
the designer to program the data rate directly by adjusting
rate and the converter’s resolution. If the sample rate is
the decimation ratio. Suppose the data rate is chosen to
kept constant, lower data rates provide high effective
reso­lution, or ENOB, at the output of the converter.
be some fraction of fS, as shown in Figure 5a. The freDS ADCs have other functions besides the basics in these
quencies from 0 to fD, which constitute the output, are in
two articles, acting as current sources, voltage sources,
the signal band. Note the noise level in the signal band.
input buffers, etc. However, examining any DS ADC will
In Figure 5a, the effective number of bits (ENOB) is
always reveal a modulator and a digital/decimation filter.
very high. Since the output-data rate (fD) is determined
by the decimator-filter function, it depends on the decimaIn choosing a DS ADC, it is best to start with the fundation ratio (DR), where DR = fS /fD . Figure 5b shows that
mentals and then see what else the device has to offer.
the value for fD, which has moved to the right, is now
References
higher. Unfortunately, there is also more noise. Most of the
For more information related to this article, you can down­
noise is in the higher frequencies, decreasing the signal-toload an Acrobat® Reader® file at www.ti.com/lit/litnumber
noise ratio and the ENOB.
and replace “litnumber” with the TI Lit. # for the
There is a way to increase the sampling speed (fS ) while
materials
listed below.
keeping the ENOB the same, and that is to increase the
master-clock rate. This will also increase fD but will not
Document Title
TI Lit. #
decrease the decimation ratio. Unfortunately, increasing
1. Bonnie Baker, “How delta-sigma ADCs work,
the master-clock rate will also increase power consumpPart 1,” Analog Applications Journal
tion. Additionally, most converters have a practical limit
(3Q 2011) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SLYT423
for fS beyond which they will not function properly.
2. “16-bit, 5MSPS analog-to-digital converter,”
Conclusion
A DS ADC fundamentally includes a modulator and a digital/decimation filter. The modulator converts the analog
signal directly into the digital domain by using a 1-bit ADC
and oversampling. The modulator topology implements a
noise-shaping function that drives the lower-frequency
quantization noise into higher frequencies. The low-pass
digital/decimation filter throws away the high-frequency
ADS1605/6 Datasheet . . . . . . . . . . . . . . . . . . . . . SBAS274
3. “Very low noise, 24-bit analog-to-digital
converter,” ADS1255/6 Datasheet . . . . . . . . . . . SBAS288
Related Web sites
dataconverter.ti.com
www.ti.com/product/ADS1256
www.ti.com/product/ADS1605
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