MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 28

• Manufacturer Part Number: MCP6271R
• Description: 170 ?a, 2 Mhz Rail-to-rail Op Amp
• Manufacturer: Microchip Technology Inc.

Sometimes Noise Can Be Good
By Bonnie C. Baker, Microchip Technology Inc.
When I started to write this article someone saw the title and
asked, “You don’t have any kids, do you?” Well, I did have the
charge of children at one time in my life, but I think people forget
two things through their lives: extreme pleasure and extreme
pain. That’s why we keep going back for more. This column is not
about going back to painful experiences such as a noisy circuit.
Rather, it is about the pleasure of tackling those difficult analog
noise problems in the digital domain.
We have all sought the perfect conversion in our mixed-signal
circuits where the converter produces a repeatable, accurate
digital result every time. We use noise-reduction techniques
such as selecting low-noise devices, a careful layout and
analog filtering to remove undesirable signals. But, another
way to approach noisy analog-to-digital conversion problems is
to “design” noise into your signal instead of out. For instance,
you can get 12-bit accuracy from a 12-bit converter if you are
diligent about applying low-noise strategies to your circuit. As an
alternative, you can allow a degree of white noise into the circuit
and follow the conversion with a processor or controller digital
filter. In this scenario, your circuit is capable of producing 14-, 15-
or even 16+-bit accuracy. If there is noise in your circuit, you can
achieve better resolution at the output of a digital filter by using
oversampling techniques.
For instance, if you use a simple rolling-average digital filter, you
can calculate the number of bits (N) that you will add to your
conversion resolution with this formula: #oversampled data =
2
bits, you need to accumulate and average 64 samples. Time is
the primary tradeoff for this increase in resolution. The rolling-
average digital-filter algorithm accumulates several samples in
order to calculate the final result. The accumulation of these
samples takes time. Finite Impulse Response (FIR) and Infinite
Impulse Response (IIR) digital filters are also well suited for this
task.
If you have the time, this sounds like a simple solution. However,
there is one more issue to consider before you embrace this
combination of analog with digital systems in your circuit. You
must know the complexion of your ADC digital output over time. A
histogram plot is an appropriate tool to use when examining your
digital code over time. The histogram plot displays the number
of occurrences of each code. For example, the histogram plot in
Figure 1 shows 1024 repetitive data samples from a 12-bit ADC
(sample rate = 20 ksps).
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Analog and Interface Guide – Volume 2
2N
. If you want to increase your resolution from 11 bits to 14
Analog-to-Digital Converters
If you want to successfully increase the resolution of your
converter, you need to ensure that the noise from the ADC is
gaussian in nature. In a histogram plot, gaussian noise looks
similar to a statistically normal distribution around a center
code. The data in Figure 1 does not follow the shape of a normal
distribution. The data in Figure 1 appears to have a bi-modal
response. In addition, the output mean of this system should
be 2236 instead of 2297. A digital filter will not “fix” this data.
The noise in this system originates in an LED array. Poor layout
and high currents through the array make the noise on the board
intolerable.
If you use a digital filter at the output of your ADC, you are not
relieved of the responsibility of knowing what kind of data you
are producing. Digital filtering will improve the resolution of your
analog-to-digital conversion, but only if you are confident that the
noise response of your data is gaussian in nature.
Figure 1: This histogram plot shows data from an ADC that is
bimodal. It is difficult, if not impossible, to apply digital filtering
to this data to recover a digital code that actually represents the
input analog voltage accurately.
References
Elsevier, 2003.
“Mixed-Signal and DSP Design Techniques”, Walt Kester,