MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 27

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

Is Your Converter Accurate?
By Bonnie C. Baker, Microchip Technology Inc.
What does analog-to-digital converter (ADC) accuracy really
mean? You might say that accuracy means the output of the
ADC code represents the actual analog-input voltage (minus
the quantization error). This makes sense, but are you seeing a
precise determination of the analog input? Does accuracy also
mean that the ADC conversion results are repeatable? Is the
converter output code repeatable from transition to transition,
with everything remaining unchanged in the circuit during
successive conversions?
In your mind, an ADC code-to-code transition may be sharp,
occurring at a unique input voltage. Actually, the transition
regions in the ADC-transfer function may be wide. In fact, these
regions may span across several digital-output codes. In
Figure 1, a transition point occurs when the digital output
switches from one code to the next with respect to a specific
analog input voltage. But, because of ADC internal noise, the
transition point is typically not a single threshold, but rather a
small region of uncertainty. Consequently, you need to define the
transition point as the statistical average of many conversions.
Stated differently, it is at the voltage input where the uncertainty
of multiple conversions averages 50 percent of the time to one
digital code, and 50 percent of the time to an adjoining digital
code. Upon closer inspection, the conversions you collect appear
to be noisy, with a gaussian probability curve.
Figure 1: This non-ideal transfer function of an 8-bit ADC
illustrates the transition noise of every code. In this graph, FS
means full-scale.
Analog-to-Digital Converters
111
110
101
100
011
010
001
000
0
Analog Input
1/2 FS
FS
One experiment that you can quickly run with your converter is to
ground the input of a good 16-bit ADC. I am assuming, of course,
that you are using good layout techniques, bypass capacitors,
etc. in your circuit. Now, collect 1,024 samples at the converter’s
specified conversion rate. You will find that you have multiple
codes in your output data. What you will witness is the transition
noise of the converter. Some manufacturers will tell you what the
rms transition noise is for their ADCs. Multiply the rms transition
noise specification by 6.6 to obtain a peak-to-peak value.
Let’s take this discussion a step further. The offset, gain,
differential-non-linearity (DNL) and integral-non-linearity (INL) are
the accuracy specifications for ADCs. Some manufacturers also
call these the DC specifications. This is because these device
tests use a DC-input voltage for the conversions. But, these
specifications do not tell you how repeatable the results are
from conversion to conversion. They only tell you that, on the
average, these errors will be no more or less than the minimum
and maximum in your ADC manufacturer’s data sheet. In order
to describe the accuracy of your converter precisely, you need to
combine the AC specifications with the DC specifications.
Within the AC specifications, you will find three types. However,
one in particular is interesting for this discussion. This AC
specification is called the (Signal-to-Noise + Distortion) ratio,
otherwise known as SINAD. The counterpart to this specification
is “effective number of bits” (ENOB). ENOB = (SINAD – 1.76dB)/
6.02. This specification, combined with the DC specifications, will
give you a stronger feel for how accurate your converter really is.
References
EDN, March 18, 2004.
Cooper, AN546, Microchip Technology Inc.
Prentice-Hall, 1986.
This article is excerpted from Bonnie Baker’s book, “A BAKER’S
DOZEN: Real World Solutions to Real World Analog Design
Problems,” published Spring 2005 by Elsevier.
“Anticipate the Accuracy of Your Converter”, Bonnie C. Baker,
“Using the Analog-to-Digital (A/D) Converter”, Mitra, D’Souza,
“Analog-Digital Conversion Handbook”, David Sheingold,
Analog and Interface Guide – Volume 2
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