Download 2.1 Make Accurate Low-Level Measurements with High

2.1 Make Accurate Low-Level
Measurements with High-Resolution
Instrumentation
Incorporating Best Practices for Ensuring Product Quality
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Types of Low-Level Measurements
• DC measurements – measuring very small signals in
an absolute sense, unrelated to signals around them
(sensitivity)
– Example: picoamps, nanovolts, and so on
• AC measurements – measuring very small signals in
the presence of very large signals (dynamic range)
– Example: 1 nV on a 10 V signal
There are two general types of low-level measurements that you might want to take: DC and
AC measurements. With DC measurements, you are measuring a very small signal in the
absolute sense (that is, not relative to any other signal). For example, you might wish to
measure an extremely small amount of current moving through a point in a DC circuit. On the
other hand, with AC measurements you are trying to measure a very small signal sitting atop a
very large signal, such as a 1 nV sine wave on top of a 10 V sine wave. To make both of these
types of measurements, you need instrumentation with specific characteristics and abilities.
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Top Concerns in DC Measurements
Taking accurate low-level DC measurements
requires high performance instrumentation in three
categories:
• Resolution
• Sensitivity
• Accuracy
For low-level DC measurements, you should care about three key attributes of your
instrument: its resolution, sensitivity, and accuracy. Based on how your instrument performs in
these categories, you will be able to accurately detect low-level DC voltages or currents. We
will look at the definitions of resolution, sensitivity and accuracy to better understand these
terms, as well as see demonstrations of these characteristics in a real-world measurement.
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What is Resolution?
Resolution refers to how many different voltage changes can
be measured
10.00
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8.75
7.50
110
6.25
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Amplitude 5.00
(volts)
100
3-bit resolution
011
3.75
010
2.50
001
1.25
0
16-bit resolution
000
|
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0
50
|
100
Time (ms)
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150
200
The best way to understand the concept of resolution is by comparison with a yardstick.
Divide a 1 meter yardstick into millimeters. What is the resolution? The smallest “tick” on the
yardstick is the resolution. Yes, you might be able to “interpolate” between these, but in the
absence of this sophisticated guessing process the resolution is 1 part out of 1,000
The resolution of a 3-bit ADC is a function of how many parts the maximum signal can be
divided to. In this case, 23 = 8. Therefore, our best resolution is 1 part out of 8, or 3 bits.
Resolution can therefore be expressed as a percentage, x parts out of y, or most conveniently,
as bits. To figure out the number of binary levels available based on the bits of resolution you
simply take 2Bits resolution.
Let us examine how a sine wave would look if it is passed through ADCs with different
resolutions. We will compare a 3-bit ADC and a 16-bit ADC. As we learned earlier, a 3-bit
ADC can represent 8 discrete voltage levels. A 16-bit ADC can represent 65,536 discrete
voltage levels. As you can see the representation of our sine wave with 3-bit resolution looks
more like a step function than a sine wave. However, the 16-bit ADC gives us a clean looking
sine wave. Keep in mind that resolution is a fixed quantity of an ADC, and it depends on the
measurement device that you use.
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What is Sensitivity?
• Sensitivity is the smallest signal detectable by an
instrument
– Example: a typical DMM has a sensitivity of 100 nV
• Sensitivity does not equal resolution
– A 12-bit DAQ board could have greater sensitivity than
a 16-bit DAQ board
Sensitivity is not resolution, nor is it accuracy, which will be explained next. The key question
with sensitivity is “How small a signal can I detect?” It refers to the smallest signal you can
detect with a particular instrument on its most sensitive range.
To illustrate how sensitivity and resolution are different, we can use the example of a 12-bit
DAQ board and a 16-bit DAQ board. At first glance, it may seem that the 16-bit DAQ board is
more sensitive because it has a higher resolution. However, this is only true if the voltage
ranges and gain settings of the two DAQ boards are the same. For a low-level measurement,
you should use as small a voltage range as possible to maximize the use of your instrument’s
resolution, as well as magnify the signal to improve your ability to detect it.
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What is Accuracy?
• Accuracy is the uncertainty of a measurement
– Typically expressed as percentage or in ppm
• Common accuracy specifications:
– XX% of Reading ±YY% of Range
– XX ppm of Reading ±YY ppm of Range
Accuracy refers to a quantitative measure of the magnitude of error. Accuracy should never be
confused with precision. Precision measures how far from the mean or average of replicated
measurements a particular measurement lies.
Accuracy is typically expressed as ± percent of full scale.
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Bounding Accuracy
Spec Limit
Error (as a % of range)
xx gain limit
yy Offset limit
Input signal
Actual
Spec Limit
Accuracy = ±(xx% of reading + yy% of range)
(Accuracy units : % or PPM – parts per million)
The classic butterfly curve helps us understand accuracy. Our specifications define
this butterfly curve. If the performance of the instrument falls within this butterfly
curve, we meet our specifications. When we produce our instruments, we assure that
the actual performance is guardbanded from these specification limits. This assures
that when the customer receives the product, uses it under his/her specific conditions,
over the calibration cycle, its performance will still fall within this butterfly curve.
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Demo – Resolution and Sensitivity
NI PXI-4071 7½-Digit Flexible Resolution DMM
• ±10 nV to 1,000 DCV (700 ACV)
• ±1 pA to 3 A
• 10 µΩ to 5 GΩ
• ±500 VDC/Vrms common-mode isolation
This demo shows the incredible resolution and sensitivity of the PXI-4071 7 ½ digit
FlexDMM, which has 26-bit resolution, can measure a 1,000 V signal, but is also sensitive
enough to detect a 10 nV signal. This DMM also can measure resistances from 10 µΩ at its
most sensitive to 5 GΩ at its maximum, and features 500 VDC of common mode voltage. You
can think of a digital multimeter as a universal instrument for DC measurements because you
can make several measurements like current, voltage, or resistance with a single device.
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Top Concern in AC Measurements
Low-level AC measurements require instrumentation
with an additional characteristic: Dynamic Range
±1 V
±0.00000001 V
We live in an analog world, so scientists and engineers will always need to measure AC
signals. With low-level AC measurements, we have a new concern we did not deal with in our
DC measurements: the instrument’s dynamic range.
In low-level AC measurements, we are typically trying to accurately take view a low-level AC
signal in the presence of a large AC signal. For example, we might be trying to measure a 10
nV sine wave in the presence of a 1 V sine wave.
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What Is Dynamic Range?
Dynamic range is the ratio of the highest signal level
a circuit can handle to the smallest signal level it
can handle Typically expressed in dB: dB = 20 log (V2/V1)
Large signal
Dynamic range
Small signal
So what is dynamic range? Dynamic range is an expression of the ratio between the highest
allowable voltage a circuit can handle to the smallest detectable voltage. In the above image,
we are examining the frequency domain of an AC signal composed of two different sine
waves: a large sine wave with a frequency of 1 MHz and smaller sine wave with a frequency
of 10 MHz. Dynamic range is typically expressed in decibels (dB), which is calculated as
20*log(V2/V1).
In this example, we can view the dynamic range as the difference between the amplitudes of
the two signals in the power spectrum.
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Demo – Intermodulation Distortion Detection
NI PXI-5922 Flexible Resolution Digitizer
• Flexible resolution from 24 bits at 500 kS/s to 16 bits at 15 MS/s
– 22 bits at 1 MS/s, 20 bits at 5 MS/s, 18 bits at 10 MS/s
• –170 dBfs/Hz noise density
• Integrated anti-alias filtering
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Summary
• For low-level DC measurements, understand the
differences between accuracy, sensitivity, and
resolution for your measurement
• For low-level AC measurements, look for products
with high dynamic range to detect small signals in
the presence of large signals
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