2.1 Make Accurate Low-Level Measurements with High-Resolution Instrumentation. Incorporating Best Practices for Ensuring Product Quality

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1 2.1 Make Accurate Low-Level Measurements with High-Resolution Instrumentation Incorporating Best Practices for Ensuring Product Quality 77

2 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. 78

3 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. 79

4 What is Resolution? Resolution refers to how many different voltage changes can be measured Amplitude (volts) bit resolution bit resolution Time (ms) 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 2 Bits 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. 80

5 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. 81

6 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. 82

7 Bounding Accuracy Spec Limit Error (as a % of range) xx gain limit Actual yy Offset limit Input signal 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. 83

8 Demo Resolution and Sensitivity NI PXI ½-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 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. 84

9 Top Concern in AC Measurements Low-level AC measurements require instrumentation with an additional characteristic: Dynamic Range ±1 V ± 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. 85

10 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. 86

11 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 87

12 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 88