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output, just as does a common LCD computer monitor. In this way, the digital oscilloscope measures
and stores voltage in a digital manner but then displays it in an analog format as a sequence of
measured points, as shown in Figure 6.8b. The amplifier and time base controls are interpreted as
with the analog oscilloscope. These common lab devices can be found as small and very portable
packages and are often a virtual software component of a 12- to 18-bit analog-to-digital converter
data acquisition system. Analog-to-digital sampling techniques used to measure voltages are
discussed in Chapter 7.
The LabView interactive program (virtual instrument) called Oscilloscope is available with the
companion software. It features a basic two-channel oscilloscope. The user can vary channel
displayed and the time sweep and gain using two built-in signals (sine wave and square wave) and an
active signal trigger.
Example 6.2
We can pick the requirements for an oscilloscope based on the intended signal to be measured. For
example, in the USB 1.1 protocol for data transmission, a single frame of data lasts for 1 ms with
data transmitted serially at 12 Mbps (million bits per second). We can simplify this as trying to
capture a 12 MHz square wave for 1 ms on the oscilloscope screen to base our requirements.
From the discussions of Chapter 2, we know that we need at least five harmonics to reconstruct a
square wave with any reasonable fidelity, so that the sampling rate required to reconstruct a 12 MHz
square wave is at least five times the fundamental frequency or at least 60 MHz (i.e., 60 Â 10
6
samples/s). So that sets the lowest value on oscilloscope response. The storage capacity of the digital
oscilloscope required to capture one frame of the USB data signal is 60 Â 10
6 samples/s  0.001 s or
60,000 samples at 60 MHz. For comparison of potential oscilloscope needs, the more common USB
2.0 transmits at a nominal 480 Mbps and the new USB 3.0 transmits at 4.8 Gbps (i.e., 4.8 Â 10
9
samples/sec).
Potentiometer
The potentiometer
2 is a device used to measure DC voltages that are in the microvolt to millivolt
range. Equivalent to a balance scale, a potentiometer balances an unknown input voltage against a
known internal voltage until both sides are equal. A potentiometer is a null balance instrument in
that it drives the loading error to essentially zero. Potentiometers have been supplanted by digital
voltmeters, which are deflection mode devices but have very high input impedances so as to keep
loading errors small, even at low-level voltages.
Voltage Divider Circuit
A general purpose component found in the potentiometer circuit is the voltage divider circuit shown
in Figure 6.9. The point labeled A in Figure 6.9 represents a sliding contact, which makes an
2 The term ‘‘potentiometer’’ is used in several different ways: a sliding contact precision variable resistor, the divider circuit
in Figure 6.9, and a high-precision circuit used as a voltage measuring instrument.
6.3 Analog Devices: Voltage Measurements 217
11:55:4 Page 217
output, just as does a common LCD computer monitor. In this way, the digital oscilloscope measures
and stores voltage in a digital manner but then displays it in an analog format as a sequence of
measured points, as shown in Figure 6.8b. The amplifier and time base controls are interpreted as
with the analog oscilloscope. These common lab devices can be found as small and very portable
packages and are often a virtual software component of a 12- to 18-bit analog-to-digital converter
data acquisition system. Analog-to-digital sampling techniques used to measure voltages are
discussed in Chapter 7.
The LabView interactive program (virtual instrument) called Oscilloscope is available with the
companion software. It features a basic two-channel oscilloscope. The user can vary channel
displayed and the time sweep and gain using two built-in signals (sine wave and square wave) and an
active signal trigger.
Example 6.2
We can pick the requirements for an oscilloscope based on the intended signal to be measured. For
example, in the USB 1.1 protocol for data transmission, a single frame of data lasts for 1 ms with
data transmitted serially at 12 Mbps (million bits per second). We can simplify this as trying to
capture a 12 MHz square wave for 1 ms on the oscilloscope screen to base our requirements.
From the discussions of Chapter 2, we know that we need at least five harmonics to reconstruct a
square wave with any reasonable fidelity, so that the sampling rate required to reconstruct a 12 MHz
square wave is at least five times the fundamental frequency or at least 60 MHz (i.e., 60 Â 10
6
samples/s). So that sets the lowest value on oscilloscope response. The storage capacity of the digital
oscilloscope required to capture one frame of the USB data signal is 60 Â 10
6 samples/s  0.001 s or
60,000 samples at 60 MHz. For comparison of potential oscilloscope needs, the more common USB
2.0 transmits at a nominal 480 Mbps and the new USB 3.0 transmits at 4.8 Gbps (i.e., 4.8 Â 10
9
samples/sec).
Potentiometer
The potentiometer
2 is a device used to measure DC voltages that are in the microvolt to millivolt
range. Equivalent to a balance scale, a potentiometer balances an unknown input voltage against a
known internal voltage until both sides are equal. A potentiometer is a null balance instrument in
that it drives the loading error to essentially zero. Potentiometers have been supplanted by digital
voltmeters, which are deflection mode devices but have very high input impedances so as to keep
loading errors small, even at low-level voltages.
Voltage Divider Circuit
A general purpose component found in the potentiometer circuit is the voltage divider circuit shown
in Figure 6.9. The point labeled A in Figure 6.9 represents a sliding contact, which makes an
2 The term ‘‘potentiometer’’ is used in several different ways: a sliding contact precision variable resistor, the divider circuit
in Figure 6.9, and a high-precision circuit used as a voltage measuring instrument.
6.3 Analog Devices: Voltage Measurements 217