E1C07 09/14/2010
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which shows how these combinations correspond to the 2
2 possible values of a 2-bit binary output.
Logic circuits transfer this information to a register.
Because all the comparators act simultaneously, the conversion occurs in a single clock count.
Thus an attraction of this converter is its speed, with typical sample rates on the order of 150 MHz at
8 bits. Its disadvantage lies in the cost associated with its 2
M
À 1 comparators and the associated
logic circuitry. For example, an 8-bit converter requires 255 comparators.
Digital Voltmeters
The digital voltmeter must convert an analog input signal to a digital output. This conversion can be
done through several basic techniques (7). The most common method used in digital meters is with
dual-ramp converters. The limits of performance for digital voltmeters can be significantly higher
than for analog meters. Because of their very high input impedance, loading errors are very small at
low-voltage levels, nearly as low as null balance instruments such as the potentiometer. The
resolution (1 LSB) of these meters may be significantly better than their accuracy, so care must be
taken in estimating uncertainty in their output. These devices are able to perform integration in both
DC and true root-mean-square (rms) AC architecture.
Example 7.6
A 0- to 10-V, three-digit digital voltmeter (DVM) is built around a 10-bit, single-ramp A/D
converter. A 100-kHz clock is used. For an applied input voltage of 6.372 V, what is the DVM output
value? How long will the conversion process take?
KNOWN
E FSR ¼ 10 V; M ¼ 10
Clock speed ¼ 100 kHz
Three significant digit readout (x.xx)
Input voltage, E i ¼ 6.372 V
FIND
Digital output display value
Conversion time
SOLUTION The resolution of the A/D converter is
Q ¼ E FSR =2
M
¼ 10 V=1024 ¼ 9:77 mV
Because each ramp step increases the counter value by one bit, the ramp converter has a slope
slope ¼ Q Â 1 bit=step ¼ 9:77 mV=step
The number of steps required for conversion is
steps required ¼
E i
slope
¼
6:372 V
0:00977 V=step
¼ 652:29 or 653 steps
so the indicated output voltage is
E 0 ¼ slope  number of steps
¼ 9:77 mV=step  653 steps ¼ 6:3769 V % 6:38 V
282 Chapter 7 Sampling, Digital Devices, and Data Acquisition
14:43:50 Page 282
which shows how these combinations correspond to the 2
2 possible values of a 2-bit binary output.
Logic circuits transfer this information to a register.
Because all the comparators act simultaneously, the conversion occurs in a single clock count.
Thus an attraction of this converter is its speed, with typical sample rates on the order of 150 MHz at
8 bits. Its disadvantage lies in the cost associated with its 2
M
À 1 comparators and the associated
logic circuitry. For example, an 8-bit converter requires 255 comparators.
Digital Voltmeters
The digital voltmeter must convert an analog input signal to a digital output. This conversion can be
done through several basic techniques (7). The most common method used in digital meters is with
dual-ramp converters. The limits of performance for digital voltmeters can be significantly higher
than for analog meters. Because of their very high input impedance, loading errors are very small at
low-voltage levels, nearly as low as null balance instruments such as the potentiometer. The
resolution (1 LSB) of these meters may be significantly better than their accuracy, so care must be
taken in estimating uncertainty in their output. These devices are able to perform integration in both
DC and true root-mean-square (rms) AC architecture.
Example 7.6
A 0- to 10-V, three-digit digital voltmeter (DVM) is built around a 10-bit, single-ramp A/D
converter. A 100-kHz clock is used. For an applied input voltage of 6.372 V, what is the DVM output
value? How long will the conversion process take?
KNOWN
E FSR ¼ 10 V; M ¼ 10
Clock speed ¼ 100 kHz
Three significant digit readout (x.xx)
Input voltage, E i ¼ 6.372 V
FIND
Digital output display value
Conversion time
SOLUTION The resolution of the A/D converter is
Q ¼ E FSR =2
M
¼ 10 V=1024 ¼ 9:77 mV
Because each ramp step increases the counter value by one bit, the ramp converter has a slope
slope ¼ Q Â 1 bit=step ¼ 9:77 mV=step
The number of steps required for conversion is
steps required ¼
E i
slope
¼
6:372 V
0:00977 V=step
¼ 652:29 or 653 steps
so the indicated output voltage is
E 0 ¼ slope  number of steps
¼ 9:77 mV=step  653 steps ¼ 6:3769 V % 6:38 V
282 Chapter 7 Sampling, Digital Devices, and Data Acquisition