E1C07 09/14/2010
14:43:50 Page 285
Lighter filtering might use three terms, while heavier filtering might use 10 or more terms. This
filtering scheme is easily accomplished within a spreadsheet program.
Amplifiers
All data-acquisition systems are input range limited; that is, there is a minimum value of a signal that
they can resolve, and a maximum value that initiates the onset of saturation. Thus, some transducer
signals need amplification or attenuation prior to conversion. Most data-acquisition systems contain
on-board instrumentation amplifiers as part of the signal conditioning stage, as depicted in Figure
7.13, with selectable gains ranging from less than to greater than unity. Gain is varied either by a
resistor jumper or by logic switches set by software, which effectively reset resistor ratios across opamplifiers. Section 6.6 in Chapter 6 discusses amplifiers.
Although instrument amplifiers offer good output impedance characteristics, voltages can also
be attenuated using a voltage divider. The output voltage from the divider circuit of Figure 7.14 is
determined by
E o ¼ E i
R 2
R 1 þ R 2
ð7:23Þ
For example, a 0- to 50-V signal can be measured by a 0- to 10-V A/D converter using R 1 ¼ 40 kV
and R 2 ¼ 10 kV.
When only the dynamic content of time-dependent signals is important, amplification may
require a strategy. For example, suppose the mean value of a voltage signal is large but the dynamic
content is small, such as with the 5-Hz signal
yðtÞ ¼ 2 þ 0:1 sin 10 pt ðVÞ
ð 7:24Þ
Setting the amplifier gain at or more than G ¼ 2.5 to improve the resolution of the dynamic content
would saturate a Æ5-VA/D converter. In such situations, you can remove the mean component from
the signal prior to amplification by (1) adding a mean voltage of equal but opposite sign, such as
À2 V here, or (2) passing the signal through a very low frequency high-pass filter (also known as AC
coupling).
Shunt Resistor Circuits
An A/D converter requires a voltage signal at its input. It is straightforward to convert current signals
into voltage signals using a shunt resistor. The circuit in Figure 7.15 provides a voltage
E o ¼ IR shunt
ð7:25Þ
R 1
R 2
S
A
D
o
t
e
c
a
f
r
e
t
n
I
r
e
c
u
d
s
n
a
r
T
E i
E o
E i
E o =
R2
R1 + R2
Figure 7.14 Voltage divider circuit for
signal amplitude attenuation.
7.7 Data-Acquisition System Components 285
14:43:50 Page 285
Lighter filtering might use three terms, while heavier filtering might use 10 or more terms. This
filtering scheme is easily accomplished within a spreadsheet program.
Amplifiers
All data-acquisition systems are input range limited; that is, there is a minimum value of a signal that
they can resolve, and a maximum value that initiates the onset of saturation. Thus, some transducer
signals need amplification or attenuation prior to conversion. Most data-acquisition systems contain
on-board instrumentation amplifiers as part of the signal conditioning stage, as depicted in Figure
7.13, with selectable gains ranging from less than to greater than unity. Gain is varied either by a
resistor jumper or by logic switches set by software, which effectively reset resistor ratios across opamplifiers. Section 6.6 in Chapter 6 discusses amplifiers.
Although instrument amplifiers offer good output impedance characteristics, voltages can also
be attenuated using a voltage divider. The output voltage from the divider circuit of Figure 7.14 is
determined by
E o ¼ E i
R 2
R 1 þ R 2
ð7:23Þ
For example, a 0- to 50-V signal can be measured by a 0- to 10-V A/D converter using R 1 ¼ 40 kV
and R 2 ¼ 10 kV.
When only the dynamic content of time-dependent signals is important, amplification may
require a strategy. For example, suppose the mean value of a voltage signal is large but the dynamic
content is small, such as with the 5-Hz signal
yðtÞ ¼ 2 þ 0:1 sin 10 pt ðVÞ
ð 7:24Þ
Setting the amplifier gain at or more than G ¼ 2.5 to improve the resolution of the dynamic content
would saturate a Æ5-VA/D converter. In such situations, you can remove the mean component from
the signal prior to amplification by (1) adding a mean voltage of equal but opposite sign, such as
À2 V here, or (2) passing the signal through a very low frequency high-pass filter (also known as AC
coupling).
Shunt Resistor Circuits
An A/D converter requires a voltage signal at its input. It is straightforward to convert current signals
into voltage signals using a shunt resistor. The circuit in Figure 7.15 provides a voltage
E o ¼ IR shunt
ð7:25Þ
R 1
R 2
S
A
D
o
t
e
c
a
f
r
e
t
n
I
r
e
c
u
d
s
n
a
r
T
E i
E o
E i
E o =
R2
R1 + R2
Figure 7.14 Voltage divider circuit for
signal amplitude attenuation.
7.7 Data-Acquisition System Components 285