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參數資料
| 型號： | AD8221-EVAL |
| 廠商： | Analog Devices, Inc. |
| 元件分類： | 運動控制電子 |
| 英文描述： | Single Supply, Rail-to-Rail, Low Cost Instrumentation Amplifier |
| 中文描述： | 單電源，軌至軌，低成本儀表放大器 |
| 文件頁數： | 17/20頁 |
| 文件大小： | 1283K |
| 代理商： | AD8221-EVAL |
AD8221
Rev. A | Page 17 of 20
+12V
+IN
–IN
0.1
μ
F
10
μ
F
0.1
μ
F
10
μ
F
–12V
R3 1k
+2.5V
R4 1k
REF
R5
499
R2
10k
R1
10k
C1
470pF
+12V
0.1
μ
F
0.1
μ
F
–12V
+12V
+5V
+5V
0.1
μ
F
10nF
0.1
μ
F
–12V
+12V
0.1
μ
F
0.1
μ
F
–12V
R6 27.4
R7 27.4
C2
220
μ
F
10
μ
F
0.1
μ
F
22
μ
F
+5V
2.5V
220nF
10nF
AD8221
AD8022
()
OP27
AD8022
()
AD7723
V
IN+
AV
DD
AGND VGND REF1 REF2
DV
DD
V
IN–
AD780
GND
V
IN
V
OUT
0
Figure 51. Interfacing to a Differential Input ADC
CONDITIONING ±10 V SIGNALS FOR A +5 V
DIFFERENTIAL INPUT ADC
There is a need in many applications to condition
±
10 V signals.
However, many of today’s ADCs and digital ICs operate on
much lower, single-supply voltages. Furthermore, new ADCs
have differential inputs because they provide better common-
mode rejection, noise immunity, and performance at low supply
voltages. Interfacing a
±
10 V, single-ended instrumentation
amplifier to a +5 V, differential ADC may be a challenge.
Interfacing the in-amp to the ADC requires attenuation and a
level shift. A solution is shown in Figure 51.
In this topology, an OP27 sets the AD8221’s reference voltage.
The in-amp’s output signal is taken across the OUT pin and the
REF pin. Two 1 kΩ resistors and a 499 Ω resistor attenuate the
±10 V signal to +4 V. An optional capacitor, C1, may serve as an
ant aliasing filter. An AD8022 is used to drive the ADC.
This topology has five benefits. In addition to level-shifting and
attenuation, very little noise is contributed to the system. Noise
from R1 and R2 is common to both of the ADC’s inputs and is
easily rejected. R5 adds a third of the dominant noise and there-
fore makes a negligible contribution to the noise of the system.
The attenuator divides the noise from R3 and R4. Likewise, its
noise contribution is negligible. The fourth benefit of this inter-
face circuit is that the AD8221’s acquisition time is reduced by a
factor of 2. With the help of the OP27, the AD8221 only needs
to deliver one-half of the full swing; therefore, signals can settle
more quickly. Lastly, the AD8022 settles quickly, which is helpful
because the shorter the settling time, the more bits that can be
resolved when the ADC acquires data. This configuration pro-
vides attenuation, a level-shift, and a convenient interface with a
differential input ADC while maintaining performance.
AC-COUPLED INSTRUMENTATION AMPLIFIER
Measuring small signals that are in the amplifier’s noise or offset
can be a challenge. Figure 52 shows a circuit that can improve
the resolution of small ac signals. The large gain reduces the
referred input noise of the amplifier to 8 nV/√Hz. Thus, smaller
signals can be measured since the noise floor is lower. DC
offsets that would have been gained by 100 are eliminated from
the AD8221’s output by the integrator feedback network.
At low frequencies, the OP1177 forces the AD8221’s output to
0 V. Once a signal exceeds f
HIGH-PASS
, the AD8221 outputs the
amplified input signal.
AD8221
OP1177
R
15.8k
+V
S
+IN
–IN
0.1
μ
F
0.1
μ
F
0.1
μ
F
0.1
μ
F
10
μ
F
10
μ
F
REF
C 1
μ
F
–V
S
–V
S
+V
S
+V
S
–V
S
R
499
1
2
π
RC
f
HIGH-PASS
=
0
Figure 52. AC-Coupled Circuit
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