AD8221-EVAL 查看數據表（PDF） - Analog Devices
零件编号
产品描述 (功能)
生产厂家
AD8221-EVAL Datasheet PDF : 20 Pages
| |||
+12V
+2.5V
10µF 0.1µF
+IN
10µF
AD8221
R1
REF 10kΩ
–IN
0.1µF
R2
10kΩ
–12V
+12V
0.1µF
OP27
0.1µF
–12V
R3 1kΩ
C1
470pF
R5
499Ω
R4 1kΩ
+12V
0.1µF
AD8022
(½)
0.1µF
–12V
+12V
0.1µF
AD8022
(½)
0.1µF
–12V
AD8221
R6 27.4Ω
C2
220µF
R7 27.4Ω
+5V
+5V
10nF
VIN+
AVDD
DVDD
AD7723
VIN–
AGND VGND REF1 REF2
220nF 10nF
+5V
10µF
0.1µF
2.5V
VIN
VOUT
AD780 22µF
GND
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.
+VS
0.1µF
+IN
R
499Ω
–IN
AD8221
REF
0.1µF
–VS
fHIGH-PASS =
1
2πRC
C 1µF
+VS
0.1µF
R
15.8kΩ
OP1177
+VS
–VS
10µF 10µF
0.1µF
–VS
Figure 52. AC-Coupled Circuit
Rev. A | Page 17 of 20
Share Link: 