OP291GP 查看數據表（PDF） - Analog Devices
零件编号
产品描述 (功能)
生产厂家
OP291GP Datasheet PDF : 20 Pages
| |||
OP191/OP291/OP491
APPLICATIONS
Single +3 V Supply, Instrumentation Amplifier
The OP291’s low supply current and low voltage operation make
it ideal for battery-powered applications such as the instrumen-
tation amplifier shown in Figure 6. The circuit utilizes the classic
two op amp instrumentation amplifier topology, with four resistors
to set the gain. The equation is simply that of a noninverting
amplifier as shown in the figure. The two resistors labeled R1
should be closely matched to each other as well as both resistors
labeled R2 to ensure good common-mode rejection performance.
Resistor networks ensure the closest matching as well as matched
drifts for good temperature stability. Capacitor C1 is included
to limit the bandwidth and, therefore, the noise in sensitive
applications. The value of this capacitor should be adjusted
depending on the desired closed-loop bandwidth of the instru-
mentation amplifier. The RC combination creates a pole at a
frequency equal to 1/(2 π × R1C1). If AC-CMRR is critical,
than a matched capacitor to C1 should be included across the
second resistor labeled R1.
+3V
58
VIN
1/2
OP291
7
VOUT
64
3
1/2
OP291 1
2
R1
R2
R2
R1
VOUT
=
(1
+
–R–1–
R2
)
VIN
C1
100pF
Figure 6. Single +3 V Supply Instrumentation Amplifier
Because the OP291 accepts rail-to-rail inputs, the input common-
mode range includes both ground and the positive supply of 3
V. Furthermore, the rail-to-rail output range ensures the widest
signal range possible and maximizes the dynamic range of the
system. Also, with its low supply current of 300 µA/device, this
circuit consumes a quiescent current of only 600 µA, yet still
exhibits a gain bandwidth of 3 MHz.
A question may arise about other instrumentation amplifier
topologies for single-supply applications. For example, a variation
on this topology adds a fifth resistor between the two inverting
inputs of the op amps for gain setting. While that topology works
well in dual-supply applications, it is inherently not appropriate
for single-supply circuits. The same could be said for the tradi-
tional three op amp instrumentation amplifier. In both cases, the
circuits simply will not work in single-supply situations unless a
false ground between the supplies is created.
Single-Supply RTD Amplifier
The circuit in Figure 7 uses three op amps of the OP491 to
develop a bridge configuration for an RTD amplifier that oper-
ates from a single +5 V supply. The circuit takes advantage of
the OP491’s wide output swing range to generate a high bridge
excitation voltage of 3.9 V. In fact, because of the rail-to-rail
output swing, this circuit will work with supplies as low as 4.0 V.
Amplifier A1 servos the bridge to create a constant excitation
current in conjunction with the AD589, a 1.235 V precision
reference. The op amp maintains the reference voltage across
the parallel combination of the 6.19 kΩ and 2.55 MΩ resistor,
which generates a 200 µA current source. This current splits
evenly and flows through both halves of the bridge. Thus, 100 µA
flows through the RTD to generate an output voltage based on
its resistance. A 3-wire RTD is used to balance the line resis-
tance in both 100 Ω legs of the bridge to improve accuracy.
26.7k⍀
200⍀
10-TURNS
26.7k⍀
100⍀
RTD
2.55M⍀
100⍀
6.19k⍀
AD589
A1
1/4
OP491
37.4k⍀
A2
1/4
OP491
GAIN = 274
+5V
A3
1/4
OP491
365⍀
365⍀
100k⍀
VOUT
100k⍀
0.01pF
ALL RESISTORS 1% OR BETTER
+5V
Figure 7. Single-Supply RTD Amplifier
Amplifiers A2 and A3 are configured in the two op amp IA
discussed above. Their resistors are chosen to produce a gain of
274, such that each 1°C increase in temperature results in a
10 mV change in the output voltage, for ease of measurement.
A 0.01 µF capacitor is included in parallel with the 100 kΩ
resistor on amplifier A3 to filter out any unwanted noise from
this high gain circuit. This particular RC combination creates a
pole at 1.6 kHz.
–14–
REV. A
Share Link: 