OP191 Datasheet, PDF (14/20 Pages) Analog Devices – Micropower Single-Supply Rail-to-Rail Input/Output Op Amps

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
instrumentation amplifier shown in Figure 59. 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 instrumentation 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
V_IN
3
5
8
1/2
7
6 ^OP291
4
V_OUT
1/2
1
2 ^OP291
R1
R2
R2
R1
V_OUT
=
(1
+
–R–1–
R2
)
V_IN
C1
100pF
Figure 59. 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 traditional 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 60 uses three op amps of the OP491 to
develop a bridge configuration for an RTD amplifier that
operates 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 resistance in both 100 Ω legs of the bridge to
improve accuracy.
26.7k
200Ω
10-TURNS
26.7k
100Ω
RTD
2.55M
100Ω
6.19k
AD589
A1
1/4
OP491
37.4k
+5V
A2
1/4
OP491
GAIN = 274
+5V
A3
1/4
OP491
365
365
100kΩ
V_OUT
100kΩ
0.01pF
NOTE:
ALL RESISTORS 1% OR BETTER
Figure 60. 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. 0