OP271_15 Datasheet, PDF (8/12 Pages) Analog Devices – High-Speed, Dual Operational Amplifier

OP271
APPLICATION INFORMATION
Capacitive Load Driving and Power Supply Considerations
When R_f > 3 k⍀, a pole created by R_f and the amplifier’s
input capacitance (3 pF) creates additional phase shift and
The OP217 is unity-gain stable and is capable of driving large
capacitive loads without oscillating. Nonetheless, good supply
reduces phase margin. A small capacitor in parallel with R_f
helps eliminate this problem.
bypassing is highly recommended. Proper supply bypassing
reduces problems caused by supply line noise and improves
the capacitive load driving capability of the OP271.
Computer Simulations
Many electronic design and analysis programs include models
for op amps which calculate AC performance from the location
In the standard feedback amplifier, the op amp’s output resistance
of poles and zeros. As an aid to designers utilizing such a
combines with the load capacitance to form a low-pass filter that
program, major poles and zeros of the OP271 are listed below.
adds phase shift in the feedback network and reduces stability. A
Their location will vary slightly between production lots.
simple circuit to eliminate this effect is shown in Figure 2. The
Typically, they will be within ؎15% of the frequency listed.
added components, C1 and R3, decouple the amplifier from the
Use of this data will enable the designer to evaluate gross
load capacitance and provide additional stability. The values of
circuit performance quickly, but should not supplant rigorous
C1 and R3 shown in Figure 8 are for a load capacitance of up to
1000 pF when used with the OP271.
V+
C2
E ^10␮F
+
T _R1
E ^VIN
C3
0.1␮F
R2
OP271
C1
200pF
R3
50⍀
C4
10␮F
+
V_OUT
C_L
1000pF
L _C5
0.1␮F
V–
PLACE SUPPLY DECOUPLING
CAPACITORS AT OP271
O _{Figure 2. Driving Large Capacitive Loads}
Unity-Gain Buffer Applications
S _{When Rf Յ 100 ⍀ and the input is driven with a fast, large-signal}
pulse (>1 V), the output waveform will look as shown in Figure
3.
During the fast feedthrough-like portion of the output, the input
B _{protection diodes effectively short the output to the input, and a}
current, limited only by the output short-circuit protection, will
be drawn by the signal generator. With R_f Ն 500 ⍀, the output is
capable of handling the current requirements (I_L Յ 20 mA at
O ^{10 V); the amplifier will stay in its active mode and a smooth}
characterization of a breadboard circuit.
POLES
15Hz
1.2 MHz
2 X 32 MHz
8 X 40 MHz
ZEROS
2.5 MHz
4 X 23 MHz
-
-
APPLICATIONS
Low Phase Error Amplifier
The simple amplifier depicted in Figure 4, utilizes a monolithic
dual operational amplifier and a few resistors to substantially
reduce phase error compared to conventional amplifier designs.
At a given gain, the frequency range for a specified phase
accuracy is over a decade greater than for a standard single op
amp amplifier.
The low phase error amplifier performs second-order frequency
compensation through the response of op amp A2 in the
feedback loop of A1. Both op amps must be extremely well
matched in frequency response. At low frequencies, the A1
feedback loop forces V₂/(K1 + 1)=V_IN. The A2 feedback loop
forces V_O/V_IN=K1 + 1. The DC gain is determined by the
resistor divider around A2. Note that, like a conventional
single op amp amplifier, the DC gain is set by resistor ratios
only. Minimum gain for the low phase error amplifier is 10.
R2
R2 = R1
R2
K1
1/2
OP271E
V₂
A2
transition will occur.
R1
R1
1/2
OP271E
R1
K1
A1
V_IN
OP271
8.5V/␮s
ASSUME: A1 AND A2 ARE MATCHED.
␻
A_O(s) = _s
V_O
V_O = (K₁+1) V_IN
Figure 4. Low Phase Error Amplifier
Figure 3. Pulsed Operation
–8–
REV. A