OP271_02 Datasheet, PDF (8/12 Pages) Analog Devices – High-Speed, Dual Operational Amplifier

OP271
APPLICATION INFORMATION
Capacitive Load Driving and Power Supply Considerations
The OP217 is unity-gain stable and is capable of driving large
capacitive loads without oscillating. Nonetheless, good supply
bypassing is highly recommended. Proper supply bypassing
reduces problems caused by supply line noise and improves
the capacitive load driving capability of the OP271.
In the standard feedback amplifier, the op amp’s output resistance
combines with the load capacitance to form a low-pass filter that
adds phase shift in the feedback network and reduces stability. A
simple circuit to eliminate this effect is shown in Figure 2. The
added components, C1 and R3, decouple the amplifier from the
load capacitance and provide additional stability. The values of
C1 and R3 shown in Figure 8 are for a load capacitance of up to
1000 pF when used with the OP271.
V+
C2
10␮F
+
R1
V_IN
C3
0.1␮F
R2
OP271
C1
200pF
R3
50⍀
C4
10␮F
+
V_OUT
C_L
1000pF
C5
0.1␮F
V–
PLACE SUPPLY DECOUPLING
CAPACITORS AT OP271
Figure 2. Driving Large Capacitive Loads
Unity-Gain Buffer Applications
When R_f 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
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
10 V); the amplifier will stay in its active mode and a smooth
transition will occur.
R1
OP271
8.5V/␮s
Figure 3. Pulsed Operation
When R_f > 3 k⍀, a pole created by R_f and the amplifier’s
input capacitance (3 pF) creates additional phase shift and
reduces phase margin. A small capacitor in parallel with R_f
helps eliminate this problem.
Computer Simulations
Many electronic design and analysis programs include models
for op amps which calculate AC performance from the location
of poles and zeros. As an aid to designers utilizing such a
program, major poles and zeros of the OP271 are listed below.
Their location will vary slightly between production lots.
Typically, they will be within ؎15% of the frequency listed.
Use of this data will enable the designer to evaluate gross
circuit performance quickly, but should not supplant rigorous
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
R1
1/2
OP271E
R1
K1
A1
V_IN
ASSUME: A1 AND A2 ARE MATCHED.
␻
A_O(s) = _s
V_O
V_O = (K₁+1) V_IN
Figure 4. Low Phase Error Amplifier
–8–
REV. A