Datasheet OP271 (Analog Devices) - 8

OP271 APPLICATION INFORMATION
When Rf > 3 k⍀, a pole created by Rf and the amplifier’s
Capacitive Load Driving and Power Supply Considerations
input capacitance (3 pF) creates additional phase shift and The OP217 is unity-gain stable and is capable of driving large reduces phase margin. A small capacitor in parallel with Rf capacitive loads without oscillating. Nonetheless, good supply helps eliminate this problem. bypassing is highly recommended. Proper supply bypassing
Computer Simulations
reduces problems caused by supply line noise and improves Many electronic design and analysis programs include models the capacitive load driving capability of the OP271. 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 ⴞ15% 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 characterization of a breadboard circuit. 1000 pF when used with the OP271.
POLES ZEROS
15Hz 2.5 MHz
V+ C2
1.2 MHz 4 X 23 MHz
10
␮
F
2 X 32 MHz -
+
8 X 40 MHz -
C3 APPLICATIONS 0.1
␮
F R2 Low Phase Error Amplifier
The simple amplifier depicted in Figure 4, utilizes a monolithic
C1
dual operational amplifier and a few resistors to substantially
200pF R1
reduce phase error compared to conventional amplifier designs.
V R3 IN 50
⍀ At a given gain, the frequency range for a specified phase
OP271 VOUT
accuracy is over a decade greater than for a standard single op
C4 CL 10
␮
F 1000pF
amp amplifier.
+
The low phase error amplifier performs second-order frequency compensation through the response of op amp A2 in the
C5 PLACE SUPPLY DECOUPLING 0.1
␮
F
feedback loop of A1. Both op amps must be extremely well
CAPACITORS AT OP271
matched in frequency response. At low frequencies, the A1
V–
feedback loop forces V2/(K1 + 1)=VIN. The A2 feedback loop Figure 2. Driving Large Capacitive Loads forces VO/VIN=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
Unity-Gain Buffer Applications
only. Minimum gain for the low phase error amplifier is 10. When Rf ⱕ 100 ⍀ and the input is driven with a fast, large-signal pulse (>1 V), the output waveform will look as shown in Figure
R2 R2 = R1
3.
R2
During the fast feedthrough-like portion of the output, the input
K1
protection diodes effectively short the output to the input, and a current, limited only by the output short-circuit protection, will
1/2 OP271E V2
be drawn by the signal generator. With R
OBSOLETE
f ⱖ 500 ⍀, the output is
A2
capable of handling the current requirements (IL ⱕ 20 mA at 10 V); the amplifier will stay in its active mode and a smooth transition will occur.
R1 1/2 R1 K1 R1 OP271E A1 VIN VO ASSUME: A1 AND A2 ARE MATCHED.
␻
8.5V/
␮
s A VO = (K1+1) VIN O(s) = OP271 s
Figure 4. Low Phase Error Amplifier Figure 3. Pulsed Operation –8– REV. A Document Outline FEATURES GENERAL DESCRIPTION PIN CONNECTIONS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ORDERING GUIDE Typical Performance Characteristics APPLICATION INFORMATION Capacitive Load Driving and Power Supply Considerations Unity-Gain Buffer Applications Computer Simulations APPLICATIONS Low Phase Error Amplifier Dual 12-Bit Voltage Output DAC Fast Current Pump OUTLINE DIMENSIONS Revision History