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參數(shù)資料
| 型號(hào)： | AD8005 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Current Feedback Amplifier(電流反饋放大器) |
| 中文描述： | 電流反饋放大器（電流反饋放大器） |
| 文件頁(yè)數(shù)： | 9/12頁(yè) |
| 文件大?。?/td> | 249K |
| 代理商： | AD8005 |
AD8005
–9–
REV. 0
APPLICATIONS
Driving Capacitive Loads
Capacitive loads interact with an op amp’s output impedance
to create an extra delay in the feedback path. This reduces
circuit stability, and can cause unwanted ringing and oscilla-
tion. A given value of capacitance causes much less ringing
when the amplifier is used with a higher noise gain.
The capacitive load drive of the AD8005 can be increased by
adding a low valued resistor in series with the capacitive load.
Introducing a series resistor tends to isolate the capacitive load
from the feedback loop thereby diminishing its influence. Fig-
ure 29 shows the effects of a series resistor on capacitive drive
for varying voltage gains. As the closed-loop gain is increased,
the larger phase margin allows for larger capacitive loads with
less overshoot. Adding a series resistor at lower closed-loop
gains accomplishes the same effect. For large capacitive loads,
the frequency response of the amplifier will be dominated by
the roll-off of the series resistor and capacitive load.
AD8005
R
G
R
F
R
S
R
L
1k
C
L
Figure 28. Driving Capacitive Loads
C
80
70
60
1
4
5
CLOSED-LO3
50
2
40
30
20
10
0
V
S
=
±
5V
2V OUTPUT STEP
WITH 30% OVERSHOOT
R
S
= 10
R
S
= 5
R
S
= 0
Figure 29. Capacitive Load Drive vs. Closed-Loop Gain
Single-Supply Level Shifter
In addition to providing buffering, many systems require that an
op amp provide level shifting. A common example is the level
shifting that is required to move a bipolar signal into the uni-
polar range of many modern analog-to-digital converters (ADCs).
In general, single supply ADCs have input ranges that are refer-
enced neither to ground nor supply. Instead the reference level
is some point in between, usually halfway between ground and
supply (+2.5 V for a single supply 5 V ADC). Because high
speed ADCs typically have input voltage ranges of 1 V to 2 V,
the op amp driving it must be single supply but not necessarily
rail-to-rail.
V
REF
+5V
R2
1.5k
V
OUT
0.01μF
10μF
+5V
R1
1.5k
V
IN
0.1μF
R4
10k
R3
30.1k
AD8005
Figure 30. Bipolar to Unipolar Level Shifter
Figure 30 shows a level shifter circuit that can move a bipolar
signal into a unipolar range. A positive reference voltage, derived
from the +5 V supply, sets a bias level of +1.25 V at the non-
inverting terminal of the op amp. In ac applications, the accuracy
of this voltage level is not important. Noise is however a serious
consideration. A 0.1
μ
F capacitor provides useful decoupling of
this noise.
The bias level on the noninverting terminal sets the input common-
mode voltage to +1.25 V. Because the output will always be
positive, the op amp may therefore be powered with a single
+5 V power supply.
The overall gain function is given by the equation:
V
OUT
=
–
R
2
R
1
V
IN
+
R
4
R
3
+
R
4
1
+
R
2
R
1
V
REF
In the above example, the equation simplifies to
V
OUT
=
–
V
IN
+
2.5
V
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