AD8061-EB Analog Devices, AD8061-EB Datasheet - Page 12

• Manufacturer Part Number: AD8061-EB
• Description: Low-Cost/ 300 MHz Rail-to-Rail Amplifiers
• Manufacturer: Analog Devices

AD8061/AD8062/AD8063
CIRCUIT DESCRIPTION
The AD8061/AD8062/AD8063 family are very high-speed volt-
age feedback op amps. The high slew rate input stage is a true
single-supply topology, capable of sensing signals at or below
the minus supply rail. The rail-to-rail output stage can pull
within 30 mV of either supply rail when driving light loads and
within 0.3 V when driving 150 . High-speed performance is
maintained at supply voltages as low as 2.7 V.
Headroom Considerations
These amplifiers are designed for use in low-voltage systems. To
obtain optimum performance, it is useful to understand the be-
havior of the amplifier as input and output signals approach the
amplifier’s headroom limits.
The AD806x’s input common-mode voltage range extends from
the negative supply voltage (actually 200 mV below this), or
“ground” for single supply operation, to within 1.8 V of the
positive supply voltage. Thus, at a gain of 2, the AD806x can
provide full “rail-to-rail” output swing for supply voltage as low
as 3.6 V, assuming the input signal swing from –V
to +V
output range down to 2.7 V total supply voltage.
Exceeding the headroom limit is not a concern for any inverting
gain on any supply voltage, as long as the reference voltage at
the amplifier’s positive input lies within the amplifier’s input
common-mode range.
S
2.4V
4.5V
2.5V
0.5V
/2. At a gain of 3, the AD806x can provide a rail-to-rail
2.6V
2.5V
0
0
1V/DIV
Figure 39. 200 mV Step Response
50mV/DIV
5
5
Figure 40. 2 V Step Response
10
10
15
15
20
20
TIME – ns
TIME – ns
25
25
30
30
35
35
V
G = 2
R
V
S
IN
L
40
40
V
G = 1
R
= 5V
= R
= 2V p-p
S
L
= 5V
= 1k
S
F
45
= 1k
(or ground)
45
50
50
–12–
The input stage will be the headroom limit for signals when the
amplifier is used in a gain of 1 for signals approaching the
positive rail. Figure 41 shows a typical offset voltage versus
input common-mode voltage for the AD806x amplifier on a
5 V supply. Accurate dc performance is maintained from about
200 mV below the minus supply to within 1.8 V of the positive
supply. For high-speed signals, however, there are other consid-
erations. Figure 42 shows –3 dB bandwidth versus dc input
voltage for a unity gain follower. As the common-mode voltage
approaches the positive supply, the amplifier holds together
well, but the bandwidth begins to drop at 1.9 V within +V
This can manifest itself in increased distortion or settling time.
Figure 12 plots the distortion of a 1 V p-p signal with the
AD806x amplifier used as a follower on a 5 V supply versus
signal common-mode voltage. Distortion performance is main-
tained until the input signal center voltage gets beyond 2.5 volts,
as the peak of the input sine wave begins to run into the upper
common-mode voltage limit. Higher frequency signals require
more headroom than the lower frequencies to maintain distor-
tion performance. Figure 43 illustrates how the rising edge
settling time for the amplifier configured as a unity gain follower
stretches out as the top of a 1 V step input approaches and exceeds
the specified input common-mode voltage limit.
Figure 41. V
Figure 42. Unity Gain Follower Bandwidth vs. Input
Common Mode, V
–0.4
–0.8
–1.2
–1.6
–2.0
–2.4
–2.8
–3.2
–3.6
–4.0
–0.5
–2
–4
–6
–8
2
0
0.1
0
OS
0.5
1
vs. Common-Mode Voltage, V
S
1.0
= 5 V
FREQUENCY – MHz
V
10
CM
1.5
– Volts
2.0
100
2.5
3.0
1000
V
V
V
V
V
CM
CM
CM
CM
CM
= 3.0
= 3.1
= 3.2
= 3.3
= 3.4
3.5
S
10000
= 5 V
4.0
REV. A
S
.