AD8061/AD8062/AD8063
CIRCUIT DESCRIPTION
The AD8061/AD8062/AD8063 family is comprised of high
speed voltage 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 Ω. High speed perform-
ance is maintained at supply voltages as low as 2.7 V.
–0.4
–0.8
–1.2
–1.6
–2.0
–2.4
–2.8
–3.2
–3.6
–4.0
HEADROOM CONSIDERATIONS
These amplifiers are designed for use in low voltage systems.
To obtain optimum performance, it is useful to understand the
behavior of the amplifier as input and output signals approach
the amplifier’s headroom limits.
–0.5
0
0.5
1.0
1.5
V
2.0
(V)
2.5
3.0
3.5
4.0
CM
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 −VS (or ground) to
+VS/2. At a gain of 3, the AD806x can provide a rail-to-rail
output range down to 2.7 V total supply voltage.
Figure 45. VOS vs. Common-Mode Voltage, VS = 5 V
2
0
V
V
V
V
V
= 3.0
= 3.1
= 3.2
= 3.3
= 3.4
CM
CM
CM
CM
CM
–2
–4
–6
–8
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.
0.1
1
10
100
1k
10k
The input stage is the headroom limit for signals when the
amplifier is used in a gain of 1 for signals approaching the
positive rail. Figure 45 shows a typical offset voltage vs.
input common-mode voltage for the AD806x amplifier on
a 5 V supply. Accurate dc performance is maintained from
approximately 200 mV below the minus supply to within
1.8 V of the positive supply. For high-speed signals, however,
there are other considerations. Figure 46 shows −3 dB
bandwidth vs. 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 +VS.
FREQUENCY (MHz)
Figure 46. Unity-Gain Follower Bandwidth vs. Input Common Mode, VS = 5 V
Higher frequency signals require more headroom than lower
frequencies to maintain distortion performance. Figure 47
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.
For signals approaching the minus supply and inverting gain
and high positive gain configurations, the headroom limit is
the output stage. The AD806x amplifiers use a common emitter
style output stage. This output stage maximizes the available
output range, limited by the saturation voltage of the output
transistors. The saturation voltage increases with the drive
current the output transistor is required to supply, due to the
output transistors’ collector resistance. The saturation voltage is
estimated using the equation VSAT = 25 mV + IO × 8 Ω, where IO
is the output current, and 8 Ω is a typical value for the output
transistors’ collector resistance.
This manifests itself in increased distortion or settling time.
Figure 16 plots the distortion of a 1 V p-p signal with the
AD806x amplifier used as a follower on a 5 V supply vs. signal
common-mode voltage. Distortion performance is maintained
until the input signal center voltage gets beyond 2.5 V, as the
peak of the input sine wave begins to run into the upper
common-mode voltage limit.
Rev. D | Page 14 of 20
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