ADA4851-1/ADA4851-2/ADA4851-4
Rev. J | Page 18 of 24
Figure 42 illustrates how the rising edge settling time for the
amplifier is configured as a unity-gain follower, stretching out
as the top of a 1 V step input that approaches and exceeds the
specified input common-mode voltage limit.
For signals approaching the negative supply and inverting gain
and high positive gain configurations, the headroom limit is the
output stage. The ADA4851-1/ADA4851-2/ADA4851-4 amplifiers
use a common emitter 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 that the output transistor is required to supply due
to the collector resistance of the output transistor.
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
0 102030405060708090100
OUTPUT VOLTAGE (V)
TIME (ns)
05143-052
3.6
V
STEP
= 2V TO 3V
V
STEP
= 2.1V TO 3.1V
V
STEP
= 2.2V TO 3.2V
V
STEP
= 2.3V TO 3.3V
V
STEP
= 2.4V TO 3.4V
G = +1
R
L
= 1kΩ
V
S
= 5V
Figure 42. Output Rising Edge for 1 V Step at Input Headroom Limits
As the saturation point of the output stage is approached, the
output signal shows increasing amounts of compression and
clipping. As in the input headroom case, higher frequency
signals require slightly more headroom than the lower fre-
quency signals. Figure 16 illustrates this point by plotting the
typical harmonic distortion vs. the output amplitude.
OVERLOAD BEHAVIOR AND RECOVERY
Input
The specified input common-mode voltage of the ADA4851-1/
ADA4851-2/ADA4851-4 is 200 mV below the negative supply
to within 2.2 V of the positive supply. Exceeding the top limit
results in lower bandwidth and increased rise time, as shown in
Figure 41 and Figure 42. Pushing the input voltage of a unity-
gain follower to less than 2 V from the positive supply leads to
the behavior shown in Figure 43—an increasing amount of output
error as well as a much increased settling time. The recovery time
from input voltages of 2.2 V or closer to the positive supply is
approximately 55 ns, which is limited by the settling artifacts
caused by transistors in the input stage coming out of saturation.
The amplifiers do not exhibit phase reversal, even for input
voltages beyond the voltage supply rails. Going more than 0.6 V
beyond the power supplies turns on protection diodes at the input
stage, which greatly increases the current draw of the devices.
OUTPUT VOLTAGE (V)
TIME (ns)
05143-051
2.00
2.25
2.50
3.00
3.25
3.50
2.75
0 100 200 300 400 500 600 700 800 900 1k
G = +1
R
L
= 1k
Ω
V
S
= 5V
V
STEP
= 2.25V TO 3.25V
V
STEP
= 2.25V TO
3.5V, 4V, AND 5V
Figure 43. Pulse Response of G = +1 Follower, Input Step Overloading
the Input Stage
Output
Output overload recovery is typically within 35 ns after the
input of the amplifier is brought to a nonoverloading value.
Figure 44 shows output recovery transients for the amplifier
configured in an inverting gain of 1 recovering from a saturated
output from the top and bottom supplies to a point at midsupply.
–2
–1
0
1
2
3
4
5
6
0 102030405060708090100
INPUT AND OUTPUT VOLTAGE (V)
TIME (ns)
05143-053
7
G = –1
R
L
= 1kΩ
V
S
= 5V
INPUT
VOLTAGE
EDGES
V
OUT
= 5V TO 2.5V
V
OUT
= 0V TO 2.5V
Figure 44. Overload Recovery
