
AD8061/AD8062/AD8063
–13–
REV. C
For signals approaching the minus supply and inverting gain
and high positive gain configurations, the headroom limit will be
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
can be estimated using the equation
V
SAT
= 25
mV
+
I
O
×
8
,
where
I
O
is the output current, and 8
is a typical value for the
output transistors’ collector resistance.
TIME
–
ns
2.0
0
O
–
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
4
8
12
16
20
24
28
32
2V TO 3V STEP
2.1V TO 3.1V STEP
2.2V TO 3.2V STEP
2.3V TO 3.3V STEP
2.4V TO 3.4V STEP
Figure 5. Output Rising Edge for 1 V Step at Input Head-
room Limits, G = 1, V
S
= 5 V, 0 V
As the saturation point of the output stage is approached, the
output signal will show increasing amounts of compression and
clipping. As in the input headroom case, the higher frequency
signals require a bit more headroom than the lower frequency
signals. TPCs 11, 12, and 13 illustrate the point, plotting typical
distortion versus output amplitude and bias for gains of 2 and 5.
Overload Behavior and Recovery
Input
The specified input common-mode voltage of the AD806x is
–200 mV below the negative supply to within 1.8 V of the posi-
tive supply. Exceeding the top limit results in lower bandwidth
and increased settling time as seen in Figures 4 and 5. Push-
ing the input voltage of a unity gain follower beyond 1.6 V within
the positive supply leads to the behavior shown in Figure 6—an
increasing amount of output error as well as much increased
settling time. Recovery time from input voltages 1.6 V or closer
to the positive supply is about 35 ns, which is limited by the
settling artifacts caused by transistors in the input stage com-
ing out of saturation.
The AD806x family does not exhibit phase reversal, even for input
voltages beyond the voltage supply rails. Going more than 0.6 V
beyond the power supplies will turn on protection diodes at the
input stage, which will greatly increase the device’s current draw.
TIME
–
ns
2.1
0
O
–
2.3
100
VOLTAGE STEP
FROM 2.4V TO 3.4V
2.5
2.7
2.9
3.1
3.3
3.5
3.7
VOLTAGE STEP
FROM 2.4V TO 3.6V
VOLTAGE STEP
FROM 2.4V TO 3.8V,
4V AND 5V
200
300
400
500
600
Figure 6. Pulse Response for G = 1 Follower, Input Step
Overloading the Input Stage
Output
Output overload recovery is typically within 40 ns after the
amplifier’s input is brought to a nonoverloading value. Figure
7 shows output recovery transients for the amplifier recovering
from a saturated output from the top and bottom supplies to a
point at midsupply.
TIME
–
ns
–
0.20
I
–
OUTPUT VOLTAGE
5V TO 2.5V
0.20
0.60
1.0
1.4
1.8
2.2
2.6
3.0
3.4
3.8
4.2
4.6
5.0
0
10
20
30
40
50
60
70
OUTPUT VOLTAGE
0V TO 2.5V
INPUT VOLTAGE
EDGES
R
5V
V
O
2.5V
R
V
IN
Figure 7. Overload Recovery, G = –1, V
S
= 5 V
CAPACITIVE LOAD DRIVE
The AD806x family is optimized for bandwidth and speed, not
for driving capacitive loads. Output capacitance will create a
pole in the amplifier’s feedback path, leading to excessive
peaking and potential oscillation. If dealing with load capaci-
tance is a requirement of the application, the two strategies to
consider are (1) using a small resistor in series with the
amplifier’s output and the load capacitance and (2) reducing
the bandwidth of the amplifier’s feedback loop by increasing the
overall noise gain.