ADA4851-4YRUZ-R7 AD [Analog Devices], ADA4851-4YRUZ-R7 Datasheet - Page 14

• Manufacturer Part Number: ADA4851-4YRUZ-R7
• Description: Low Cost, High Speed, Rail-to-Rail Output Op Amps
• Manufacturer: AD [Analog Devices]

ADA4851-1/ADA4851-2/ADA4851-4
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 minus 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 output transistor’s collector
resistance.
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 a bit more headroom than the lower frequency
signals. Figure 16 illustrates this point by plotting the typical
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 seen 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 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.
Figure 42. Output Rising Edge for 1 V Step at Input Headroom Limits
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
3.6
0
G = +1
R
V
L
S
= 1kΩ
= 5V
10
20
30
V
V
V
V
V
STEP
STEP
STEP
STEP
STEP
40
= 2.1V TO 3.1V
= 2V TO 3V
= 2.2V TO 3.2V
= 2.3V TO 3.3V
= 2.4V TO 3.4V
TIME (ns)
50
60
70
80
90
100
Rev. C | Page 14 of 20
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
Output overload recovery is typically within 35 ns after the
amplifier’s input 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.
3.50
3.25
3.00
2.75
2.50
2.25
2.00
–1
–2
6
5
4
3
2
1
0
7
0
0
G = +1
R
V
INPUT
VOLTAGE
EDGES
L
S
100
10
= 1kΩ
= 5V
Figure 43. Pulse Response of G = 1 Follower,
Input Step Overloading the Input Stage
200
20
Figure 44. Overload Recovery
300
30
V
STEP
400
40
= 2.25V TO 3.25V
TIME (ns)
TIME (ns)
V
3.5V, 4V, AND 5V
STEP
V
500
50
OUT
= 2.25V TO
= 5V TO 2.5V
600
60
V
OUT
700
70
= 0V TO 2.5V
800
80
G = –1
R
V
L
S
900
= 1kΩ
= 5V
90
100
1k