ADA4851-1/ADA4851-2/ADA4851-4

Rev. C | Page 14 of 20

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.

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

0

1020

3040

50

6070

8090

100

TIME (ns)

3.6

G = +1

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 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.

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.

TIME (ns)

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

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

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.

–2

–1

0

1

2

3

4

5

6

0

1020

3040

50

6070

8090

100

TIME (ns)

7

G = –1

INPUT

VOLTAGE

EDGES

Figure 44. Overload Recovery