ADA4850-1/ADA4850-2

Rev. B | Page 12 of 16

CIRCUIT DESCRIPTION

The ADA4850-1/ADA4850-2 feature a high slew rate input

stage that is a true single-supply topology, capable of sensing

signals at or below the negative supply rail. The rail-to-rail

output stage can swing to within 80 mV of either supply rail

when driving light loads and within 0.17 V when driving 150 Ω.

High speed performance is maintained at supply voltages as low

as 2.7 V.

HEADROOM AND OVERDRIVE RECOVERY

CONSIDERATIONS

Input

The ADA4850-1/ADA4850-2 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. The input

common-mode voltage range extends 200 mV below the

negative supply voltage or ground for single-supply operation

to within 2.2 V of the positive supply voltage. Therefore, in a

gain of +3, the ADA4850-1/ADA4850-2 can provide full rail-

to-rail output swing for supply voltage as low as 3.3 V, assuming

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.

The input stage sets the headroom limit for signals when the

amplifier is used in a gain of +1 for signals approaching the

positive rail. For high speed signals, however, there are other

considerations. Figure 38 shows −3 dB bandwidth vs. dc input

voltage for a unity-gain follower. As the common-mode voltage

approaches the positive supply, the bandwidth begins to drop

distortion or settling time.

2

–6

0.1

1000

FREQUENCY (MHz)

1

0

–1

–2

–3

–4

–5

1

10

100

G = +1

Figure 38. Unity-Gain Follower Bandwidth vs.

Frequency for Various Input Common-Mode

Higher frequency signals require more headroom than the

lower frequencies to maintain distortion performance. Figure 39

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.

3.6

1.8

0

100

TIME (ns)

3.4

3.2

3.0

2.8

2.6

2.4

2.2

2.0

10

20

30

40

50

60

70

80

90

G = +1

Figure 39. Pulse Response, Input Headroom Limits

The recovery time from input voltages 2.2 V or closer to the

positive supply is approximately 50 ns, which is limited by the

settling artifacts caused by transistors in the input stage coming

out of saturation.

The ADA4850-1/ADA4850-2 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 increase the current draw of the devices.

Output

For signals approaching the negative supply and inverting gain,

and high positive gain configurations, the headroom limit is the

output stage. The ADA4850-1/ADA4850-2 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 drive

current, due to the output transistor 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.

Output overload recovery is typically within 40 ns after the

amplifier’s input is brought to a nonoverloading value.