ADA4851-1/ADA4851-2/ADA4851-4

Rev. E | Page 13 of 20

CIRCUIT DESCRIPTION

The ADA4851-1/ADA4851-2/ADA4851-4 feature a high slew

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

sensing signals at or below the minus supply rail. The rail-to-rail

output stage can pull within 60 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 CONSIDERATIONS

These amplifiers are designed for use in low voltage systems.

To obtain optimum performance, it is useful to understand the

behavior of the amplifiers as input and output signals approach

the headroom limits of the amplifiers. The input common-mode

voltage range of the amplifiers extends from the negative supply

voltage (actually 200 mV below this), or from ground for single-

supply operation, to within 2.2 V of the positive supply voltage.

Therefore, at a gain of 3, the amplifiers can provide full rail-to-

rail output swing for supply voltages as low as 3.3 V and down

to 3 V for a gain of 4.

Exceeding the headroom limit is not a concern for any inverting

gain on any supply voltage, as long as the reference voltage at

the positive input of the amplifier lies within the input common-

mode range of the amplifier.

The input stage is the headroom limit for signals approaching

the positive rail. Figure 40 shows a typical offset voltage vs. the

input common-mode voltage for the ADA4851-1/ADA4851-2/

ADA4851-4 amplifiers on a ±5 V supply. Accurate dc performance

is maintained from approximately 200 mV below the negative

supply to within 2.2 V of the positive supply. For high speed

signals, however, there are other considerations. Figure 41

shows −3 dB bandwidth vs. input common-mode voltage for a

unity-gain follower. As the common-mode voltage gets within 2

V of positive supply, the amplifier responds well but the

bandwidth begins to drop as the common-mode voltage

approaches the positive supply. This can manifest itself in

increased distortion or settling time. Higher frequency signals

require more headroom than the lower frequencies to maintain

distortion performance.

600

580

560

540

520

500

480

460

440

–6

–5

–4

–3

–2

–1

0

1

2

3

4

–6

–5

–4

–3

–2

–1

0

1

0.1

10

1

100

FREQUENCY (MHz)

2

1000

G = +1

Figure 41. Unity-Gain Follower Bandwidth vs. Input Common-Mode