ADA4851-1/ADA4851-2/ADA4851-4

Rev. D | Page 13 of 20

CIRCUIT DESCRIPTION

The ADA4851-1, ADA4851-2, and 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 amplifiers’ headroom limits. The amplifiers’ input common-

mode voltage range 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 amplifier’s positive input lies within the amplifier’s input

common-mode range.

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 minus

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. dc input 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