ADA4851-1YRJZ-R2 View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADA4851-1YRJZ-R2

Low Cost, High Speed, Rail-to-Rail, Output Op Amps

Analog Devices 

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 negative 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 the negative supply), 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 approaches

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.

ADA4851-1/ADA4851-2/ADA4851-4

440

460

480

500

520

540

560

580

600

–6 –5 –4 –3 –2 –1 0 1 2 3 4

VCM (V)

Figure 40. VOS vs. Common-Mode Voltage, VS = ±5 V

2

G = +1

1

RL = 1kΩ

VS = 5V

0

–1

VCM = 3.0V

VCM = 3.1V

–2

VCM = 3.2V

–3

VCM = 3.3V

–4

–5

–6

0.1

1

10

100

1000

FREQUENCY (MHz)

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

Rev. J | Page 17 of 24

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