ADA4853-1/ADA4853-2/ADA4853-3

Rev. C | Page 14 of 16

CIRCUIT DESCRIPTION

The ADA4853-1/ADA4853-2/ADA4853-3 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 100 mV of either supply rail

when driving light loads and within 200 mV when driving

150 Ω. High speed performance is maintained at supply

voltages as low as 2.65 V.

HEADROOM CONSIDERATIONS

The ADA4853-1/ADA4853-2/ADA4853-3 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 their headroom limits. The amplifiers’

input common-mode voltage range extends from the negative

supply voltage (actually 200 mV below this) to within 1.2 V of

the positive supply voltage.

Exceeding the headroom limits is not a concern for any

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

voltage at the amplifiers’ positive input lies within the

amplifiers’ input common-mode range.

The input stage is the headroom limit for signals approaching

the positive rail. Figure 50 shows a typical offset voltage vs. the

input common-mode voltage for the ADA4853-1/ADA4853-2/

ADA4853-3 on a 5 V supply. Accurate dc performance is

maintained from approximately 200 mV below the negative

supply to within 1.2 V of the positive supply. For high speed

signals, however, there are other considerations. As the

common-mode voltage gets within 1.2 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.

–0.6

–0.8

–1.0

–1.2

–1.4

–1.6

–1.8

–2.0

–1.0 –0.5

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

For signals approaching the negative supply, inverting gain, and

high positive gain configurations, the headroom limit is the

output stage. The ADA4853-1/ADA4853-2/ADA4853-3 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.

As the saturation point of the output stage is approached, the

output signal shows increasing amounts of compression and

clipping. For the input headroom case, higher frequency signals

require a bit more headroom than the lower frequency signals.

Figure 27 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 ADA4853-1/

ADA4853-2/ADA4853-3 is 200 mV below the negative supply

to within 1.2 V of the positive supply. Exceeding the top limit

results in lower bandwidth and increased rise time. Pushing the

input voltage of a unity-gain follower to less than 1.2 V from the

positive supply leads to an increasing amount of output error as

well as increased settling time. The recovery time from input

voltages 1.2 V or closer to the positive supply is approximately

40 ns; this is limited by the settling artifacts caused by transis-

tors 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, greatly increasing the current draw of the

devices.