AD8061ART datasheet(12/16 Pages) AD

12 / 16 page

AD8061/AD8062/AD8063

–12–

REV. C

50mV/DIV

VS = 5V

G = 1

RL = 1k

2.6V

TIME – ns

2.5V

2.4V

TPC 37. 200 mV Step Response

1V/DIV

VS = 5V

G = 2

RL = RF = 1k

VIN = 2V p-p

TIME – ns

4.5V

2.5V

0.5V

TPC 38. 2 V Step Response

CIRCUIT DESCRIPTION

The AD8061/AD8062/AD8063 family are very high-speed volt-

age feedback op amps. The high slew rate input stage 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 30 mV of either supply rail when driving light loads and

within 0.3 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 ampliﬁer as input and output signals approach

the ampliﬁer’s headroom limits.

The AD806x’s input common-mode voltage range extends from

the negative supply voltage (actually 200 mV below this), or

“ground” for single supply operation, to within 1.8 V of the

positive supply voltage. Thus, at a gain of 2, the AD806x can

provide full “rail-to-rail” output swing for supply voltage as low

as 3.6 V, assuming the input signal swing from –VS (or ground)

to +VS/2. At a gain of 3, the AD806x can provide a rail-to-rail

output range down to 2.7 V total supply voltage.

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 will be the headroom limit for signals when the

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

positive rail. Figure 3 shows a typical offset voltage versus

input common-mode voltage for the AD806x ampliﬁer on a

5 V supply. Accurate dc performance is maintained from about

200 mV below the minus supply to within 1.8 V of the positive

supply. For high-speed signals, however, there are other consid-

erations. Figure 4 shows –3 dB bandwidth versus dc input

VCM – V

–4.0

–3.6

–3.2

–2.8

–2.4

–2.0

–1.6

–1.2

–0.8

–0.4

–0.5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Figure 3. VOS vs. Common-Mode Voltage, VS = 5 V

VCM = 3.0

FREQUENCY – MHz

–8

0.1

–4

–2

–6

100

1000

10000

VCM = 3.1

VCM = 3.2

VCM = 3.3

VCM = 3.4

Figure 4. Unity Gain Follower Bandwidth vs. Input

Common Mode, VS = 5 V

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

approaches the positive supply, the ampliﬁer holds together

well, but the bandwidth begins to drop at 1.9 V within +VS.

This can manifest itself in increased distortion or settling time.

TPC 10 plots the distortion of a 1 V p-p signal with the AD806x

ampliﬁer used as a follower on a 5 V supply versus signal common-

mode voltage. Distortion performance is maintained until the

input signal center voltage gets beyond 2.5 V, as the peak of the

input sine wave begins to run into the upper common-mode

voltage limit. Higher frequency signals require more headroom

than the lower frequencies to maintain distortion performance.

Figure 5 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.

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