ADA4851-4YRUZ-R7 AD [Analog Devices], ADA4851-4YRUZ-R7 Datasheet - Page 13

• Manufacturer Part Number: ADA4851-4YRUZ-R7
• Description: Low Cost, High Speed, Rail-to-Rail Output Op Amps
• Manufacturer: AD [Analog Devices]

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.
Rev. C | Page 13 of 20
Figure 41. Unity-Gain Follower Bandwidth vs. Input Common-Mode
440
460
480
500
520
540
560
580
600
–1
–2
–3
–4
–5
–6
1
0
2
0.1
–6
G = +1
R
V
Figure 40. V
ADA4851-1/ADA4851-2/ADA4851-4
L
S
–5
= 1kΩ
= 5V
–4
1
OS
–3
vs. Common-Mode Voltage, V
FREQUENCY (MHz)
–2
V
CM
–1
10
(V)
0
1
100
2
S
V
V
V
V
= ±5 V
CM
CM
CM
CM
= 3.0V
= 3.1V
= 3.3V
= 3.2V
3
1000
4