ADA4850-1/ADA4850-2 Data Sheet
Rev. D | Page 12 of 14
CIRCUIT DESCRIPTION
The ADA4850-1/ADA4850-2 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 swing to within 80 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 AND OVERDRIVE RECOVERY
CONSIDERATIONS
Input
The ADA4850-1/ADA4850-2 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 headroom limits of the amplifier. The
input common-mode voltage range extends 200 mV below the
negative supply voltage or ground for single-supply operation to
within 2.2 V of the positive supply voltage. Therefore, in a gain
of +3, the ADA4850-1/ADA4850-2 can provide full rail-to-rail
output swing for supply voltage as low as 3.3 V, assuming the
input signal swing is from −V
S
(or ground) to 1.1 V.
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 sets the headroom limit for signals when the
amplifier is used in a gain of +1 for signals approaching the
positive rail. For high speed signals, however, there are other
considerations. Figure 38 shows −3 dB bandwidth vs. dc input
voltage for a unity-gain follower. As the common-mode voltage
approaches the positive supply, the bandwidth begins to drop
when within 2 V of +V
S
. This can manifest itself in increased
distortion or settling time.
Figure 38. Unity-Gain Follower Bandwidth vs.
Frequency for Various Input Common-Mode
Higher frequency signals require more headroom than the
lower frequencies to maintain distortion performance. Figure 39
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.
Figure 39. Pulse Response, Input Headroom Limits
The recovery time from input voltages 2.2 V or closer to the
positive supply is approximately 50 ns, which is limited by the
settling artifacts caused by transistors in the input stage coming
out of saturation.
The ADA4850-1/ADA4850-2 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, which greatly increase the current draw of the devices.
Output
For signals approaching the negative supply and inverting gain,
and high positive gain configurations, the headroom limit is the
output stage. The ADA4850-1/ADA4850-2 amplifiers 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 drive
current, due to the output transistor collector resistance.
As the saturation point of the output stage is approached, the
output signal shows increasing amounts of compression and
clipping. As in the input headroom case, higher frequency signals
require a bit more headroom than the lower frequency signals.
Output overload recovery is typically within 40 ns after the
input of the amplifier is brought to a nonoverloading value.
2
–6
0.1
1000
05320-096
FREQUENCY (MHz)
GAIN (dB)
1
0
–1
–2
–3
–4
–5
1 10 100
V
S
= 5V
G = +1
R
L
= 1kΩ
V
OUT
= 0.1V p-p
V
CM
= 3V
V
CM
= 3.1V
V
CM
= 3.2V
V
CM
= 3.3V
3.6
1.8
0
100
05320-061
TIME (ns)
OUTPUT VOLTAGE (V)
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
10 20
30 40
50 60
70
80 90
V
S
= 5V
G = +1
R
L
= 1k
Ω
V
STEP
= 2V TO 3V
V
STEP
= 2.4V TO 3.4V
V
STEP
= 2.3V TO 3.3V
V
STEP
= 2.2V TO 3.2V
V
STEP
= 2.1V TO 3.1V
