AD8099_13 Datasheet, PDF (16/29 Pages) Analog Devices – Ultralow Distortion, High Speed 0.95 nV/Hz Voltage Noise Op Amp

Data Sheet
THEORY OF OPERATION
The AD8099 is a voltage feedback op amp that employs a new
highly linear low noise input stage. With this input stage, the
AD8099 can achieve better than 90 dB distortion for a 2 V p-p,
10 MHz output signal with an input referred voltage noise of
less than 1 nV/√Hz. This noise level and distortion
performance has been previously achievable only with fully
uncompensated amplifiers. The AD8099 achieves this level of
performance for gains as low as +2. This new input stage also
triples the achievable slew rate for comparably compensated
1 nV/√Hz amplifiers.
The simplified AD8099 topology is shown in Figure 58. The
amplifier is a single gain stage with a unity gain output buffer
fabricated in Analog Devices’ extra fast complimentary bipolar
process (XFCB). The AD8099 has 85 dB of open-loop gain and
maintains precision specifications such as CMRR, PSRR, VOS,
and VOS/T to levels that are normally associated with
topologies having two or more gain stages.
gm
R1
BUFFER
C_C
V_OUT
R_L
Figure 58. AD8099 Topology
The AD8099 can be externally compensated down to a gain of 2
through the use of an RC network. Above gains of 15, no exter-
nal compensation network is required. To realize the full gain
bandwidth product of the AD8099, no PCB trace should be
connected to or within close proximity of the external compen-
sation pin for the lowest possible capacitance.
External compensation allows the user to optimize the closed-
loop response for minimal peaking while increasing the gain
bandwidth product in higher gains, lowering distortion errors
that are normally more prominent with internally compensated
parts in higher gains. For a fixed gain bandwidth, wideband
distortion products would normally increase by 6 dB going
from a closed-loop gain of 2 to 4. Increasing the gain bandwidth
product of the AD8099 eliminates this effect with increasing
closed-loop gain.
The AD8099 is available in both a SOIC and an LFCSP, each of
which has a thermal pad for lower operating temperature. To
help avoid this pad in board layout, both packages have an extra
output pin on the opposite side of the package for ease in con-
necting a feedback network to the inputs. The secondary output
pin also isolates the interaction of any capacitive load on the
AD8099
output and self-inductance of the package and bond wire from
the feedback loop. While using the secondary output for feed-
back, inductance in the primary output will now help to isolate
capacitive loads from the output impedance of the amplifier.
Since the SOIC has greater inductance in its output, the SOIC
will drive capacitive loads better than the LFCSP. Using the
primary output for feedback with both packages will result in
the LFCSP driving capacitive load better than the SOIC.
The LFCSP and SOIC pinouts are identical, except for the
rotation of all pins counterclockwise by one pin on the LFCSP.
This isolates the inputs from the negative power supply pin,
removing a mutually inductive coupling that is most prominent
while driving heavy loads. For this reason, the LFCSP second
harmonic, while driving a heavy load, is significantly better
than that of the SOIC.
A three-state input pin is provided on the AD8099 for a high
impedance power-down and an optional input bias current
cancellation circuit. The high impedance output allows several
AD8099s to drive the same ADC or output line time inter-
leaved. Pulling the DISABLE pin low activates the high
impedance state. See Table 5 for threshold levels. When the
DISABLE pin is left floating, the AD8099 operates normally.
With the DISABLE pin pulled within 0.7 V of the positive
supply, an optional input bias current cancellation circuit is
turned on, which lowers the input bias current to less than 200
nA. In this mode, the user can drive the AD8099 with a high dc
source impedance and still maintain minimal output referred
offset without having to use impedance matching techniques. In
addition, the AD8099 can be ac-coupled while setting the bias
point on the input with a high dc impedance network. The
input bias current cancellation circuit will double the input
referred current noise, but this effect is minimal as long as
wideband impedance is kept low (see Figure 48 and Figure 51).
A pair of internally connected diodes limits the differential
voltage between the noninverting input and the inverting input
of the AD8099. Each set of diodes has two series diodes, which
are connected in anti-parallel. This limits the differential
voltage between the inputs to approximately 1.8 V. All of the
AD8099 pins are ESD protected with voltage limiting diodes
connected between both rails. The protection diodes can handle
5 mA of steady state current. Currents should be limited to
5 mA or less through the use of a series limiting resistor.
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