AD8008 Analog Devices, AD8008 Datasheet - Page 15

• Manufacturer Part Number: AD8008
• Description: Ultralow Distortion, High Speed Amplifiers
• Manufacturer: Analog Devices

Specifications of AD8008

• -3db Bandwidth: 650MHz
• Slew Rate: 1kV/µs
• Vos: 500µV
• Ib: 4µA
• # Opamps Per Pkg: 2
• Input Noise (nv/rthz): 2.7nV/rtHz
• Vcc-vee: 5V to 12V
• Isy Per Amplifier: 10.2mA
• Packages: SOIC,SOP

THEORY OF OPERATION
The AD8007 (single) and AD8008 (dual) are current feedback
amplifiers optimized for low distortion performance. A simplified
conceptual diagram of the AD8007 is shown in Figure 51. It
closely resembles a classic current feedback amplifier comprised
of a complementary emitter-follower input stage, a pair of signal
mirrors, and a diamond output stage. However, in the case of
the AD8007/AD8008, several modifications were made to improve
the distortion performance over that of a classic current feedback
topology.
The signal mirrors were replaced with low distortion, high
precision mirrors. In Figure 51, they are shown as M1 and M2.
Their primary function from a distortion standpoint is to reduce
the effect of highly nonlinear distortion caused by capacitances,
CJ1 and CJ2. These capacitors represent the collector-to-base
capacitances of the output devices of the mirrors.
A voltage imbalance arises across the output stage, as measured
from the high impedance node, high-Z, to the output node, OUT.
This imbalance is a result of delivering high output currents and
is the primary cause of output distortion. Circuitry is included
to sense this output voltage imbalance and generate a compensating
current, I
distortion that could be generated at the output stage. Similarly, the
nonlinear voltage imbalance across the input stage (measured from
the noninverting to the inverting input) is sensed, and a current,
I
The design and layout are strictly top-to-bottom symmetric to
minimize the presence of even-order harmonics.
IN+
DI
, is injected to compensate for input-generated distortion.
D1
D2
I
I
2
1
–
–
DO
. When injected into the circuit, I
I
DI
Figure 51. Simplified Schematic of AD8007
Q1
Q2
IN–
R
G
+V
–V
S
S
C
C
J
J
M1
1
2
M2
HIGH-Z
R
F
–
–
Q3
Q4
I
I
4
3
DO
I
DO
reduces the
Q5
Q6
OUT
+V
–V
S
S
Rev. E | Page 15 of 20
USING THE AD8007/AD8008
Supply Decoupling for Low Distortion
Decoupling for low distortion performance requires careful
consideration. The commonly adopted practice of returning the
high frequency supply decoupling capacitors to physically separate
(and possibly distant) grounds can lead to degraded even-order
harmonic performance. This situation is shown in Figure 52 using
the AD8007 as an example; however, it is not recommended. For a
sinusoidal input, each decoupling capacitor returns to its ground a
quasi-rectified current carrying high even-order harmonics.
The decoupling scheme shown in Figure 53 is recommended.
In Figure 53, the two high frequency decoupling capacitors are
first tied together at a common node and are then returned to
the ground plane through a single connection. By first adding
the two currents flowing through each high frequency decoupling
capacitor, this ensures that the current returned into the ground
plane is only at the fundamental frequency.
Figure 53. High Frequency Capacitors Returned to Ground at a Single Point
Figure 52. High Frequency Capacitors Returned to Physically Separate
499Ω
499Ω
R
IN
R
IN
G
G
Grounds (Not Recommended)
200Ω
200Ω
R
R
+V
–V
+V
–V
S
S
0.1µF
0.1µF
(Recommended)
S
S
S
S
10µF
AD8007
AD8007
GND 1
GND 2
0.1µF
0.1µF
499Ω
+
+
499Ω
+
+
10µF
10µF
R
10µF
R
F
F
AD8007/AD8008
OUT
OUT