AD8021
Rev. F | Page 21 of 28
THEORY OF OPERATION
The AD8021 is fabricated on the second generation of Analog
Devices proprietary High Voltage eXtra-Fast Complementary
Bipolar (XFCB) process, which enables the construction of PNP
and NPN transistors with similar f
T
s in the 3 GHz region. The
transistors are dielectrically isolated from the substrate (and
each other), eliminating the parasitic and latch-up problems
caused by junction isolation. It also reduces nonlinear capaci-
tance (a source of distortion) and allows a higher transistor, f
T
,
for a given quiescent current. The supply current is trimmed,
which results in less part-to-part variation of bandwidth, slew
rate, distortion, and settling time.
As shown in
Figure 61, the AD8021 input stage consists of an
NPN differential pair in which each transistor operates at a
0.8 mA collector current. This allows the input devices a high
transconductance; thus, the AD8021 has a low input noise of
2.1 nV/√Hz @ 50 kHz. The input stage drives a folded cascode
that consists of a pair of PNP transistors. The folded cascode
and current mirror provide a differential-to-single-ended
conversion of signal current. This current then drives the high
impedance node (Pin 5), where the C
C
external capacitor is
connected. The output stage preserves this high impedance with
a current gain of 5000, so that the AD8021 can maintain a high
open-loop gain even when driving heavy loads.
Two internal diode clamps across the inputs (Pin 2 and Pin 3)
protect the input transistors from large voltages that could
otherwise cause emitter-base breakdown, which would result in
degradation of offset voltage and input bias current.
+IN
–IN
C
INTERNAL
1.5pF
C
COMP
C
C
–V
S
+V
S
OUTPUT
01888-061
Figure 61. Simplified Schematic
PCB LAYOUT CONSIDERATIONS
As with all high speed op amps, achieving optimum performance
from the AD8021 requires careful attention to PC board layout.
Particular care must be exercised to minimize lead lengths
between the ground leads of the bypass capacitors and between
the compensation capacitor and the negative supply. Otherwise,
lead inductance can influence the frequency response and even
cause high frequency oscillations. Use of a multilayer printed
circuit board, with an internal ground plane, reduces ground
noise and enables a compact component arrangement.
Due to the relatively high impedance of Pin 5 and low values of
the compensation capacitor, a guard ring is recommended. The
guard ring is simply a PC trace that encircles Pin 5 and is
connected to the output, Pin 6, which is at the same potential as
Pin 5. This serves two functions. It shields Pin 5 from any local
circuit noise generated by surrounding circuitry. It also
minimizes stray capacitance, which would tend to otherwise
reduce the bandwidth. An example of a guard ring layout is
shown in
Figure 62.
Also shown in
Figure 62, the compensation capacitor is located
immediately adjacent to the edge of the AD8021 package, spanning
Pin 4 and Pin 5. This capacitor must be a high quality surface-
mount COG or NPO ceramic. The use of leaded capacitors is
not recommended. The high frequency bypass capacitor(s)
should be located immediately adjacent to the supplies,
Pin 4 and Pin 7.
To achieve the shortest possible lead length at the inverting
input, the feedback resistor R
F
is located beneath the board and
spans the distance from the output, Pin 6, to inverting input
Pin 2. The return node of Resistor R
G
should be situated as close
as possible to the return node of the negative supply bypass
capacitor connected to Pin 4.
DISABLE
V
OUT
8
7
6
1
2
3
LOGIC REFERENCE
–IN
+IN
–V
S
4
+V
S
5
C
COMP
GROUND
PLANE
BYPASS
CAPACITOR
COMPENSATION
CAPACITOR
GROUND
PLANE
BYPASS
APACITO
METAL
(TOP VIEW)
01888-062
Figure 62. Recommended Location of
Critical Components and Guard Ring
