Datasheet
AD8045
Rev. A | Page 17 of 24
THEORY OF OPERATION
The AD8045 is a high speed voltage feedback amplifier fabri-
cated on ADI’s second generation eXtra Fast Complementary
Bipolar (XFCB) process. An H-bridge input stage is used to
attain a 1400 V/µs slew rate and low distortion in addition to a
low 3 nV/√Hz input voltage noise. Supply current and offset
voltage are laser trimmed for optimum performance.
FREQUENCY RESPONSE
The AD8045’s open-loop response over frequency can be
approximated by the integrator response shown in Figure 63.
f
CROSSOVER
= 400MHz
FREQUENCY (MHz)
V
OUT
/V
IN
(dB)
0
1 10 100
f
1000
V
OUT
/V
IN
=
f
CROSSOVER
V
IN
V
OUT
04814-0-008
Figure 63. Open-Loop Response
The closed-loop transfer function for the noninverting configu-
ration is shown in Figure 64 and is written as
()
GCROSSOVERG
F
F
GCROSSOVER
IN
OUT
RfRR
RRf
V
V
××++
+××
=
π2s
)(π2
where:
s is (2 πj)f.
f
CROSSOVER
is the frequency where the amplifier’s open-loop gain
equals 1 (0 dB).
DC gain is therefore
()
G
F
G
IN
OUT
R
RR
V
V +
=
Closed-loop −3 dB bandwidth equals
()
F
G
G
CROSSOVER
IN
OUT
RR
R
f
V
V
+
×=
The closed-loop bandwidth is inversely proportional to the
noise gain of the op amp circuit, (R
F
+ R
G
)/R
G
. This simple
model can be used to predict the −3 dB bandwidth for noise
gains above +2. The actual bandwidth of circuits with noise
gains at or below +2 is higher due to the influence of other
poles present in the real op amp.
04814-0-009
R
G
R
F
V
IN
V
OUT
R
S
+–
Figure 64. Noninverting Configuration
DC ERRORS
Figure 65 shows the dc error contributions. The total output
error voltage is
⎟
⎠
⎞
⎜
⎝
⎛
+
++
⎟
⎠
⎞
⎜
⎝
⎛
+
−=
−+
G
F
G
OS
F
B
G
F
G
S
B
(ERROR)OUT
R
RR
VRI
R
RR
RIV
04814-0-010
R
G
R
F
V
OS
V
OUT
R
S
+–
I
B
+
I
B
–
Figure 65. Amplifier DC Errors
The voltage error due to I
B
+ and I
B
− is minimized if R
S
= R
F
||R
G
.
To include the effects of common-mode and power supply
rejection, model V
OS
as
CMR
V
PSR
V
VV
CMS
OSOS
nom
∆
+
∆
+=
where:
nom
os
V is the offset voltage at nominal conditions.
Δ
V
S
is the change in the power supply voltage from nominal
conditions.
PSR is the power supply rejection.
C
MR is the common-mode rejection.
Δ
V
CM
is the change in common-mode voltage from nominal
conditions.