AD8045ACPZ-REEL7(RevB) View Datasheet(PDF) - Analog Devices
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Description
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AD8045ACPZ-REEL7 Datasheet PDF : 26 Pages
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Data Sheet
THEORY OF OPERATION
The AD8045 is a high speed voltage feedback amplifier
fabricated on the Analog Devices 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 open-loop response of the AD8045 over the frequency can
be approximated by the integrator response shown in Figure 63.
VIN
VOUT
VOUT/VIN
=
fCROSSOVER
f
fCROSSOVER = 400MHz
0
1
10
100
FREQUENCY (MHz)
1000
Figure 63. Open-Loop Response
The closed-loop transfer function for the noninverting
configuration is shown in Figure 64 and is written as
VOUT
VIN
=
2 π × fCROSSOVER ×(RG + RF )
(RF + RG )s + 2 π × fCROSSOVER × RG
where:
s is (2 πj)f.
fCROSSOVER is the frequency where the open-loop gain of the
amplifier equals 1 (0 dB).
DC gain is therefore
VOUT = (RG + RF )
VIN
RG
Closed-loop −3 dB bandwidth equals
( ) VOUT
VIN
=
fCROSSOVER ×
RG
RG + RF
The closed-loop bandwidth is inversely proportional to the
noise gain of the op amp circuit, (RF + RG)/RG. 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.
AD8045
RS
VIN
+ VOUT –
RF
RG
Figure 64. Noninverting Configuration
DC ERRORS
Figure 65 shows the dc error contributions. The total output
error voltage is
VOUT
(ERROR)
=
−I
B
+
RS
RG + RF
RG
+
IB− RF
+ VOS
RG + RF
RG
RS
VOS
IB+
+ VOUT –
IB–
RF
RG
Figure 65. Amplifier DC Errors
The voltage error due to IB+ and IB− is minimized if RS = RF||RG.
To include the effects of common-mode and power supply
rejection, model VOS as
VOS
= VOS nom
+
∆VS
PSR
+
∆VCM
CMR
where:
Vos nom is the offset voltage at nominal conditions.
ΔVS is the change in the power supply voltage from nominal
conditions.
PSR is the power supply rejection.
CMR is the common-mode rejection.
ΔVCM is the change in common-mode voltage from nominal
conditions.
Rev. B | Page 17 of 24
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