ADA4941-1YR-EBZ Просмотр технического описания (PDF) - Analog Devices
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ADA4941-1YR-EBZ Datasheet PDF : 24 Pages
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FREQUENCY RESPONSE VS. CLOSED-LOOP GAIN
The operational amplifiers used in the ADA4941-1 are voltage
feedback with an open-loop frequency response that can be
approximated with the integrator response, as shown in Figure 53.
100
80
60
40
20
fcr = 50MHz
0
0.001
0.01
0.1
1
10
100
FREQUENCY (MHz)
Figure 53. ADA4941-1 Op Amp Open-Loop Gain vs. Frequency
For each amplifier, the frequency response can be approximated
by the following equations:
VO _A1 = VIN
× 1 +
RF
RG
×
1
+
1
RF
+ RG
RG
×
f
fcr
(15)
(Noninverting Response)
VO _A2
= VIN
×
− RF
RG
×
1
+
1
RF
+ RG
RG
×
f
fcr
(16)
(Inverting Response)
fCR is the gain-bandwidth frequency of the amplifier (where the
open-loop gain shown in Figure 53 equals 1). fCR for both
amplifiers is about 50 MHz.
ADA4941-1
The inverting amplifier A2 has a fixed feedback network. The
transfer function is approximately
VO
_
A2
=
− VIN
×
1
+
1
2× f
50 MHz
=
− VOP
×
1
+
1
f
25 MHz
(17)
A1’s frequency response depends on the external feedback
network as indicated by Equation 15. The overall differential
output voltage is therefore
VO, dm = VOP − VON = VOP + VOP ×
1
1
+
25
f
MHz
(18)
VO ,
dm
=
VIN
×
1 +
RF
RG
×
1
+
1
RF + RG
RG
×
50
f
MHz
×
(19)
1
+
1
+
1
f
25 MHz
Multiplying the terms and neglecting negligible terms leads to
the following approximation:
VO
, dm
=
VIN 1 +
RF
RG
×
(20)
2
1
+
RF
+ RG
RG
×
50
f
MHz
×
1
+
25
f
MHz
There are two poles in this transfer function, and the lower
frequency pole limits the bandwidth of the differential
amplifier. If VOP is shorted to IN− (A1 is a unity gain follower),
the 25 MHz closed-loop bandwidth of the inverting channel
limits the overall bandwidth. When A1 is operating with higher
noise gains, the bandwidth is limited by A1’s closed-loop
bandwidth, which is inversely proportional to the noise gain
(1 + RF/RG). For instance, if the external feedback network
provides a noise gain of 10, the bandwidth drops to 5 MHz.
Rev. C | Page 19 of 24
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