Datasheet
ADA4941-1
Rev. C | Page 19 of 24
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
0
0.001 100
FREQUENCY (MHz)
OPEN-LOOP GAIN (dB)
05704-062
80
60
40
20
0.01 0.1 1 10
fcr = 50MHz
Figure 53. ADA4941-1 Op Amp Open-Loop Gain vs. Frequency
For each amplifier, the frequency response can be approximated
by the following equations:
×
+
+
×
+×=
fcr
f
R
RR
R
R
VIN_A1V
G
G
F
G
F
O
1
1
1
(15)
(Noninverting Response)
×
+
+
×
−
×=
fcr
f
R
RR
R
R
VIN_A2V
G
G
F
G
F
O
1
1
(16)
(Inverting Response)
f
CR
is the gain-bandwidth frequency of the amplifier (where the
open-loop gain shown in Figure 53 equals 1). f
CR
for both
amplifiers is about 50 MHz.
The inverting amplifier A2 has a fixed feedback network. The
transfer function is approximately
+
×−=
×
+
×−=
MHz25
1
1
MHz50
2
1
1
2_
f
VOP
f
VINAV
O
(17)
A1’s frequency response depends on the external feedback
network as indicated by Equation 15. The overall differential
output voltage is therefore
V
O
, dm = VOP − VON = VOP + VOP ×
+
MHz25
1
1
f
(18)
+
+
×
×
+
+
×
+×=
MHz25
1
1
1
MHz50
1
1
1
f
f
R
RR
R
R
VIN, dmV
G
G
F
G
F
O
(19)
Multiplying the terms and neglecting negligible terms leads to
the following approximation:
+×
×
+
+
×
+=
MHz25
1
MHz50
1
2
1,
ff
R
RR
R
R
VINdmV
G
G
F
G
F
O
(20)
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 + R
F
/R
G
). For instance, if the external feedback network
provides a noise gain of 10, the bandwidth drops to 5 MHz.