Datasheet

Data Sheet ADA4891-1/ADA4891-2/ADA4891-3/ADA4891-4
Rev. E | Page 15 of 24
APPLICATIONS INFORMATION
USING THE ADA4891
Understanding the subtleties of the ADA4891 family of amplifiers
provides insight into how to extract the peak performance from
the device. The following sections describe the effect of gain,
component values, and parasitics on the performance of the
ADA4891. The wideband, noninverting gain configuration of
the ADA4891 is shown in Figure 50; the wideband, inverting
gain configuration of the ADA4891 is shown in Figure 51.
WIDEBAND, NONINVERTING GAIN OPERATION
08054-023
ADA4891
R
F
R
G
R
T
50
Ω
SOURCE
R
L
+V
S
–V
S
10µF
0.1µF
V
I
V
O
10µF
0.1µF
Figure 50. Noninverting Gain Configuration
In Figure 50, R
F
and R
G
denote the feedback and gain resistors,
respectively. Together, R
F
and R
G
determine the noise gain of the
amplifier. The value of R
F
defines the 0.1 dB bandwidth (for
more information, see the Effect of R
F
on 0.1 dB Gain Flatness
section). Typical R
F
values range from 549 Ω to 698 Ω for the
ADA4891-1/ADA4891-2. Typical R
F
values range from 301 Ω
to 453 Ω for the ADA4891-3/ADA4891-4.
In a controlled impedance signal path, R
T
is used as the input
termination resistor designed to match the input source imped-
ance. Note that R
T
is not required for normal operation. R
T
is
generally set to match the input source impedance.
WIDEBAND, INVERTING GAIN OPERATION
08054-024
ADA4891
R
F
R
T
R
G
50Ω
SOURCE
R
L
+V
S
–V
S
V
I
V
O
10µF
0.1µF
10µF
0.1µF
Figure 51. Inverting Gain Configuration
Figure 51 shows the inverting gain configuration. For the
inverting gain configuration, set the parallel combination of
R
T
and R
G
to match the input source impedance.
Note that a bias current cancellation resistor is not required in
the noninverting input of the amplifier because the input bias
current of the ADA4891 is very low (less than 2 pA). Therefore,
the dc errors caused by the bias current are negligible.
For both noninverting and inverting gain configurations, it is
often useful to increase the R
F
value to decrease the load on the
output. Increasing the R
F
value improves harmonic distortion at
the expense of reducing the 0.1 dB bandwidth of the amplifier.
This effect is discussed further in the Effect of R
F
on 0.1 dB Gain
Flatness section.
RECOMMENDED VALUES
Table 5 and Table 6 provide a quick reference for various configu-
rations and show the effect of gain on the −3 dB small-signal
bandwidth, slew rate, and peaking of the ADA4891-1/ADA4891-2/
ADA4891-3/ADA4891-4. Note that as the gain increases, the
small-signal bandwidth decreases, as is expected from the gain
bandwidth product relationship. In addition, the phase margin
improves with higher gains, and the amplifier becomes more
stable. As a result, the peaking in the frequency response is
reduced (see Figure 7 and Figure 10).
Table 5. Recommended Component Values and Effect of Gain on ADA4891-1/ADA4891-2 Performance (R
L
= 1 kΩ)
Feedback Network Values −3 dB Small-Signal Bandwidth (MHz) Slew Rate (V/µs)
Peaking (dB)
Gain R
F
(Ω) R
G
(Ω) V
OUT
= 200 mV p-p t
R
t
F
−1 604 604 118 188 192 1.3
+1 0 Open 240 154 263 2.6
[phone] 210 1.4
+5 604 151 32.5 149 154 0
+10 604 67.1 12.7 71 72 0