Data Sheet ADA4898-1/ADA4898-2
APPLICATIONS INFORMATION
HIGHER FEEDBACK RESISTOR GAIN OPERATION
The ADA4898-1/ADA4898-2 schematic for the noninverting
gain configuration shown in Figure 45 is nearly a textbook
example. The only exception is the feedback capacitor in
parallel with the feedback resistor, R
F
, but this capacitor is
recommended only when using a large R
F
value (>300 Ω).
Figure 46 shows the difference between using a 100 Ω resistor
and a 1 kΩ feedback resistor. Due to the high input capacitance in
the ADA4898-1/ADA4898-2 when using a higher feedback
resistor, more peaking appears in the closed-loop gain. Using
the lower feedback resistor resolves this issue; however, when
running at higher supplies (±15 V) with an R
F
of 100 Ω, the
system draws a lot of extra current into the feedback network.
To avoid this problem, a higher feedback resistor can be used
with a feedback capacitor in parallel. Figure 46 shows the effect of
placing a feedback capacitor in parallel with a larger R
F
. In this
gain-of-2 configuration, R
F
= R
G
= 1 kΩ and C
F
= 2.7 pF. When
using C
F
, the peaking drops from 6 dB to less than 2 dB.
07037-043
V
IN
V
OUT
10µF
+V
S
–V
S
R
T
R
L
+
10µF
+
C
F
R
F
R
F
0.1µF
0.1µF
Figure 45. Noninverting Gain Schematic
–15
–12
–9
–6
–3
0
3
6
9
12
07037-044
FREQUENCY (Hz)
CLOSED-LOOP GAIN (dB)
100k
10M
1M
100M
R
F
= 1kΩ, C
F
= 2.7pF
R
F
= 1kΩ
R
F
= 100Ω
G = +2
R
L
= 1kΩ
V
S
= ±15V
Figure 46. Small Signal Frequency Response for
Various Feedback Impedances
RECOMMENDED VALUES FOR VARIOUS GAINS
Table 8 provides a useful reference for determining various gains
and associated performance. R
F
is set to 100 Ω for gains greater
than 1. A low feedback R
F
resistor value reduces peaking and
minimizes the contribution to the overall noise performance
of the amplifier.
Table 8. Gains and Recommended Resistor Values Associated with Them (Conditions: V
S
= ±5 V, T
A
= 25°C, R
L
= 1 kΩ, R
T
= 49.9 Ω)
Gain R
F
(Ω) R
G
(Ω)
−3 dB SS BW (MHz),
V
OUT
= 100 mV p-p
Slew Rate (V/µs),
V
OUT
= 2 V Step
ADA4898-1/ADA4898-2
Voltage Noise (nV/√Hz), RTO
Total System Noise
(nV/√Hz), RTO
+1 0 Not applicable 65 55 0.9 1.29
+2 100 100 30 50 1.8 3.16
+5 100 24.9 9 45 4.5 7.07
Rev. E | Page 15 of 20
