Datasheet

Data Sheet ADA4898-1/ADA4898-2
Rev. D | Page 15 of 20
APPLICATIONS INFORMATION
HIGHER FEEDBACK RESISTOR GAIN OPERATION
The ADA4898 schematic for the noninverting gain
configuration shown in Figure 47 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 48 shows the
difference between using a 100 Ω resistor and a 1 kΩ feedback
resistor. Due to the high input capacitance in the ADA4898 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 48
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 47. Noninverting Gain Schematic
–15
–12
–9
–6
–3
0
3
6
9
12
07037-044
FREQUENCY (Hz)
CLOSED-LOOP GAIN (dB)
100k 10M1M 100M
R
F
= 1kΩ, C
F
= 2.7pF
R
F
= 1kΩ
R
F
= 100Ω
G = +2
R
L
= 1kΩ
V
S
= ±15V
Figure 48. 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 Voltage
Noise (nV/√Hz), RTO
Total System Noise
(nV/√Hz), RTO
+1 0 N/A 65 55 0.9 1.29
[phone] 1.8 3.16
+5 100 24.9 9 45 4.5 7.07