Datasheet

ADA4899-1
Rev. B | Page 14 of 20
APPLICATIONS
UNITY-GAIN OPERATION
The ADA4899-1 schematic for unity-gain configuration is
nearly a textbook example (see
Figure 46). The only exception is
the small 24.9 Ω series resistor at the noninverting input. The
series resistor is only required in unity-gain configurations;
higher gains negate the need for the resistor. In
Table 4, it can be
seen that the overall noise contribution of the amplifier and the
24.9 Ω resistor is equivalent to the noise of a single 87 Ω resistor.
Figure 47 shows the small signal frequency response for the
unity-gain amplifier shown in
Figure 46.
05720-037
V
OUT
+
V
S
–V
S
0.1µF
24.9Ω
V
IN
0.1µF
Figure 46. Unity-Gain Schematic
05720-063
10000110010 1000
–12
–9
–6
–3
0
3
G = +1
R
L
= 100Ω
CLOSED-LOOP GAIN (dB)
FREQUENCY (MHz)
25mV p-p
50mV p-p
200mV p-p
100mV p-p
Figure 47. Small Signal Frequency Response for Various Output Voltages
RECOMMENDED VALUES FOR VARIOUS GAINS
Table 4 provides a handy reference for determining various
gains and associated performance. For noise gains greater than
one, the Series Resistor R
S
is not required. Resistors R
F
and R
G
are kept low to minimize their contribution to the overall noise
performance of the amplifier.
Table 4. Conditions: V
S
= ±5 V, T
A
= 25°C, R
L
= 1 kΩ
Gain R
F
(Ω) R
G
(Ω) R
S
(Ω)
−3 dB SS BW (MHz)
(25 mV p-p)
Slew Rate (V/μs)
(2 V Step)
ADA4899-1 Voltage
Noise (nV/√Hz)
Total Voltage
Noise (nV/√Hz)
+1 0 NA 24.9 605 274 1 1.2
−1 100 100 0 294 265 2 2.7
[phone] 2.7
+5 200 49.9 0 77 227 5 6.5
+10 453 49.9 0 37 161 10 13.3