4 0 0 MHz, Ult ra -Lo w -Dis t o rt io n Op Am p s
18/MAX4309
Inverting and Noninverting Configurations
_____________________P in De s c rip t io n
The values of the gain-setting feedback and input resis-
tors are important design considerations. Large resistor
values will increase voltage noise, and will interact with
the amplifier’s input and PC board capacitance to gen-
erate undesirable poles and zeros, which can decrease
bandwidth or cause oscillations. For example, a nonin-
verting gain of +2, using 1kΩ resistors combined with
2pF of input capacitance and 0.5pF of board capaci-
tance, will cause a feedback pole at 128MHz. If this
pole is within the anticipated amplifier bandwidth, it will
jeopardize stability. Reducing these 1kΩ resistors to
100Ω will extend the pole frequency to 1.28GHz, but
could limit output swing by adding 200Ω in parallel with
the amplifier’s load. Clearly, the selection of resistor val-
ues must be tailored to the specific application.
PIN
1
NAME
N.C.
IN-
FUNCTION
No Connection. Not internally connected.
Inverting Input
2
3
IN+
Noninverting Input
4, 5
6
V
EE
Negative Power Supply, connect to -5V
.
DC
OUT
Amplifier Output
Positive Power Supply, connect to +5V
7, 8
V
CC
.
DC
_______________De t a ile d De s c rip t io n
Ch o o s in g Re s is t o r Va lu e s
Unity-Gain Configuration
The MAX4108 is internally compensated for unity gain.
When configured for unity gain, the device requires a
small resistor in series with the feedback path. This
resistor improves the AC response by reducing the Q
of the tank circuit, which is formed by parasitic feed-
back inductance and capacitance.
The MAX4108/MAX4109/MAX4308/MAX4309 are ultra-
low-distortion, high-bandwidth op amps. The output
distortion will be degraded as the total load resistance
seen by the amplifier decreases. To minimize distortion
products, keep the input and gain-setting resistors rela-
tively large. A 500Ω feedback resistor combined with
an appropriate input resistor to set the gain will provide
excellent AC performance without significantly increas-
ing distortion.
R
G
R
F
25
MAX4108
20
MAX4308
15
R
S
V
IN
R
L
C
L
MAX4109/MAX4309
10
5
0
PART
R (Ω)
R (Ω) GAIN (V/V)
G
F
MAX4108
MAX4109
MAX4308
MAX4309
24
∞
1
2
500
500
500
500
125
56
10 40
70
100 130 160 190 220
5
CAPACITANCE (pF)
10
Figure 1a. Using an Isolation Resistor for High Capacitive
Loads
Figure 1b. Optimal Isolation Resistor (R ) vs. Capacitive Load
S
_______________________________________________________________________________________
9