Applications information – Rainbow Electronics MAX4249_MAX4257 User Manual
Page 11
MAX4249–MAX4257
UCSP, Single-Supply, Low-Noise,
Low-Distortion, Rail-to-Rail Op Amps
______________________________________________________________________________________
11
Using a Feed-Forward Compensation
Capacitor, C
Z
The amplifier’s input capacitance is 11pF. If the resis-
tance seen by the inverting input is large (feedback
network), this can introduce a pole within the amplifier’s
bandwidth, resulting in reduced phase margin.
Compensate the reduced phase margin by introducing
a feed-forward capacitor (C
Z
) between the inverting
input and the output (Figure 1). This effectively cancels
the pole from the inverting input of the amplifier.
Choose the value of C
Z
as follows:
C
Z
= 11 x (R
F
/ R
G
) [pF]
In the unity-gain stable MAX4250–MAX4254, the use of
a proper C
Z
is most important for A
V
= 2V/V, and A
V
=
-1V/V. In the decompensated MAX4249/MAX4255
/MAX4256/MAX4257, C
Z
is most important for A
V
=
10V/V. Figures 2a and 2b show transient response both
with and without C
Z
.
Using a slightly smaller C
Z
than suggested by the for-
mula above achieves a higher bandwidth at the
expense of reduced phase and gain margin. As a gen-
eral guideline, consider using C
Z
for cases where R
G
||
R
F
is greater than 20k
Ω (MAX4250–MAX4254) or
greater than 5k
Ω (MAX4249/MAX4255/MAX4256/
MAX4257).
Applications Information
The MAX4249–MAX4257 combine good driving capa-
bility with ground-sensing input and rail-to-rail output
operation. With their low distortion, low noise and low-
power consumption, these devices are ideal for use in
portable instrumentation systems and other low-power,
noise-sensitive applications.
Ground-Sensing and Rail-to-Rail Outputs
The common-mode input range of these devices
extends below ground, and offers excellent common-
mode rejection. These devices are guaranteed not to
undergo phase reversal when the input is overdriven
(Figure 3).
Figure 4 showcases the true rail-to-rail output operation
of the amplifier, configured with A
V
= 10V/V. The output
swings to within 8mV of the supplies with a 10k
Ω load,
making the devices ideal in low-supply-voltage applica-
tions.
Output Loading and Stability
Even with their low quiescent current of 400µA, these
amplifiers can drive 1k
Ω loads while maintaining excel-
lent DC accuracy. Stability while driving heavy capaci-
tive loads is another key feature.
V
OUT
V
IN
R
ISO
C
L
MAX4250
MAX4251
MAX4252
MAX4253
MAX4254
4.25V
4.45V
-200mV
0
0
V
OUT
V
IN
A
V
= 1
V
DD
= 5V
R
L
= 10k
Ω
20
µs/div
0
5V
V
DD
= 5V
R
L
= 10k
Ω
A
V
= 10
f = 1kHz
200
µs/div
V
OUT
1V/div
Figure 3. Overdriven Input Showing No Phase Reversal
Figure 4. Rail-to-Rail Output Operation
Figure 5. Capacitive-Load Driving Circuit