MAX4256 Maxim, MAX4256 Datasheet - Page 11

• Manufacturer Part Number: MAX4256
• Description: The MAX4249–MAX4257 low-noise, low-distortion operational amplifiers offer rail-to-rail outputs and single-supply operation down to 2
• Manufacturer: Maxim

Figure 3. Overdriven Input Showing No Phase Reversal
Figure 4. Rail-to-Rail Output Operation
Figure 5. Capacitive-Load Driving Circuit
4.45V
V
4.25V
IN
0
0
5V
0
V
R
A
f = 1kHz
DD
L
V
= 10
= 10kΩ
= 5V
______________________________________________________________________________________
200µs/div
20µs/div
MAX4250
MAX4251
MAX4252
MAX4253
MAX4254
Low-Distortion, Rail-to-Rail Op Amps
R
ISO
A
V
R
V
DD
L
= 1
= 10kΩ
UCSP, Single-Supply, Low-Noise,
= 5V
-200mV
V
V
V
1V/div
OUT
IN
C
OUT
L
V
OUT
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
input and the output (Figure 1). This effectively cancels
the pole from the inverting input of the amplifier.
Choose the value of C
In the unity-gain stable MAX4250–MAX4254, the use of
a proper C
A
MAX4255/MAX4256/MAX4257, C
for A
response both with and without C
Using a slightly smaller C
mula above achieves a higher bandwidth at the
expense of reduced phase and gain margin. As a gen-
eral guideline, consider using C
R
greater than 5kΩ (MAX4249/MAX4255/MAX4256/
MAX4257).
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.
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
swings to within 8mV of the supplies with a 10kΩ load,
making the devices ideal in low-supply-voltage applica-
tions.
Even with their low quiescent current of 400µA, these
amplifiers can drive 1kΩ loads while maintaining excel-
lent DC accuracy. Stability while driving heavy capaci-
tive loads is another key feature.
V
F
Ground-Sensing and Rail-to-Rail Outputs
is greater than 20kΩ (MAX4250–MAX4254) or
= -1V/V. In the decompensated MAX4249/
V
= 10V/V. Figures 2a and 2b show transient
Using a Feed-Forward Compensation
Z
is most important for A
Applications Information
C
Z
Output Loading and Stability
= 11 x (R
Z
as follows:
Z
than suggested by the for-
F
Z
/ R
) between the inverting
Z
G
Z
V
for cases where R
) [pF]
Z
.
= 10V/V. The output
is most important
Capacitor, C
V
= 2V/V, and
G
11
Z
||