
Ground-Sensing and Rail-to-Rail Outputs
The common-mode input range of these devices extends
below ground, and offers excellent common-mode rejec-
tion. 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 AV = 5V/V. The output
swings to within 8mV of the supplies with a 10kΩ load,
making the devices ideal in low-supply voltage applica-
tions.
Power Supplies and Layout
The MAX4475–MAX4478/MAX4488/MAX4489 operate
from a single +2.7V to +5.5V power supply or from dual
supplies of ±1.35V to ±2.75V. For single-supply opera-
tion, bypass the power supply with a 0.1µF ceramic
Low Noise
The amplifier’s input-referred noise-voltage density is
dominated by flicker noise at lower frequencies, and by
thermal noise at higher frequencies. Because the thermal
noise contribution is affected by the parallel combination
of the feedback resistive network (RF || RG, Figure 1),
these resistors should be reduced in cases where the
system bandwidth is large and thermal noise is dominant.
This noise contribution factor decreases, however, with
increasing gain settings.
For example, the input noise-voltage density of the
circuit with RF = 100kΩ, RG = 11kΩ (AV = +5V/V) is
en = 14nV/√Hz, en can be reduced to 6nV/√Hz by choos-
ing RF = 10kΩ, RG = 1.1kΩ (AV = +5V/V), at the expense
of greater current consumption and potentially higher
distortion. For a gain of 100V/V with RF = 100kΩ, RG =
1.1kΩ, the en is still a low 6nV/√Hz.
Using a Feed-Forward Compensation
Capacitor, CZ
The amplifier’s input capacitance is 10pF. If the resistance
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 (CZ) between the inverting input and the out-
put (Figure 1). This effectively cancels the pole from the
inverting input of the amplifier. Choose the value of CZ
as follows:
CZ = 10 x (RF / RG) [pF]
In the unity-gain stable MAX4475–MAX4478, the use
of a proper CZ is most important for AV = +2V/V, and
AV = -1V/V. In the decompensated MAX4488/MAX4489,
CZ is most important for AV = +10V/V. Figures 2a and 2b
show transient response both with and without CZ.
Using a slightly smaller CZ than suggested by the formula
above achieves a higher bandwidth at the expense of
reduced phase and gain margin. As a general guideline,
consider using CZ for cases where RG || RF is greater
than 20kΩ (MAX4475–MAX4478) or greater than 5kΩ
(MAX4488/MAX4489).
Applications Information
The MAX4475–MAX4478/MAX4488/MAX4489 combine
good driving capability with ground-sensing input and
rail-to-rail output operation. With their low distortion and
low noise, they are ideal for use in ADC buffers, medical
instrumentation systems and other noise-sensitive appli-
cations.
Figure 4. Rail-to-Rail Output Operation
Figure 3. Overdriven Input Showing No Phase Reversal
VOUT
1V/div
5V
0V
20ms/div
VOUT
2V/div
VIN
2V/div
0V
AV = +1
VDD = +5V
RL = 10kΩ
40µs/div
www.maximintegrated.com Maxim Integrated
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13
MAX4475–MAX4478/
MAX4488/MAX4489
SOT23, Low-Noise, Low-Distortion,
Wide-Band, Rail-to-Rail Op Amps