SOT23, 3V/5V, Single-Supply, Rail-to-Rail
Instrumentation Amplifiers
Power-Supply Bypass and Layout
Applications Information
Good layout technique optimizes performance by
decreasing the amount of stray capacitance at the
instrumentation amplifier’s gain-setting pins. Excess
capacitance produces peaking in the amplifier’s fre-
quency response. To decrease stray capacitance, min-
imize trace lengths by placing external components as
close to the instrumentation amplifier as possible. For
best performance, bypass each power supply to
ground with a separate 0.1µF capacitor.
Setting the Gain (MAX4460)
The MAX4460 gain is set by connecting a resistive-
divider from OUT to GND, with the center tap connect-
ed to FB (Figure 2). The gain is calculated by:
Gain = 1 + R2 / R1
Because FB has less than 100pA IB, high-valued resis-
tors can be used without significantly affecting the gain
accuracy. The sum of resistors (R1 + R2) near 100kΩ is
a good compromise. Resistor accuracy directly affects
gain accuracy. Resistor sum less than 20kΩ should not
be used because their loading can slightly affect output
accuracy.
Microphone Amplifier
The MAX4462’s bipolar output, along with its excellent
common-mode rejection ratio, makes it suitable for pre-
cision microphone amplifier applications. Figure 7 illus-
trates one such circuit. In this case, the electret
microphone is resistively biased to the supply voltage
through a 2.2kΩ pullup resistor. The MAX4462 directly
senses the output voltage at its noninverting input, and
indirectly senses the microphone’s ground through an
AC-coupling capacitor. This technique provides excel-
lent rejection of common-mode noise picked up by the
microphone lead wires. Furthermore, ground noise from
distantly located microphones is reduced.
Capacitive-Load Stability
The MAX4460/MAX4461/MAX4462 are capable of dri-
ving capacitive loads up to 100pF.
Applications needing higher capacitive drive capability
may use an isolation resistor between OUT and the
load to reduce ringing on the output signal. However
this reduces the gain accuracy due to the voltage drop
across the isolation resistor.
The single-ended output of the MAX4462 is converted to
differential through a single op amp, the MAX4335. The
op amp forces the midpoint between OUT+ and OUT- to
be equal to the reference voltage. The configuration
does not change the MAX4662T’s fixed gain of 10.
Output Loading
For best performance, the output loading should be to
the potential seen at REF for the MAX4462 or to ground
for the MAX4460/MAX4461.
REF Input (MAX4462)
The REF input of the MAX4462 can be connected to any
voltage from (V + 0.1V) to (V
- 1.7V). A buffered
DD
SS
voltage-divider with sink and source capability works
well to center the output swing at VDD/2. Unbuffered
resistive dividers should be avoided because the 100kΩ
(typ) input impedance of REF causes amplitude-depen-
dent variations in the divider’s output.
Bandgap references, either series or shunt, can be
used to drive REF. This provides a voltage and temper-
ature invariant reference. This same reference voltage
can be used to bias bridge sensors to eliminate supply
voltage ratiometricity. For proper operation, the refer-
ence must be able to sink and source at least 25µA.
In many applications, the MAX4462 is connected to a
CODEC or other device with a reference voltage out-
put. In this case, the receiving device’s reference out-
put makes an ideal reference voltage. Verify the
reference output of the device is capable of driving the
MAX4462’s REF input.
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
