much less than 2.1V (triple-diode drop), input resistance is
typically 4MΩ. For differential voltages greater than 2.1V,
input resistance is around 10.6kΩ, and the input bias cur-
rent can be approximated by the following equation:
IB = (VDIFF - 2.1V)/10.6kΩ
In the region where the differential input voltage approach-
es 2.1V, the input resistance decreases exponentially
from 4MΩ to 10.6kΩ as the diodes begin to conduct. It fol-
lows that the bias current increases with the same curve.
In unity-gain configuration, high slew-rate input signals
may capacitively couple to the output through the triple-
diode stacks.
Rail-to-Rail Output Stage
The MAX4245/MAX4246/MAX4247 can drive a 2kΩ load
and still typically swing within 35mV of the supply rails.
Figure 3 shows the output voltage swing of the MAX4245
configured with AV = -1V/V.
Applications Information
Power-Supply Considerations
The MAX4245/MAX4246/MAX4247 operate from a single
+2.5V to +5.5V supply (or dual ±1.25V to ±2.75V sup-
plies) and consume only 320µA of supply current per
amplifier. A 90dB power-supply rejection ratio allows the
amplifiers to be powered directly off a decaying battery
voltage, simplifying design and extending battery life.
Power-Up
The MAX4245/MAX4246/MAX4247 output typically set-
tles within 4µs after power-up. Figure 4 shows the output
voltage on power-up and power-down.
Shutdown Mode
The MAX4245/MAX4247 feature a low-power shutdown
mode. When SHDN_ is pulled low, the supply current
drops to 50nA per amplifier, the amplifier is disabled, and
the output enters a high-impedance state. Pulling SHDN_
high enables the amplifier. Figure 5 shows the MAX4245/
MAX4247’s shutdown waveform.
Due to the output leakage currents of three-state devices
and the small internal pullup current for SHDN_, do not
leave SHDN_ open/high-impedance. Leaving SHDN_
open may result in indeterminate logic levels, and could
adversely affect op amp operation. The logic threshold for
SHDN_ is referred to VSS. When using dual supplies, pull
SHDN_ to VSS, not GND, to shut down the op amp.
Driving Capacitive Loads
The MAX4245/MAX4246/MAX4247 are unity-gain stable
for loads up to 470pF. Applications that require greater
capacitive drive capability should use an isolation resistor
Figure 2. Input Protection Circuit
Figure 3. Rail-to-Rail Input/Output Voltage Range
Figure 4. Power-Up/Power-Down Waveform
IN-
5.3kΩ
IN+
5.3kΩOUT
IN 2V/div
2V/div
400µs/div
OUT
VDD
2V/div
2V/div
10µs/div
MAX4245/MAX4246/
MAX4247
Ultra-Small, Rail-to-Rail I/O with Disable,
Single-/Dual-Supply, Low-Power Op Amps
www.maximintegrated.com Maxim Integrated
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