Detailed Description

Input Protection Circuit

The LMX321/LMX358/LMX324’s inputs are protected

from large differential input voltages by internal 3.5k

Ω

series resistors and back-to-back triple diode stacks

across the inputs (Figure 1). For differential input volt-

ages (much less than 1.8V), input resistance is typically

3M

Ω. For differential input voltages greater than 1.8V,

input resistance is around 7k

Ω, and the input bias cur-

rent can be approximated by the following equation:

In the region where the differential input voltage

approaches 1.8V, input resistance decreases exponen-

tially from 3M

Ω to 7kΩ as the diode block begins con-

ducting. Inversely, the bias current increases with the

same curve.

Rail-to-Rail Output Stage

The LMX321/LMX358/LMX324 drive 2k

Ω loads and still

typically swing within 40mV of the supply rails. Figure 2

shows the output voltage swing of the LMX321 config-

Driving Capacitive Loads

Driving a capacitive load can cause instability in many

op amps, especially those with low quiescent current.

The LMX321/LMX358/LMX324 are unity-gain stable for

a range of capacitive loads to above 400pF. Figure 4

shows the response of the LMX321 with an excessive

capacitive load. Adding a series resistor between the

output and the load capacitor (Figure 5) improves the

circuit’s response by isolating the load capacitance

from the op amp’s output.

Applications Information

Power-Up

The LMX321/LMX358/LMX324 outputs typically settle

within 10µs after power-up. Figure 6 shows the output

voltage on power-up and power-down.

Single/Dual/Quad, General-Purpose,

Low-Voltage, Rail-to-Rail Output Op Amps

_______________________________________________________________________________________

9

3.5k

Ω

3.5k

Ω

Figure 1. Input Protection Circuit

200

µs/div

2.5V

-2.5V

1V/div

2V/div

Figure 2. Rail-to-Rail Output Swing

LMX321

LMX358

LMX324

Figure 3. Capacitive-Load-Driving Circuit

4

µs/div

2.6V

2.4V

2.6V

2.4V

100mV/div

100mV/div

Figure 4. Output With Excessive Capacitive Load