High-Output-Drive, Precision, Low-Power, Single-

Supply, Rail-to-Rail I/O Op Amps with Shutdown

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11

Since the input stage switches between the NPN and

PNP pairs, the input bias current changes polarity as the

input voltage passes through the transition region. Match

the effective impedance seen by each input to reduce the

offset error caused by input bias currents flowing through

external source impedances (Figures 4 and 5).

High source impedances, together with input capaci-

tance, can create a parasitic pole that produces an

underdamped signal response. Reducing the input

impedance or placing a small (2pF to 10pF) capacitor

across the feedback resistor improves response.

The MAX4165–MAX4169’s inputs are protected from large

differential input voltages by 1k

Ω series resistors and

back-to-back triple diodes across the inputs (Figure 6).

For differential voltages less than 1.8V, input resistance is

typically 500k

Ω. For differential input voltages greater

than 1.8V, input resistance is approximately 2k

Ω. The

input bias current is given by the following equation:

Ω

Rail-to-Rail Output Stage

The minimum output is within millivolts of ground for

single-supply operation, where the load is referenced

and the output voltage swing of a MAX4165 connected

as a voltage follower. The maximum output voltage

swing is load dependent; however, it is guaranteed to

with maximum load (25

Ω to ground).

Driving Capacitive Loads

The MAX4165–MAX4169 have a high tolerance for

capacitive loads. They are stable with capacitive loads

up to 250pF. Figure 8 is a graph of the stable operating

region for various capacitive loads vs. resistive loads.

Figures 9 and 10 show the transient response with

excessive capacitive loads (1500pF), with and without

the addition of an isolation resistor in series with the

output. Figure 11 shows a typical noninverting capaci-

tive-load-driving circuit in the unity-gain configuration.

The resistor improves the circuit’s phase margin by iso-

lating the load capacitor from the op amp’s output.

1k

1k

Figure 6. Input Protection Circuit

IN

(1V/div)

OUT

(1V/div)

TIME (5

µs/div)

Figure 7. Rail-to-Rail Input/Output Range