Applications Information

Rail-to-Rail Input Stage

Devices in the MAX4122–MAX4129 family of high-

speed amplifiers have rail-to-rail input and output

stages designed for low-voltage single-supply opera-

tion. The input stage consists of separate NPN and

PNP differential stages, which combine to provide an

input common-mode range extending 0.2V beyond the

supply rails. The PNP stage is active for input voltages

close to the negative rail, and the NPN stage is active

for input voltages near the positive rail. The input offset

voltage is typically below 200µV. The switchover transi-

extended to minimize the slight degradation in CMRR

caused by the mismatch of the input pairs. Their low

offset voltage, high bandwidth, and rail-to-rail common-

mode range make these op amps excellent choices for

precision low-voltage data-acquisition systems.

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.

To reduce the offset error caused by input bias cur-

rents flowing through external source impedances,

match the effective impedance seen by each input

(Figures 1a, 1b). High source impedances, together

with the input capacitance, 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 the response.

The MAX4122–MAX4129’s inputs are protected from

large differential input voltages by 1k

Ω series resistors

and back-to-back triple diodes across the inputs

(Figure 2). For differential input voltages less than 1.8V

the input resistance is typically 500k

Ω. For differential

input voltages greater than 1.8V the input resistance is

approximately 2k

Ω, and the input bias current is

determined by the following equation:

Rail-to-Rail Output Stage

The minimum output voltage will be within millivolts of

ground for single-supply operation where the load is

voltage range and output voltage swing of a MAX4123

connected as a voltage follower. With a +3V supply

and the load tied to ground, the output swings from

0.00V to 2.90V. The maximum output voltage swing

depends on the load, but will be within 150mV of a +3V

supply, even with the maximum load (500

Ω to ground).

Driving a capacitive load can cause instability in most

high-speed op amps, especially those with low quiescent

current. The MAX4122–MAX4129 have a high tolerance

for capacitive loads. They are stable with capacitive

loads up to 500pF. Figure 4 gives the stable operating

region for capacitive loads. Figures 5–8 show the

response with capacitive loads, and the results of adding

an isolation resistor in series with the output (Figure 9).

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

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

I

=

V

- 1.8V

2k

BIAS

DIFF

Ω

Single/Dual/Quad, Wide-Bandwidth, Low-Power,

Single-Supply Rail-to-Rail I/O Op Amps

______________________________________________________________________________________

11

IN

TIME (2

µs/div)

OUT

Figure 3. Rail-to-Rail Input /Output Voltage Range

10,000

100

100

1k

100k

10k

RESISTIVE LOAD (

Ω)

1000

UNSTABLE

REGION

Figure 4. Capacitive-Load Stability