Single/Dual/Quad, Low-Power, Single-Supply,

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

10

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_______________Detailed Description

Rail-to-Rail Input Stage

The MAX4330–MAX4334 have rail-to-rail input and out-

put stages that are specifically designed for low-

voltage, single-supply operation. The input stage con-

sists of separate NPN and PNP differential stages,

which operate together to provide a common-mode

range extending to 0.25V beyond both supply rails. The

caused by mismatched input pairs. The input offset volt-

age is typically 250µV. Low offset voltage, high band-

width, rail-to-rail common-mode input range, and

rail-to-rail outputs make this family of op amps an excel-

lent choice for precision, low-voltage data-acquisition

systems.

Since the input stage consists of NPN and PNP pairs,

the input bias current changes polarity as the input volt-

age passes through the crossover region. Match the

effective impedance seen by each input to reduce the

offset error due to input bias currents flowing through

external source impedances (Figures 1a and 1b). The

combination of high source impedance with input

capacitance (amplifier input capacitance plus stray

capacitance) creates a parasitic pole that produces an

underdamped signal response. Reducing input capaci-

tance or placing a small capacitor across the feedback

resistor improves response.

The MAX4330–MAX4334’s inputs are protected from

large differential input voltages by internal 1k

Ω series

resistors and back-to-back triple diode stacks across

the inputs (Figure 2). For differential input voltages

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

2.3M

Ω. For differential input voltages greater than 1.8V,

input resistance is around 2k

Ω, and the input bias cur-

rent can be approximated by the following equation:

Ω

In the region where the differential input voltage

approaches 1.8V, input resistance decreases exponen-

tially from 2.3M

Ω to 2kΩ as the diode block begins con-

ducting. Inversely, the bias current increases with the

same curve.

R3

R3 = R1

R2

R1

R2

MAX4330

MAX4331

MAX4332

MAX4333

MAX4334

Figure 1a. Reducing Offset Error Due to Bias Current

(Noninverting)

R3

R3 = R1

R2

R1

R2

MAX4330

MAX4331

MAX4332

MAX4333

MAX4334

Figure 1b. Reducing Offset Error Due to Bias Current

(Inverting)