MAX40089 datasheet(13/18 Pages) AD

13 / 18 page

Since there is a differential protection scheme used in

these family of op amps, these amplifiers cannot be used

as comparators in open loop, which is often a possibility

on an unused channel of op amp.

Using a Feed-Forward Compensation

Capacitor, CZ

The amplifier’s input capacitance is 7pF and if the

resistance seen by the inverting input is large (

Figure

1) as a result of feedback network, this resistance and

capacitance combination can introduce a pole within the

amplifier’s bandwidth resulting in reduced phase margin.

Compensate the reduced phase margin by introducing

a feed-forward capacitor (CZ) between the inverting

input and the output (shown in

Figure 1). This effectively

cancels the pole from the inverting input of the amplifier.

Choose the value of CZ as follows:

CZ = 10 x (RF/RG) [pF]

Intheunity-gainstable:MAX40079/MAX40077/MAX40078,

the use of correct value CZ is most important for closed

loop non-inverting gain AV = +2V/V, and inverting gain AV

= -1V/V.

In the de-compensated MAX40087/MAX40089, CZ is

most important for closed loop gain AV = +10V/V.

Using a slightly smaller CZ than suggested by the formula

above achieves a higher bandwidth at the expense of

reduced phase and gain margin. As a general guideline,

consider using CZ for cases where RG||RF is greater than

20kΩ (for MAX40079/MAX40077/MAX40078) and greater

than 5kΩ (for MAX40087/MAX40089).

Applications Information

The

MAX40079/MAX40087/MAX40077/MAX40089/

MAX40078 family of op amps combine good driving

capability that can also support ground/low-side sensing

input and rail-to-rail output operation. With their low distor-

tion and low noise, they are ideal for use in ADC buffers,

DAC output buffers, medical instrumentation systems and

other noise-sensitive applications.

However, there are two main application areas where

these ultra-low input bias current op amps find place and

they are to measure high impedance measurements.

High Impedance measurements can be interfacing either

Current output sensors or voltage output sensors that

would need very high output resistance to be interfaced

with. These op amps offer just that as the input imped-

ance of these amplifiers is in the range of

1000GΩ.

Voltage output sensors readout can be accomplished with

unity gain buffer configuration and current output sensors

like photo-diodes current read out can be accomplished

in transimpedance amplifier configuration discussed later

in this data sheet.

Ground-Sensing and Rail-to-Rail Outputs

The common-mode input range of these devices extends

below ground over temperature that offers excellent com-

mon mode rejection and can be used in low side current

sensing applications. These devices are guaranteed not

to undergo phase reversal when the input is overdriven

over input common mode voltage range as shown in

Figure 2.

Figure 3 showcases the true rail-to-rail output operation of

the amplifier, configured with AV = 5V/V. The output swings

to within 8mV of the supplies with a 10kΩ load, making the

devices ideal in low-supply voltage applications.

Figure 1. Adding Feed-Forward Compensation

SHDN =5V

VSS = 0V

VDD = 5V

IN-

IN+

MAX40079/

½MAX40077

VOUT

VIN

www.maximintegrated.com

Maxim Integrated │ 13

MAX40079/MAX40087/

MAX40077/MAX40089/

MAX40078

Single/Dual/Quad Ultra-Low

Input Bias Current,

Low-Noise Amplifiers

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