+

-

OUT+

IN-

IN+

OUT-

V+

V

ocm

V-

EN

V

ocm

LMP8350

www.ti.com

SNOSB80C – FEBRUARY 2011 – REVISED OCTOBER 2015

7 Detailed Description

7.1 Overview

The LMP8350 is a fully-differential voltage feedback amplifier designed to drive precision differential ADC

converters. The LMP8350, though fully integrated for ultimate balance and distortion performance, functionally

provides three channels. Two of these channels are the V+ and V− signal path channels, which function similarly

to inverting mode operational amplifiers and are the primary signal paths.

The third channel is the common-mode (VOCM) feedback circuit. This is the circuit that sets the output common

mode as well as driving the V+ and V− outputs to be equal magnitude and opposite phase, even when only one

of the two input channels is driven. The common-mode feedback circuit allows for single-ended to differential

operation. The output common-mode voltage is set by applying the appropriate voltage to the VOCM pin.

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Full Bandwidth Limitations

Although the LMP8350 has a unity gain bandwidth of over 200 MHz, it is primarily intended for lower sample

rate, high-precision ADCs with baseband analog input signal bandwidths in the DC to <1 MHz range (not to be

confused with sampling rate). The high open-loop bandwidth of the LMP8350 is used to provide ultra low

distortion and fast settling times. Maximum power bandwidth is limited by the internal output common-mode

feedback path, which is limited to 1 MHz to 5 MHz. Operation with input signals above 1 MHz with near full

output swings can cause random shifts in the output common mode and possible AC instabilities. For this

reason, the LMP8350 is not intended to be used wide bandwidth (> 1 MHz) signal paths. Single-ended inputs

rely on the common-mode signal path and will have a bandwidth limited to that of the internal common-mode

buffer.

7.3.2 ESD Protection

The LMP8350 is protected against electrostatic discharge (ESD) on all pins. The LMP8350 will survive 2000-V

human body model and 200-V machine model events. Under normal operation, the ESD diodes have no effect

on circuit performance. There are occasions, however, when the ESD diodes will be evident. If the LMP8350 is

driven by a large signal while the device is powered down the ESD diodes will conduct. The current that flows

through the ESD diodes will either exit the chip through the supply pins or will flow through the device, hence it is

possible to power up a chip with a large signal applied to the input pins. Using the shutdown mode is one way to

conserve power and still prevent unexpected operation.

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