LTC6362 datasheet(24/42 Pages) AD

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LTC6363 Family

Rev. C

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APPLICATIONS INFORMATION

Functional Description

The LTC6363 family consists of four fully differential, low

power, low noise, precision amplifiers. The LTC6363 is an

unconstrained, fully differential amplifier, typically used

withfourexternalresistors.TheLTC6363-0.5,LTC6363-1,

and LTC6363-2 (gains of 0.5, 1, and 2 respectively) are

fully-differentialfixedgainblocksfeaturingprecision, laser

trimmed, matched internal resistors for accurate, stable

gain and excellent CMRR. The entire LT6363 family is

optimized to convert a fully differential or single-ended

signal to a low impedance, balanced differential output

suitable for driving high performance, low power differ-

ential sigma-delta or SAR ADCs. The balanced differential

nature of the amplifier also provides even-order harmonic

distortion cancellation, and low susceptibility to common

mode noise (e.g. power supply noise).

The outputs of the LTC6363 family are capable of swing-

ing rail-to-rail and can source up to 90mA or sink up to

40mA of current. The LTC6363 family is optimized for high

bandwidthandlowpowerapplications.Loadcapacitances

above 50pF to ground or 25pF differentially should be

decoupled with 10Ω to 50Ω of series resistance from

each output to prevent oscillation or ringing.

SHDN Pin

TheLTC6363familyhasaSHDNpinwhich,whentiedtoV–or

driven to below VIL, will shut down amplifier operation such

that only 20µA (at VS = 3V) to 70µA (at VS = 10V) is drawn

from the supplies. Pull-down circuitry should be capable of

sinking at least 12µA to guarantee complete shutdown over

all conditions. For normal amplifier operation, the SHDN pin

should be left floating or tied to V+ or driven to above VIH.

General Amplifier Applications

In Figure 1, the gain to VOUTDIFF from VINP and VINM is

given by:

VOUTDIFF = V+OUT − V–OUT ≈

⎛

⎝

⎜

⎜⎜

⎞

⎠

⎟

⎟⎟ • VINP – VINM

(

)

Note from the previous equation, the differential output

voltage(V+OUT–V–OUT)isindependentofinputandoutput

common mode voltages, or the voltage at the common

mode pin. This makes the LTC6363 family ideally suited

Figure 1. Definitions and Terminology

–

V–OUT

V+

V–

V+OUT

VOCM

VINP

VINM

VOCM

6363 F01

V–IN

V+IN

+–

VCM +

–

= RF

for pre-amplification, pre-attenuation, level shifting and

conversion of single-ended signals to differential output

signals for driving differential input ADCs or other devices.

Output Common Mode and VOCM Pin

The output common mode voltage is defined as the aver-

age of the two outputs:

VOUTCM =

V+OUT + V–OUT

⎛

⎝

⎜

⎞

⎠

⎟

⎟ = VOCM

As the equation shows, the output common mode voltage

is independent of the input common mode voltage, and

is instead determined by the voltage on the VOCM pin, by

means of an internal common mode feedback loop.

The VOCM input connects to the base of a PNP transistor

and an internal resistor divider network. If the VOCM pin

is left open, the resistor divider creates a default voltage

approximately halfway between V+ and V–. The VOCM pin

can be overdriven to another voltage if desired for greater

accuracy or flexibility. For example, when driving an ADC,

if the ADC makes a reference available for setting the

common mode voltage, it can be directly tied to the VOCM

pin, as long as the ADC is capable of driving the 1.8M

input resistance presented by the VOCM pin. The Electrical

Characteristics table specifies the valid range that can be

applied to the VOCM pin (VOUTCMR).

Input Common Mode Voltage Range

For all versions of the LTC6363, the input common mode

voltage range, VICMR, specification refers to the voltage at

the input pins of the part. The input common mode voltage

range of the LTC6363-0.5, LTC6363-1 and LTC6363-2 are

extended beyond that of the LTC6363 due to the resistor

divider action of the on-chip resistors.

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