LTC6419 datasheet(13/24 Pages) AD

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LTC6419

6419fa

For more information www.linear.com/LTC6419

APPLICATIONS INFORMATION

Assuming infinite open loop gain, the differential output

relationship is given by the equation:

VOUTDIFF = V+OUT – V–OUT ≈ VINDIFF •

VCM •

∆β

AVG

– VOCM •

∆β

AVG

where RF is the average of RF1, and RF2, and RI is the

average of RI1, and RI2.

VOUTDIFF = V+OUT – V–OUT ≈(VCM – VOCM) •

∆β

AVG

(3)

In general, the degree of feedback pair mismatch is a

source of common mode to differential conversion of

both signals and noise. Using 0.1% resistors or better

will mitigate most problems and will provide about 54dB

worst case of common mode rejection. A low impedance

ground plane should be used as a reference for both the

input signal source and the VOCM pin.

There may be concern on how feedback factor mismatch

affects distortion. Feedback factor mismatch from using

1% resistors or better, has a negligible effect on distortion.

However, in single supply level shifting applications where

there is a voltage difference between the input common

mode voltage and the output common mode voltage,

resistor mismatch can make the apparent voltage offset

of the amplifier appear worse than specified.

The apparent input referred offset induced by feedback

factor mismatch is derived from Equation (3):

VOSDIFF(APPARENT) ≈ (VCM – VOCM) • Δβ

Using the LTC6419 in a single 5V supply application with

0.1% resistors, the input common mode grounded, and

the VOCM pin biased at 1.25V, the worst case mismatch

can induce 1.25mV of apparent offset voltage.

Noise and Noise Figure

The LTC6419’s differential input referred voltage and cur-

rent noise densities are 1.1nV/√Hz and 8.8pA/√Hz, respec-

tively. In addition to the noise generated by the amplifier,

the surrounding feedback resistors also contribute noise.

A simplified noise model is shown in Figure 4. The output

noise generated by both the amplifier and the feedback

components is given by the equation:

eno =

eni • 1+





















+ 2 • i

n • RF

(

)2 +

2 • enRI •









+ 2 • e

nRF

βAVG is defined as the average feedback factor from the

outputs to their respective inputs:

AVG =

•

RI1

RI1 +RF1

RI2

RI2 +RF2









β is defined as the difference in the feedback factors:

∆β =

RI2

RI2 +RF2

–

RI1

RI1 +RF1

Here, VCM and VINDIFF are defined as the average and

the difference of the two input voltages VINP and VINM,

respectively:

VCM =

VINP + VINM

VINDIFF = VINP – VINM

When the feedback ratios mismatch (Δ

β), common mode

to differential conversion occurs. Setting the differential

input to zero (VINDIFF = 0), the degree of common mode

to differential conversion is given by the equation:

Figure 3. Real-World Application with Feedback

Resistor Pair Mismatch

–

RF2

V–OUT

V+OUT

VVOCM

VOCM

6419 F03

RF1

RI2

RI1

–

VINP

–

VINM

V–IN

V+IN

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