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LTC6404

6404f

APPLICATIONS INFORMATION

VINCM is deﬁned as the average of the two input voltages

VINP, and VINM (also called the source-referred input com-

mon mode voltage):

INCM

INP

INM

()

•

and VINDIFF is deﬁned as the difference of the input

voltages:

VINDIFF = VINP – VINM

When the feedback ratios mismatch (

Δβ), common mode

to differential conversion occurs.

Setting the differential input to zero (VINDIFF = 0), the de-

gree of common mode to differential conversion is given

by the equation:

OUTDIFF

OUT

INCM

OCM

AVG

≈

()

+ –

–•

–

⏐

NDIFF = 0

In general, the degree of feedback pair mismatch is a

source of common mode to differential conversion of both

signals and noise. Using 1% resistors or better will mitigate

most problems, and will provide about 34dB worst-case of

common mode rejection. Using 0.1% resistors will provide

about 54dB 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. A direct short of

VOCM to this ground or bypassing the VOCM with a high

quality 0.1μF ceramic capacitor to this ground plane, will

further prevent common mode signals from being con-

verted to differential.

There may be concern on how feedback ratio mismatch

affects distortion. Distortion caused by feedback ratio mis-

match using 1% resistors or better is negligible. 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 higher than speciﬁed.

The apparent input referred offset induced by feedback

ratio mismatch is derived from the following equation:

VOSDIFF(APPARENT) ≈ (VICM – VOCM) • Δβ

Using the LTC6404-1 in a single supply application on a

single 5V supply with 1% resistors, and the input common

mode grounded, with the VOCM pin biased at mid-supply,

the worst-case DC offset can induce 25mV of apparent

offset voltage. With 0.1% resistors, the worst case appar-

ent offset reduces to 2.5mV.

Input Impedance and Loading Effects

The input impedance looking into the VINP or VINM input

of Figure 1 depends on whether the sources VINP and

VINM are fully differential. For balanced input sources

(VINP = –VINM), the input impedance seen at either input

is simply:

RINP = RINM = RI

For single ended inputs, because of the signal imbalance

at the input, the input impedance increases over the bal-

anced differential case. The input impedance looking into

either input is:

INP

INM

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

–•

Input signal sources with non-zero output impedances can

also cause feedback imbalance between the pair of feedback

networks. For the best performance, it is recommended

that the source’s output impedance be compensated for.

If input impedance matching is required by the source,

R1 should be chosen (see Figure 4):

INM

1 =

•

–

Figure 4. Optimal Compensation for Signal Source Impedance

–

RINM

R2 = RS || R1

R1 CHOSEN SO THAT R1 || RINM = RS

R2 CHOSEN TO BALANCE R1 || RS

6404 F04

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