LTC6409 datasheet(11/24 Pages) AD

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LTC6409

6409fb

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APPLICATIONS INFORMATION

by the source, a termination resistor RT should be chosen

(see Figure 2) such that:

RT =

RINM •RS

RINM – RS

According to Figure 2, the input impedance looking into

the differential amp (RINM) reflects the single-ended

source case, given above. Also, R2 is chosen as:

R2 = RT ||RS =

RT •RS

RT +RS

Figure 2. Optimal Compensation for Signal Source Impedance

Δb is defined as the difference in the feedback factors:

∆b =

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 (Δb), 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:

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

∆b

bAVG

(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 distor-

tion. However, in single supply level shifting applications

where there is a voltage difference between the input com-

mon mode voltage and the output common mode voltage,

–

RINM

R2 = RS || RT

RT CHOSEN SO THAT RT || RINM = RS

R2 CHOSEN TO BALANCE RT || RS

6409 F02

Effects of Resistor Pair Mismatch

Figure 3 shows a circuit diagram which takes into consid-

eration that real world resistors will not match perfectly.

Assuming infinite open loop gain, the differential output

relationship is given by the equation:

VOUTDIFF = V+OUT – V–OUT ≈ VINDIFF •

VCM •

∆b

bAVG

– VOCM •

∆b

bAVG

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

average of RI1, and RI2.

bAVG is defined as the average feedback factor from the

outputs to their respective inputs:

bAVG =

•

RI1

RI1 +RF1

RI2

RI2 +RF2

















Figure 3. Real-World Application with Feedback

Resistor Pair Mismatch

–

RF2

V–OUT

V+OUT

VVOCM

VOCM

6409 F03

RF1

RI2

RI1

–

VINP

–

VINM

V–IN

V+IN

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