LTC6409

11

6409fa

applicaTions inForMaTion

a termination resistor R

T

should be chosen (see Figure

2) such that:

R

T

=

R

INM

•R

S

R

INM

– R

S

According to Figure 2, the input impedance looking into

the differential amp (R

INM

) reflects the single-ended source

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

R2

=R

T

||R

S

=

R

T

•R

S

R

T

+R

S

Figure 2. Optimal Compensation for Signal Source Impedance

Δ

b is defined as the difference in the feedback factors:

∆β =

R

I2

R

I2

+R

F2

R

I1

R

I1

+R

F1

Here, V

CM

and V

INDIFF

are defined as the average and

the difference of the two input voltages V

INP

and V

INM

respectively:

V

CM

=

V

INP

+ V

INM

2

V

INDIFF

= V

INP

– V

INM

When the feedback ratios mismatch (Δ

b), common mode

to differential conversion occurs. Setting the differential

input to zero (V

INDIFF

= 0), the degree of common mode

to differential conversion is given by the equation:

V

OUTDIFF

= V

+OUT

– V

–OUT

≈(V

CM

– V

OCM

) • ∆β

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 V

OCM

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,

V

S

+

+

R

F

R

F

R

I

R

INM

R

S

R

I

R2 = R

S

|| R

T

R

T

CHOSEN SO THAT R

T

|| R

INM

= R

S

R2 CHOSEN TO BALANCE R

T

|| R

S

R

T

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:

V

OUTDIFF

= V

+OUT

– V

–OUT

≈ V

INDIFF

R

F

R

I

+

V

CM

• ∆β

β

AVG

– V

OCM

• ∆β

β

AVG

where R

F

is the average of R

F1

, and R

F2

, and R

I

is the

average of R

I1

, and R

I2

.

b

AVG

is defined as the average feedback factor from the

outputs to their respective inputs:

β

AVG

=

1

2

R

I1

R

I1

+R

F1

+

R

I2

R

I2

+R

F2





Figure 3. Real-World Application with Feedback

Resistor Pair Mismatch

+

R

F2

V

–OUT

V

+OUT

V

VOCM

V

OCM

6409 F03

R

F1

R

I2

R

I1

+

V

INP

+

V

INM

V

–IN

V

+IN

DC1591A Datasheet Related Products:
DC1591A Information:
Part No.
DC1591A
Description
DEMO BRD FOR LTC6409 ADC
File Size
449356 bytes
Page Size
612 x 792 pts (letter)
All Pages
24
Manufacturer
Linear Technology
Homepage
http://www.linear.com/
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