CHOOSING THE PROPER VALUE FOR THE

APPLICATION CIRCUITS USING FULLY

Gain

V

OD

V

IN

THS4504

THS4505

www.ti.com ......................................................................................................................................................... SLOS363D – AUGUST 2002 – REVISED MAY 2008

requirements, determine the value of

the

gain

FEEDBACK AND GAIN RESISTORS

resistors, directly impacting the input impedance of

the entire circuit. While there are no strict rules about

The selection of feedback and gain resistors impacts

resistor selection, these trends can provide qualitative

circuit performance in a number of ways. The values

design guidance.

in this section provide the optimum high-frequency

performance

(lowest

distortion,

flat

frequency

response). Since the THS4500 family of amplifiers is

DIFFERENTIAL AMPLIFIERS

developed with a voltage-feedback architecture, the

choice of resistor values does not have a dominant

Fully differential amplifiers provide designers with a

effect

on

bandwidth,

unlike

a

current-feedback

great

deal

of

flexibility

in

a

wide

variety

of

amplifier.

However,

resistor

choices

do

have

applications. This section provides an overview of

second-order effects. For optimal performance, the

some common circuit configurations and gives some

following feedback resistor values are recommended.

design guidelines. Designing the interface to an ADC,

In higher gain configurations (gain greater than two),

driving lines differentially, and filtering with fully

the feedback resistor values have much less effect on

differential amplifiers are a few of the circuits that are

the high-frequency performance. Example feedback

covered.

and gain resistor values are given in the section on

basic design considerations (Table 3).

Amplifier loading, noise, and the flatness of the

frequency response are three design parameters that

should

be

considered

when

selecting

feedback

resistors. Larger resistor values contribute more noise

and can induce peaking in the ac response in low

gain configurations, and smaller resistor values can

load the amplifier more heavily, resulting in a

reduction in distortion performance. In addition,

feedback

resistor

values,

coupled

with

gain

Table 2. Midrail Referenced

Gain (V/V)

VIN+ (V)

VIN– (V)

VIN (VPP)

VOCM (V)

VOD (VPP)

VNMIN (V)

VNMAX (V)

1

0.5 to 4.5

2.5

4

2.5

4

2

3

2

1.5 to 3.5

2.5

2

2.5

4

2.16

2.83

4

2.0 to 3.0

2.5

1

2.5

4

2.3

2.7

8

2.25 to 2.75

2.5

0.5

2.5

4

2.389

2.61

Table 3. Resistor Values for Balanced Operation in Various Gain Configurations

R2 & R4 (

Ω)

R1 (

Ω)

R3 (

Ω)

RT (Ω)

1

392

412

383

54.9

1

499

523

487

53.6

2

392

215

187

60.4

2

1.3 k

665

634

52.3

5

1.3 k

274

249

56.2

5

3.32 k

681

649

52.3

10

1.3 k

147

118

64.9

10

6.81 k

698

681

52.3

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