THS4501CDG4 datasheet(24/48 Pages) TI1

24 / 48 page

Gain

VOCM

Vout+

Vout-

–

1–

2(1)K)

K +

R2 + R4

R3 + R1 * Rs || RT

1 +

R1 ) R2

2 +

R3 ) RT || RS

R3 ) RT || RS ) R4

+ 2

1–

) β2

) RS

+ 2

1–

) β2

THS4500

THS4501

SLOS350F

– APRIL 2002 – REVISED OCTOBER 2011

www.ti.com

Table 2. Midrail Referenced

Gain

VIN+

VIN–

VIN

VOCM

VOD

VNMIN

VNMAX

(V/V)

(V)

(VPP)

(V)

(VPP)

(V)

0.5 to 4.5

2.5

1.5 to 3.5

2.5

2.16

2.83

2.0 to 3.0

2.5

2.3

2.7

2.25 to 2.75

2.5

0.5

2.5

2.389

2.61

Table 3. Resistor Values for Balanced Operation

CHOOSING THE PROPER VALUE FOR THE

in Various Gain Configurations

FEEDBACK AND GAIN RESISTORS

The selection of feedback and gain resistors impacts

R2 and R4

R1 (

Ω)

R3 (

Ω)

RT (Ω)

circuit performance in a number of ways. The values

(

Ω)

presented in this section provide the optimum

high-frequency performance (lowest distortion, flat

392

412

383

54.9

frequency response). Since the THS4500 family of

499

523

487

53.6

amplifiers is developed with a voltage feedback

392

215

187

60.4

architecture, the choice of resistor values does not

1.3 k

665

634

52.3

have a dominant effect on bandwidth, unlike a

current-feedback amplifier. However, resistor choices

1.3 k

274

249

56.2

have

second-order

effects.

For

optimal

3.32 k

681

649

52.3

performance, the following feedback resistor values

1.3 k

147

118

64.9

are recommended. In higher gain configurations (gain

6.81 k

698

681

52.3

greater than two), the feedback resistor values have

much less effect on the high-frequency performance.

Example feedback and gain resistor values are given

the

section

basic

design

considerations

(Table 3).

Amplifier loading, noise, and the flatness of the

frequency response are three design parameters that

should

considered

when

selecting

feedback

resistors. Larger resistor values contribute more noise

and can induce peaking in the ac response in low

gain configurations; smaller resistor values can load

the amplifier more heavily, resulting in a reduction in

distortion performance. In addition, feedback resistor

Figure 101. Diagram for Design Calculations

values, coupled with gain requirements, determine

the value of the gain resistors and directly impact the

Equations for calculating fully differential amplifier

input impedance of the entire circuit. While there are

resistor values in order to obtain balanced operation

no strict rules about resistor selection, these trends

in the presence of a 50-

Ω source impedance are

can provide qualitative design guidance.

given in Equation 6 through Equation 9.

APPLICATION CIRCUITS USING FULLY

DIFFERENTIAL AMPLIFIERS

Fully differential amplifiers provide designers with a

(6)

great

deal

flexibility

wide

variety

applications. This section provides an overview of

some common circuit configurations and gives some

(7)

design guidelines. Designing the interface to an

analog-to-digital

converter

(ADC),

driving

lines

differentially,

and

filtering

with

fully

differential

(8)

amplifiers are a few of the circuits that are covered.

(9)

BASIC DESIGN CONSIDERATIONS

The circuits in Figure 98 through Figure 101 are used

to highlight basic design considerations for fully

differential amplifier circuit designs.

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