BASIC DESIGN CONSIDERATIONS

1

R =

T

1 -

K

2(1+K)

R3

-

1

R

S

K=

R2

R1

R2=R4

R3=R1

(R ||R )

-

S

T

R1

R1+R2

b =

1

R3+R ||R

T

S

R3+R ||

+R4

T

S

R

b =

2

(7)

V

OD

V

S

=2

1 - b2

b

b2

1

+

R

T

R

R

S

T

+

(8)

V

OD

V

IN

=2

1 - b2

b

b2

1

+

(9)

INTERFACING TO AN ANALOG-TO-DIGITAL

THS4504

THS4505

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

• Use separate analog and digital power supplies

and grounds. Noise (bounce) in

the

power

The circuits in Figure 76 through Figure 78 are used

supplies (created by digital switching currents) can

to highlight basic design considerations for fully

couple

directly

into

the

signal

path,

and

differential amplifier circuit designs.

power-supply noise can create higher distortion

products as well.

Equations for calculating fully differential amplifier

resistor values in order to obtain balanced operation

• Use care when filtering. While an RC low-pass

in the presence of a 50-

Ω source impedance are

filter may be desirable on the output of the

given in Equation 6 through Equation 9.

amplifier to filter broadband noise, the excess

loading

can

negatively

impact

the

amplifier

linearity. Filtering in the feedback path does not

have this effect.

• AC-coupling allows easier circuit design. If

dc-coupling is required, be aware of the excess

(6)

power

dissipation

that

can

occur

due

to

level-shifting

the

output

through

the

output

common-mode voltage control.

• Do not terminate the output unless required. Many

open-loop,

class-A

amplifiers

require

50-

Ω

termination for proper operation, but closed-loop

fully differential amplifiers drive a specific output

voltage regardless of the load impedance present.

Terminating the output of a fully differential

amplifier with a heavy load adversely effects the

amplifier's linearity.

For more detailed information about balance in fully

• Comprehend the V

OCM input drive requirements.

differential

amplifiers,

see

the

Fully

Differential

Determine if the ADC voltage reference can

Amplifiers, referenced at the end of this data sheet.

provide the required amount of current to move

VOCM to the desired value. A buffer may be

needed.

CONVERTER

• Decouple the V

OCM pin to eliminate the antenna

The THS4500 family of amplifiers are designed

effect. VOCM is a high-impedance node that can

specifically

to

interface

to

today's

act as an antenna. A large decoupling capacitor

highest-performance

analog-to-digital

converters.

on this node eliminates this problem.

This section highlights the key concerns when

• Be cognizant of the input common-mode range. If

interfacing

to

an

ADC

and

provides

example

the input signal is referenced around the negative

ADC/fully differential amplifier interface circuits.

power supply rail (e.g., around ground on a single

Key

design

concerns

when

interfacing

to

an

5 V supply), then the THS4500/1 accommodates

analog-to-digital converter:

the input signal. If the input signal is referenced

• Terminate

the

input

source

properly.

In

around midrail, choose the THS4502/3 for the

high-frequency

receiver

chains,

the

source

best operation.

feeding the fully differential amplifier requires a

• Packaging makes a difference at higher

specific load impedance (for example, 50

Ω).

frequencies. If possible, choose the smaller,

• Design a symmetric printed-circuit board (PCB)

thermally-enhanced MSOP package for the best

layout. Even-order distortion products are heavily

performance.

As

a

rule,

lower

junction

influenced by layout, and careful attention to a

temperatures

provide

better

performance.

If

symmetric layout will minimize these distortion

possible, use a thermally-enhanced package,

products.

even if the power dissipation is relatively small

compared to the maximum power dissipation

• Minimize inductance in power-supply decoupling

rating to achieve the best results.

traces

and

components.

Poor

power-supply

decoupling can have a dramatic effect on circuit

• Comprehend the effect of the load impedance

performance. Since the outputs are differential,

seen by the fully differential amplifier when

differential currents exist in the power-supply pins.

performing

system-level

intercept

point

Thus, decoupling capacitors should be placed in a

calculations.

Lighter

loads

(such

as

those

manner that minimizes the impedance of the

presented by an ADC) allow smaller intercept

current loop.

points to support the same level of intermodulation

distortion performance.

22

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