THS4501CD datasheet(24/37 Pages) TI

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THS4500

THS4501

SLOS350D − APRIL 2002 − REVISED JANUARY 2004

www.ti.com

INTERFACING TO AN ANALOG-TO-DIGITAL

CONVERTER

The

THS4500

family

amplifiers

are designed

specifically to interface to today’s highest-performance

analog-to-digital converters. This section highlights the

key concerns when interfacing to an ADC and provides

example ADC/fully differential amplifier interface circuits.

Key

design

concerns

when

interfacing

analog-to-digital converter:

D Terminate the input source properly. In high-frequency

receiver chains, the source feeding the fully

differential

amplifier

requires

specific

load

impedance (e.g., 50

Ω).

D Design a symmetric printed-circuit board layout.

Even-order distortion products are heavily influenced

by layout, and careful attention to a symmetric layout

will minimize these distortion products.

D Minimize inductance in power supply decoupling

traces

and

components.

Poor

power

supply

decoupling can have a dramatic effect on circuit

performance. Since the outputs are differential,

differential currents exist in the power supply pins.

Thus, decoupling capacitors should be placed in a

manner that minimizes the impedance of the current

loop.

D Use separate analog and digital power supplies and

grounds. Noise (bounce) in the power supplies

(created by digital switching currents) can couple

directly into the signal path, and power supply noise

can create higher distortion products as well.

D Use care when filtering. While an RC low-pass filter

may be desirable on the output of the amplifier to filter

broadband noise, the excess loading can negatively

impact the amplifier linearity. Filtering in the feedback

path does not have this effect.

D AC-coupling allows easier circuit design. If

dc-coupling is required, be aware of the excess power

dissipation that can occur due to level-shifting the

output through the output common-mode voltage

control.

D 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

the

load

impedance

present.

Terminating the output of a fully differential amplifier

with a heavy load adversely effects the amplifier’s

linearity.

D Comprehend the VOCM input drive requirements.

Determine if the ADC’s voltage reference can provide

the required amount of current to move VOCM to the

desired value. A buffer may be needed.

D Decouple the VOCM pin to eliminate the antenna

effect. VOCM is a high-impedance node that can act as

an antenna. A large decoupling capacitor on this node

eliminates this problem.

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

input signal is referenced around the negative power

supply rail (e.g., around ground on a single 5 V

supply), then the THS4500/1 accommodates the

input signal. If the input signal is referenced around

midrail, choose the THS4502/3 for the best operation.

D Packaging makes a difference at higher frequencies.

If possible, choose the smaller, thermally enhanced

MSOP package for the best performance. As a rule,

lower

junction

temperatures

provide

better

performance. If possible, use a thermally enhanced

package, even if the power dissipation is relatively

small compared to the maximum power dissipation

rating to achieve the best results.

D Comprehend the effect of the load impedance seen by

the fully differential amplifier when performing

system-level intercept point calculations. Lighter

loads (such as those presented by an ADC) allow

smaller intercept points to support the same level of

intermodulation distortion performance.

EXAMPLE ANALOG-TO-DIGITAL

CONVERTER DRIVER CIRCUITS

The THS4500 family of devices is designed to drive

high-performance ADCs with extremely high linearity,

allowing for the maximum effective number of bits at the

output of the data converter. Two representative circuits

shown below highlight single-supply operation and split

supply operation. Specific feedback resistor, gain resistor,

and feedback capacitor values are not specified, as their

values depend on the frequency of interest. Information on

calculating these values can be found in the applications

material above.

Figure 101

−

VOCM

12 Bit/80 MSps

5 V

−5 V

µF

0.1

µF

µF 0.1 µF

THS4503

µF

0.1

µF

ADS5410

Using the THS4503 With the ADS5410

Riso

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