ADS5541IPAPR datasheet(23/37 Pages) TI1

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0.1

µF

100

Ω

0.1

µF

1000pF

1:1

100

Ω

OPA695

400

Ω

V = 8V/V

(18dB)

57.5

Ω

INP

INM

−5V

+5V

Ω

ADS5541

Production Data

ADS5541

SBAS307C – MAY 2004 – REVISED FEBRUARY 2007

When it is necessary to buffer or apply a gain to the

Figure 41 illustrates how RIN and CIN can be placed

incoming analog signal, it is possible to combine

to isolate the signal source from the switching inputs

single-ended operational amplifiers with an RF

of the ADC and to implement a low-pass RC filter to

transformer, or to use a differential input/output

limit the input noise in the ADC. It is recommended

amplifier without a transformer, to drive the input of

that these components be included in the ADS5541

the ADS5541. Texas Instruments offers a wide

circuit layout when any of the amplifier circuits

selection

single-ended

operational

amplifiers

discussed previously are used. The components

(including the THS3201, THS3202, OPA847, and

allow fine-tuning of the circuit performance. Any

OPA695) that can be selected depending on the

mismatch between the differential lines of the

application. An RF gain block amplifier, such as

ADS5541

input

produces

degradation

Texas Instruments THS9001, can also be used with

performance

high

input

frequencies,

mainly

transformer

for

high

input

frequency

characterized by an increase in the even-order

applications. The THS4503 is a recommended

harmonics. In this case, special care should be taken

differential input/output amplifier. Table 4 lists the

to keep as much electrical symmetry as possible

recommended amplifiers.

between both inputs.

When

using

single-ended

operational

amplifiers

Another possible configuration for lower-frequency

(such as the THS3201, THS3202, OPA847, or

signals

the

use

differential

input/output

OPA695) to provide gain, a three-amplifier circuit is

amplifiers that can simplify the driver circuit for

recommended with one amplifier driving the primary

applications

requiring

dc-coupling

the

input.

of an RF transformer and one amplifier in each of the

Flexible in their configurations (see Figure 42), such

legs of the secondary driving the two differential

amplifiers can be used for single-ended-to-differential

inputs of the ADS5520. These three amplifier circuits

conversion signal amplification.

minimize even-order harmonics. For high frequency

inputs, an RF gain block amplifier can be used to

drive a transformer primary; in this case, the

transformer secondary connections can drive the

input

the

ADS5541

directly,

shown

Figure 40, or with the addition of the filter circuit

shown in Figure 41.

Table 4. Recommended Amplifiers to Drive the Input of the ADS5520

INPUT SIGNAL FREQUENCY

RECOMMENDED AMPLIFIER

TYPE OF AMPLIFIER

USE WITH TRANSFORMER

DC to 20MHz

THS4503

Differential In/Out Amp

DC to 50MHz

OPA847

Operational Amp

Yes

OPA695

Operational Amp

Yes

10MHz to 120MHz

THS3201

Operational Amp

Yes

THS3202

Operational Amp

Yes

Over 100MHz

THS9001

RF Gain Block

Yes

Figure 41. Converting a Single-Ended Input Signal to a Differential Signal Using an RF Transformer

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