ADS5522IPAP datasheet(21/32 Pages) TI1

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ADS5522

50W

25W

10W

0.1

25W

400 A x

f (inMSPS)

(5)

ADS5522

SBAS320C – MAY 2004 – REVISED FEBRUARY 2007

This differential input topology produces a high level

Where:

of ac-performance for high sampling rates. It also

fS > 2MSPS.

results in a very high usable input bandwidth,

This equation helps to design the output capability

especially important for high intermediate-frequency

and impedance of the driving circuit accordingly.

(IF) or undersampling applications. The ADS5522

requires each of the analog inputs (INP, INM) to be

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

externally biased around the common-mode level of

incoming analog signal, it is possible to combine

the internal circuitry (CM, pin 17). For a full-scale

single-ended operational amplifiers with an RF

differential input, each of the differential lines of the

transformer, or to use a differential input/output

input signal (pins 19 and 20) swings symmetrically

amplifier without a transformer, to drive the input of

between CM + 0.575 V and CM –0.575 V. This

the ADS5522. Texas Instruments offers a wide

means that each input is driven with a signal of up to

selection

single-ended

operational

amplifiers

0.575 V, so that each input has a maximum

(including the THS3201, THS3202, OPA847, and

differential signal of 1.15 V

PP for a total differential

OPA695) that can be selected depending on the

input signal swing of 2.3 V

PP. The maximum swing is

application. An RF gain block amplifier, such as TI'ss

determined by the two reference voltages, the top

THS9001, can also be used with an RF transformer

reference (REFP, pin 29), and the bottom reference

for high input frequency applications. The THS4503

(REFM, pin 30).

is a recommended differential input/output amplifier.

Table 4 lists the recommended amplifiers.

The ADS5522 obtains optimum performance when

the analog inputs are driven differentially. The circuit

When

using

single-ended

operational

amplifiers

shown

Figure

illustrates

one

possible

(such as the THS3201, THS3202, OPA847, or

configuration using an RF transformer.

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

recommended with one amplifier driving the primary

of an RF transformer and one amplifier in each of the

legs of the secondary driving the two differential

inputs of the ADS5522. These three amplifier circuits

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

ADS5522

directly,

shown

Figure 38, or with the addition of the filter circuit

shown in Figure 39.

Figure 39 illustrates how RIN and CIN can be placed

to isolate the signal source from the switching inputs

Figure 38. Transformer Input to Convert

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

Single-Ended Signal to Differential Signal

limit the input noise in the ADC. It is recommended

that these components be included in the ADS5522

The single-ended signal is fed to the primary winding

circuit layout when any of the amplifier circuits

of an RF transformer. Since the input signal must be

discussed previously are used. The components

biased around the common-mode voltage of the

allow fine-tuning of the circuit performance. Any

internal circuitry, the common-mode voltage (VCM)

mismatch between the differential lines of the

from the ADS5522 is connected to the center-tap of

ADS5522

input

produces

degradation

the secondary winding. To ensure a steady low-noise

performance

high

input

frequencies,

mainly

VCM reference, best performance is attained when

characterized by an increase in the even-order

the CM output (pin 17) is filtered to ground with a

harmonics. In this case, special care should be taken

0.1-

µF and 0.001-µF low-inductance capacitors,

to keep as much electrical symmetry as possible

between both inputs.

Output VCM (pin 17) is designed to directly drive the

Another possible configuration for lower-frequency

ADC input. When providing a custom CM level, be

signals

the

use

differential

input/output

aware that the input structure of the ADC sinks a

amplifiers that can simplify the driver circuit for

common-mode current in the order of 400

µA (200

applications

requiring

dc-coupling

the

input.

µA per input) at 80 MSPS. Equation 5 describes the

Flexible in their configurations (see Figure 40), such

dependency of the common-mode current and the

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

sampling frequency:

conversion signal amplification.

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