R

0

50

Ω

Z

0

50

Ω

1:1

INP

INM

CM

ADT1−1WT

R

50

Ω

1nF

0.1

µF

AC Signal

Source

10

Ω

25

Ω

25

Ω

ADS5520

600mA

f

125 MSPS

(1)

ADS5520

This differential input topology produces a high level of ac-performance for high sampling rates. It also results in

a very high usable input bandwidth, especially important for high intermediate-frequency (IF) or undersampling

applications. The ADS5520 requires each of the analog inputs (INP, INM) to be externally biased around the

common-mode level of the internal circuitry (CM, pin 17). For a full-scale differential input, each of the differential

lines of the input signal (pins 19 and 20) swings symmetrically between CM + 0.575 V and CM – 0.575 V. This

means that each input is driven with a signal of up to CM ±0.575 V, so that each input has a maximum

determined by the two reference voltages, the top reference (REFP, pin 29), and the bottom reference (REFM,

pin 30).

The ADS5520 obtains optimum performance when the analog inputs are driven differentially. The circuit shown

in Figure 40 illustrates one possible configuration using an RF transformer.

Figure 40. Transformer Input to Convert Single-Ended Signal to Differential Signal

The single-ended signal is fed to the primary winding of an RF transformer. Placing a 25-

Ω resistor in series with

INP and INM is recommended to dampen ringing due to ADC kickback.

Since the input signal must be biased around the common-mode voltage of the internal circuitry, the

to ground with a 10-

Ω series resistor and parallel 0.1-

µF and 0.001-µF low-inductance capacitors, as illustrated in

Figure 39.

that the input structure of the ADC sinks a common-mode current in the order of 600

µA (300 µA per input).

Equation 1 describes the dependency of the common-mode current and the sampling frequency:

Where:

This equation helps to design the output capability and impedance of the driving circuit accordingly.

When it is necessary to buffer or apply a gain to the incoming analog signal, it is possible to combine

single-ended operational amplifiers with an RF transformer, or to use a differential input/output amplifier without a

transformer, to drive the input of the ADS5520. Texas Instruments offers a wide selection of single-ended

operational amplifiers (including the THS3201, THS3202, OPA695, and OPA847) that can be selected

depending on the application. An RF gain block amplifier, such as Texas Instruments THS9001, can also be

used with an RF transformer for high input frequency applications. The THS4503 is a recommended differential

input/output amplifier. Table 5 lists the recommended amplifiers.

Copyright © 2004–2008, Texas Instruments Incorporated

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