ADS5541IPAPR datasheet(22/37 Pages) TI1

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500mA

S (in MSPS)

105 MSPS

(5)

Ω

1:1

INP

INM

ADT1−1WT

Ω

1nF

0.1

µF

AC Signal

Source

Ω

ADS5541

Production Data

ADS5541

SBAS307C – MAY 2004 – REVISED FEBRUARY 2007

This differential input topology produces a high level

The single-ended signal is fed to the primary winding

of ac performance for high sampling rates. It also

of an RF transformer. Placing a 25

Ω resistor in

results in a very high usable input bandwidth,

series with INP and INM is recommended to dampen

especially important for high intermediate-frequency

ringing because of ADC kickback. Since the input

(IF) or undersampling applications. The ADS5541

signal must be biased around the common-mode

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

voltage of the internal circuitry, the common-mode

externally biased around the common-mode level of

voltage (VCM) from the ADS5541 is connected to the

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

center-tap of the secondary winding. To ensure a

differential input, each of the differential lines of the

steady low-noise VCM reference, best performance is

input signal (pins 19 and 20) swing symmetrically

attained when the CM output (pin 17) is filtered to

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

ground with a 10

Ω series resistor and parallel 0.1µF

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

and 0.001

µF low-inductance capacitors as illustrated

CM ± 0.575V, so that each input has a maximum

in Figure 39.

differential signal of 1.15V

PP for a total differential

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

input signal swing of 2.3V

PP. The maximum swing is

ADC input. When providing a custom CM level, be

determined by the two reference voltages, the top

aware that the input structure of the ADC sinks a

reference (REFP, pin 29) and the bottom reference

common-mode current in the order of 500

µA (250µA

(REFM, pin 30).

per input) at 105MSPS. Equation 5 describes the

The ADS5541 gives optimum performance when the

dependency of the common-mode current and the

analog inputs are driven differentially. The circuit

sampling frequency:

shown

Figure

illustrates

one

possible

configuration using an RF transformer.

Where:

fS > 2MSPS.

This equation helps to design the output capability

and impedance of the driving circuit accordingly.

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

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