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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

ADS5510

600mA

f

S (in MSPS)

125 MSPS

(1)

ADS5510

SLAS499 – JANUARY 2007

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 ADS5510 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

differential signal of 1.15 V

PP for a total differential input signal swing of 2.3 VPP. The maximum swing is

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

pin 30).

The ADS5510 obtains optimum performance when the analog inputs are driven differentially. The circuit shown

in Figure 25 illustrates one possible configuration using an RF transformer.

Figure 25. 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

common-mode voltage (VCM) from the ADS5510 is connected to the center-tap of the secondary winding.

To ensure a steady low-noise VCM reference, best performance is attained when the CM output (pin 17) is

filtered to ground with a 10-

Ω series resistor and parallel 0.1-µF and 0.001-µF low-inductance capacitors, as

illustrated in Figure 24.

Output VCM (pin 17) is designed to directly drive the ADC input. When providing a custom CM level, be aware

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:

fS > 2 MSPS.

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 ADS5510. 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 4 lists the recommended amplifiers.

19

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