AD9608 datasheet(25/41 Pages) AD

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AD9608

Data Sheet

Rev. C | Page 24 of 40

Input Common Mode

The analog inputs of the AD9608 are not internally dc-biased.

Therefore, in ac-coupled applications, the user must provide

a dc bias externally. Setting the device so that VCM = AVDD/2

is recommended for optimum performance, but the device can

function over a wider range with reasonable performance, as

shown in Figure 43.

An on-board, common-mode voltage reference is included in

the design and is available from the VCM pin. The VCM pin

must be decoupled to ground by a 0.1 µF capacitor, as described

in the Applications Information section.

100

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

INPUT COMMON-MODE VOLTAGE (V)

SFDR (dBc)

SNR (dBFS)

Figure 43. SNR/SFDR vs. Input Common-Mode Voltage,

fIN = 70 MHz, fS = 125 MSPS

Differential Input Configurations

Optimum performance is achieved while driving the AD9608 in

a differential input configuration. For baseband applications,

the AD8138, ADA4937-2, and ADA4938-2 differential drivers

provide excellent performance and a flexible interface to the ADC.

The output common-mode voltage of the ADA4938-2 is easily

set with the VCM pin of the AD9608 (see Figure 44), and the

driver can be configured in a Sallen-Key filter topology to

provide band limiting of the input signal.

AVDD

VIN

76.8

Ω

120

Ω

0.1µF

Ω

10pF

200

Ω

200

Ω

ADA4938

ADC

VIN–x

VIN+x

VCM

Figure 44. Differential Input Configuration Using the ADA4938-2

For baseband applications below ~10 MHz where SNR is a key

parameter, differential transformer coupling is the recommended

input configuration (see Figure 45). To bias the analog input,

the VCM voltage can be connected to the center tap of the

secondary winding of the transformer.

2V p-p

49.9

Ω

0.1µF

ADC

VCM

VIN+x

VIN–x

Figure 45. Differential Transformer-Coupled Configuration

The signal characteristics must be considered when selecting

a transformer. Most RF transformers saturate at frequencies that

are below a few megahertz (MHz). Excessive signal power can

also cause core saturation, which leads to distortion.

At input frequencies in the second Nyquist zone and above, the

noise performance of most amplifiers is not adequate to achieve

the true SNR performance of theAD9608. For applications above

~10 MHz where SNR is a key parameter, differential double balun

coupling is the recommended input configuration (see Figure 46).

An alternative to using a transformer-coupled input at frequencies

in the second Nyquist zone is to use the AD8352 differential driver

(see Figure 47). See the AD8352 data sheet for more information.

ADC

0.1µF

2V p-p

VCM

0.1µF

Ω

VIN+x

VIN–x

Figure 46. Differential Double Balun Input Configuration

AD8352

Ω

0.1µF

8, 13

VCC

200

Ω

200

Ω

ANALOG INPUT

ADC

VCM

VIN+x

VIN–x

Figure 47. Differential Input Configuration Using the AD8352

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