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AD9246

Rev. A | Page 16 of 44

DIFFERENTIAL INPUT CONFIGURATIONS

Optimum performance is achieved by driving the AD9246 in

a differential input configuration. For baseband applications,

the AD8138 differential driver provides excellent performance

and a flexible interface to the ADC. The output common-mode

voltage of the AD8138 is easily set with the CML pin of the

AD9246 (see Figure 37), and the driver can be configured in

a Sallen-Key filter topology to provide band limiting of the

input signal.

AVDD

1V p-p

49.9Ω

523Ω

0.1µF

499Ω

AD8138

AD9246

VIN+

VIN–

CML

Figure 37. Differential Input Configuration Using the AD8138

For baseband applications where SNR is a key parameter,

differential transformer coupling is the recommended input

configuration (see Figure 38). The CML voltage can be

connected to the center tap of the secondary winding of the

transformer to bias the analog input.

The signal characteristics must be considered when selecting

a transformer. Most RF transformers saturate at frequencies

below a few MHz, and excessive signal power can cause core

saturation, which leads to distortion.

2V p-p

49.9Ω

0.1µF

AD9246

VIN+

VIN–

CML

Figure 38. Differential Transformer-Coupled Configuration

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 the AD9246. For applications where

SNR is a key parameter, transformer coupling is the recom-

mended input.

For applications where SFDR is a key parameter, differential

double balun coupling is the recommended input configuration

(see Figure 40).

As an alternative to using a transformer-coupled input at

frequencies in the second Nyquist zone, the AD8352 differential

driver can be used (see Figure 41).

In any configuration, the value of the shunt capacitor, C, is

dependent on the input frequency and source impedance and

may need to be reduced or removed. Table 8 displays recom-

mended values to set the RC network. However, these values are

dependent on the input signal and should only be used as a

starting guide.

Table 8. RC Network Recommended Values

Frequency Range (MHz)

R Series (Ω) C Differential (pF)

0 to 70

70 to 200

200 to 300

>300

Open

Single-Ended Input Configuration

Although not recommended, it is possible to operate the

AD9246 in a single-ended input configuration, as long as the

input voltage swing is within the AVDD supply. Single-ended

operation can provide adequate performance in cost-sensitive

applications.

In this configuration, SFDR and distortion performance

degrade due to the large input common-mode swing. If the

source impedances on each input are matched, there should be

little effect on SNR performance. Figure 39 details a typical

single-ended input configuration.

1V p-p

49.9Ω

0.1µF

10µF

0.1µF

AVDD

1kΩ

AD9246

AVDD

VIN+

VIN–

Figure 39. Single-Ended Input Configuration

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