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AD9249

Data Sheet

Rev. 0 | Page 18 of 36

Differential Input Configurations

There are several ways to drive the AD9249, either actively or

passively. However, optimum performance is achieved by driving

the analog inputs differentially. Using a differential double balun

configuration to drive the AD9249 provides excellent performance

and a flexible interface to the ADC (see Figure 35) for baseband

applications. Similarly, differential transformer coupling also

provides excellent performance (see Figure 36).

Because the noise performance of most amplifiers is not adequate

to achieve the true performance of the AD9249, use of these

passive configurations is recommended wherever possible.

Regardless of the configuration, the value of the shunt capacitor, C,

is dependent on the input frequency and may need to be reduced

or removed.

It is recommended that the AD9249 inputs not be driven single-

ended.

VOLTAGE REFERENCE

A stable and accurate 1.0 V voltage reference is built into the

AD9249. Configure VREF using either the internal 1.0 V

reference or an externally applied 1.0 V reference voltage. The

various reference modes are summarized in the Internal Reference

Connection section and the External Reference Operation section.

Bypass the VREF pin to ground externally, using a low ESR,

1.0 μF capacitor in parallel with a low ESR, 0.1 μF ceramic

capacitor.

Internal Reference Connection

A comparator within the AD9249 detects the potential at the

SENSE pin and configures the reference into two possible

modes, which are summarized in Table 9. If SENSE is grounded,

the reference amplifier switch is connected to the internal resistor

divider (see Figure 34), setting VREF to 1.0 V.

Table 9. Reference Configuration Summary

Selected Mode

SENSE

Voltage (V)

Resulting

VREF (V)

Resulting

Differential

Span (V p-p)

Fixed Internal

Reference

GND to 0.2

1.0 internal

2.0

Fixed External

Reference

AVDD

1.0 applied

to external

VREF pin

2.0

Figure 34. Internal Reference Configuration

Figure 35. Differential Double Balun Input Configuration for Baseband Applications

Figure 36. Differential Transformer Coupled Configuration for Baseband Applications

VREF

SENSE

0.5V

ADC

SELECT

LOGIC

0.1µF

1.0µF

VIN–x1, VIN–x2

VIN+x1, VIN+x2

ADC

CORE

ADC

0.1µF

2V p-p

VCM1,

VCM2

*C1

0.1µF

33Ω

200Ω

33Ω

ET1-1-I3

5pF

*C1 IS OPTIONAL

VIN+x1,

VIN+x2

VIN–x1,

VIN–x2

2V p-p

*C1

*C1 IS OPTIONAL

49.9Ω

0.1μF

ADT1-1WT

1:1 Z RATIO

ADC

*C1

VCM1,

VCM2

33Ω

200Ω

0.1µF

5pF

VIN–x1,

VIN–x2

VIN+x1,

VIN+x2

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