AD9633 datasheet(24/42 Pages) AD

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Data Sheet

AD9633

Rev. B | Page 23 of 41

Differential Input Configurations

There are several ways to drive the AD9633 either actively or

passively. However, optimum performance is achieved by driving

the analog inputs differentially. Using a differential double balun

configuration to drive the AD9633 provides excellent perfor-

mance and a flexible interface to the ADC (see Figure 57) for

baseband applications.

For applications where SNR is a key parameter, differential trans-

former coupling is the recommended input configuration (see

Figure 58), because the noise performance of most amplifiers is

not adequate to achieve the true performance of the AD9633.

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 not recommended to drive the AD9633 inputs single-ended.

VOLTAGE REFERENCE

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

AD9633. VREF can be configured 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. The VREF pin should be externally decoupled to

ground with 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 AD9633 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 56), 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

AGND to 0.2

1.0 internal

2.0

Fixed

External

Reference

AVDD

1.0 applied

to external

VREF pin

2.0

VREF

SENSE

0.5V

ADC

SELECT

LOGIC

0.1µF

1.0µF

VIN–A/VIN–B

VIN+A/VIN+B

ADC

CORE

Figure 56. Internal Reference Configuration

ADC

0.1µF

2V p-p

VCM

*C1

0.1µF

33Ω

200Ω

33Ω

VIN+x

VIN–x

ET1-1-I3

5pF

*C1 IS OPTIONAL

Figure 57. Differential Double Balun Input Configuration for Baseband Applications

2V p-p

*C1

*C1 IS OPTIONAL

49.9Ω

0.1μF

ADT1-1WT

1:1 Z RATIO

VIN–x

ADC

VIN+x

*C1

VCM

33Ω

200Ω

0.1µF

5pF

Figure 58. Differential Transformer-Coupled Configuration

for Baseband Applications

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