AD9637 datasheet(20/41 Pages) AD

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Data Sheet

AD9637

Rev. A | Page 19 of 40

Differential Input Configurations

There are several ways to drive the AD9637 either actively or

passively. However, optimum performance is achieved by driving

the analog input differentially. Using a differential double balun

configuration to drive the AD9637 provides excellent performance

and a flexible interface to the ADC (see Figure 46) for baseband

applications.

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

former coupling is the recommended input configuration (see

Figure 47), because the noise performance of most amplifiers is

not adequate to achieve the true performance of the AD9637.

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 AD9637 input single-ended.

VOLTAGE REFERENCE

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

AD9637. 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 sections that

follow. 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 AD9637 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 45), 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– x

VIN+ x

ADC

CORE

Figure 45. 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 46. 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 47. Differential Transformer-Coupled Configuration for Baseband Applications

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