AD9681 datasheet(20/40 Pages) AD

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AD9681

Data Sheet

Rev. A | Page 20 of 40

Differential Input Configurations

There are several ways to drive the AD9681, either actively or

passively. However, optimum performance is achieved by driving

the analog inputs differentially. Using a differential double balun

configuration to drive the AD9681 provides excellent performance

and a flexible interface to the ADC (see Figure 38) for baseband

applications. Similarly, differential transformer coupling also

provides excellent performance (see Figure 39). Because the noise

performance of most amplifiers is not adequate to achieve the true

performance of the AD9681, 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 AD9681 inputs not be driven single-

ended.

VOLTAGE REFERENCE

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

AD9681. 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 AD9681 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 37), 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

VREF

SENSE

0.5V

ADC

SELECT

LOGIC

0.1µF

1.0µF

VIN–A/VIN–B

VIN+A/VIN+B

ADC

CORE

Figure 37. Internal Reference Configuration

ADC

0.1µF

2V p-p

VCM

*C1

0.1µF

33Ω

200Ω

33Ω

ET1-1-I3

5pF

*C1 IS OPTIONAL

VIN–x1,

VIN–x2

VIN+x1,

VIN+x2

Figure 38. 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

ADC

*C1

VCM

33Ω

200Ω

0.1µF

5pF

VIN–x1,

VIN–x2

VIN+x1,

VIN+x2

Figure 39. Differential Transformer Coupled Configuration for Baseband Applications

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