AD9645 datasheet(20/36 Pages) AD

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AD9645

Data Sheet

Rev. 0 | Page 20 of 36

Differential Input Configurations

There are several ways to drive the AD9645 either actively or

passively. However, optimum performance is achieved by driving

the analog inputs differentially. Using a differential double balun

configuration to drive the AD9645 provides excellent performance

and a flexible interface to the ADC for baseband applications

(see Figure 55).

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

former coupling is the recommended input configuration (see

Figure 56) because the noise performance of most amplifiers is

not adequate to achieve the true performance of the AD9645.

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 AD9645 inputs single-ended.

VOLTAGE REFERENCE

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

AD9645. 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.

If the internal reference of the AD9645 is used to drive multiple

converters to improve gain matching, the loading of the reference

by the other converters must be considered. Figure 53 shows how

the internal reference voltage is affected by loading. Figure 54

shows the typical drift characteristics of the internal reference

in 1.0 V mode.

The internal buffer generates the positive and negative full-scale

references for the ADC core.

–0.5

–1.0

–1.5

–2.0

–2.5

–3.0

–3.5

–4.0

–4.5

–5.0

3.0

2.5

2.0

1.5

1.0

0.5

LOAD CURRENT (mA)

INTERNAL VREF = 1V

Figure 53. VREF Error vs. Load Current

–8

–40

TEMPERATURE (°C)

–6

–4

–2

–15

Figure 54. Typical VREF Drift

ADC

0.1µF

2V p-p

VCM

*C1

0.1µF

33Ω

200Ω

33Ω

VINx+

VINx–

ET1-1-I3

5pF

*C1 IS OPTIONAL

Figure 55. 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

VINx–

ADC

VINx+

*C1

VCM

33Ω

200Ω

0.1µF

5pF

Figure 56. Differential Transformer-Coupled Configuration for Baseband Applications

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