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AD9635

Data Sheet

Rev. B | Page 20 of 36

Differential Input Configurations

There are several ways to drive the AD9635 either actively or

passively. However, optimum performance is achieved by driving

the analog inputs differentially. Using a differential double balun

configuration to drive the AD9635 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 AD9635.

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 AD9635 inputs single-ended.

VOLTAGE REFERENCE

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

AD9635. 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.

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.

Figure 53. VREF Error vs. Load Current

Figure 54. Typical VREF Drift

Figure 55. Differential Double Balun Input Configuration for Baseband Applications

Figure 56. Differential Transformer-Coupled Configuration for Baseband Applications

–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

–8

–40

TEMPERATURE (°C)

–6

–4

–2

–15

ADC

0.1µF

2V p-p

VCM

*C1

0.1µF

33Ω

200Ω

33Ω

VINx+

VINx–

ET1-1-I3

5pF

*C1 IS OPTIONAL

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

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