AD6673 datasheet(20/46 Pages) AD

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Data Sheet

AD6673

Rev. C | Page 19 of 45

Input Common Mode

The analog inputs of the AD6673 are not internally dc biased.

In ac-coupled applications, the user must provide this bias

externally. Setting the device so that VCM = 0.5 × AVDD (or

0.9 V) is recommended for optimum performance. An on-board

common-mode voltage reference is included in the design and is

available from the VCM pin. Using the VCM output to set the

input common mode is recommended. Optimum performance

is achieved when the common-mode voltage of the analog input

is set by the VCM pin voltage (typically 0.5 × AVDD). Decouple

the VCM pin to ground by using a 0.1 µF capacitor, as described

in the Applications Information section. Place this decoupling

capacitor close to the pin to minimize the series resistance and

inductance between the part and this capacitor.

Differential Input Configurations

Optimum performance is achieved while driving the AD6673 in a

differential input configuration. For baseband applications, the

AD8138, ADA4937-2, ADA4938-2, and ADA4930-2 differential

drivers provide excellent performance and a flexible interface to

the ADC.

The output common-mode voltage of the ADA4930-2 is easily

set with the VCM pin of the AD6673 (see Figure 28), and the

driver can be configured in a Sallen-Key filter topology to

provide band-limiting of the input signal.

VIN

76.8

Ω

120

Ω

0.1µF

200

Ω

200

Ω

0.1µF

AVDD

Ω

5pF

15pF

ADC

VIN–

VIN+

VCM

ADA4930-2

Figure 28. Differential Input Configuration Using the ADA4930-2

For baseband applications where SNR is a key parameter,

differential transformer coupling is the recommended input

configuration. An example is shown in Figure 29. To bias the

analog input, the VCM voltage can be connected to the center

tap of the secondary winding of the transformer.

2V p-p

49.9

Ω

0.1µF

ADC

VIN+

VIN–

VCM

0.1µF

Ω

Figure 29. Differential Transformer-Coupled Configuration

Consider the signal characteristics when selecting a transformer.

Most RF transformers saturate at frequencies below a few

megahertz. Excessive signal power can also cause core

saturation, which leads to distortion.

At input frequencies in the second Nyquist zone and above, the

noise performance of most amplifiers is not adequate to achieve

the true SNR performance of the AD6673. For applications where

SNR is a key parameter, differential double balun coupling is

the recommended input configuration (see Figure 30). In this

configuration, the input is ac-coupled and the VCM voltage is

provided to each input through a 33 Ω resistor. These resistors

compensate for losses in the input baluns to provide a 50 Ω

impedance to the driver.

In the double balun and transformer configurations, the value

of the input capacitors and resistors is dependent on the input

frequency and source impedance. Based on these parameters,

the value of the input resistors and capacitors may need to be

adjusted or some components may need to be removed. Table 9

displays recommended values to set the RC network for different

input frequency ranges. However, these values are dependent on

the input signal and bandwidth and should be used only as a

starting guide. Note that the values given in Table 9 are for each R1,

R2, C1, C2, and R3 components shown in Figure 29 and Figure 30.

Table 9. Example RC Network

Frequency

Range

(MHz)

Series

(Ω)

Differential

(pF)

Series

(Ω)

Shunt

(pF)

Shunt

(Ω)

0 to 100

8.2

24.9

100 to 300

3.9

8.2

24.9

ADC

0.1µF

2V p-p

VIN+

VIN–

VCM

0.1µF

Ω

0.1µF

Ω

Figure 30. Differential Double Balun Input Configuration

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