AD6672 Ver la hoja de datos (PDF) - Analog Devices

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componentes Descripción

Fabricante

AD6672

IF Receiver

Analog Devices 

Data Sheet

AD6672

Differential Input Configurations

Optimum performance can be achieved when driving the AD6672

in a differential input configuration. For baseband applications, the

AD8138, ADA4937-1, and ADA4930-1 differential drivers provide

excellent performance and a flexible interface to the ADC.

The output common-mode voltage of the ADA4930-1 is easily

set with the VCM pin of the AD6672 (see Figure 25), and the

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

provide band-limiting of the input signal.

15pF

VIN

76.8Ω

0.1µF

200Ω

90Ω

33Ω

15Ω

5pF

ADA4930-1

120Ω

200Ω

33Ω

15Ω

15pF

VIN– AVDD

ADC

VIN+

VCM

0.1µF

Figure 25. Differential Input Configuration Using the ADA4930-1

For baseband applications where SNR is a key parameter,

differential transformer coupling is the recommended input

configuration. An example is shown in Figure 26. To bias the

analog input, connect the VCM voltage to the center tap of the

secondary winding of the transformer.

2V p-p

49.9Ω

C2

R3

R2

VIN+

R1

C1

ADC

R1

R2

VIN–

VCM

the true SNR performance of the AD6672. For applications where

SNR is a key parameter, differential double balun coupling is

the recommended input configuration (see Figure 28). In this

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

provided to the input through a 33 Ω resistor. This resistor

compensates 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, C2, and R3 component shown in Figure 26

and Figure 28.

Table 9. Example RC Network

Frequency

Range

(MHz)

R1

C1

Series Differential

(Ω) (pF)

0 to 100 33

8.2

100 to 300 15

3.9

R2

Series

(Ω)

0

0

C2

Shunt

(pF)

15

8.2

R3

Shunt

(Ω)

49.9

49.9

An alternative to using a transformer-coupled input at

frequencies in the second Nyquist zone is to use an amplifier

with variable gain. The AD8375 digital variable gain amplifier

(DVGA) provides good performance for driving the AD6672.

Figure 27 shows an example of the AD8375 driving the AD6672

through a band-pass antialiasing filter.

1000pF 180nH 220nH

0.1µF

R3

C2

0.1µF

Figure 26. Differential Transformer-Coupled Configuration

The signal characteristics must be considered 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

2V p-p

0.1µF

PA

SS

0.1µF

33Ω

P

0.1µF 33Ω

1µH

AD8375

1µH

VPOS

301Ω

1nF

165Ω

5.1pF 3.9pF

165Ω

15pF

VCM

1nF

AD6672

2.5kΩ║2pF

68nH

NOTES

1000pF 180nH 220nH

1. ALL INDUCTORS ARE COILCRAFT® 0603CS COMPONENTS

WITH THE EXCEPTION OF THE 1µH CHOKE INDUCTORS (0603LS).

2. FILTER VALUES SHOWN ARE FOR A 20MHz BANDWIDTH FILTER

CENTERED AT 140MHz.

Figure 27. Differential Input Configuration Using the AD8375

C2

R3

R1

R2

VIN+

C1

0.1µF

R1

R2

ADC

VIN–

VCM

R3

C2

Figure 28. Differential Double Balun Input Configuration

Rev. C | Page 17 of 30

0.1µF

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