AD9652BBCZ-310 datasheet(23/37 Pages) AD

23 / 37 page

AD9652

Data Sheet

Rev. B | Page 22 of 36

For baseband applications where SNR is a key parameter,

differential transformer coupling is the recommended input

configuration. An example is shown in Figure 54. 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

49.9Ω

0.1µF

ADC

VIN+x

VIN–x

VCM

0.1µF

33Ω

Figure 54. 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

the true SNR performance of the AD9652. For applications

where SNR is a key parameter, differential double balun coupling is

the recommended input configuration (see Figure 56). 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 11

displays recommended values to set the RC network for different

input frequency ranges. However, these values are dependent on

the input signal; use the bandwidth only as a starting guide.

Note that the values given in Table 11 are for each R1, R2, C1,

C2, and R3 component shown in Figure 54 and Figure 56.

Table 11. Example RC Network

Frequency

Range

(MHz)

Series

(Ω)

Differential

(pF)

Series

(Ω)

Shunt

(pF)

Shunt

(Ω)

0 to 100

Open

49.9

100 to 300

Open

2.7

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 or AD8376 digital variable gain

amplifier (DVGA) provides good performance for driving the

AD9652. Figure 55 shows an example of the AD8376 driving

the AD9652 through a band-pass antialiasing filter.

AD8376

AD9652

1µH

1nF

VPOS

VCM

15pF

68nH

54kΩ║2.9pF

301Ω

165Ω

5.1pF

3.9pF

180nH

1000pF

NOTES

1. ALL INDUCTORS ARE COILCRAFT® 0603CS COMPONENTS WITH THE

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

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

CENTERED AT 140MHz.

180nH

220nH

Figure 55. Differential Input Configuration Using the AD8376

ADC

0.1µF

2V p-p

VIN+x

VIN–x

0.1µF

33Ω

0.1µF

33Ω

VCM

Figure 56. Differential Double Balun Input Configuration

Table 12. VREF Configuration Options

Selected Mode

SENSE Voltage

Resulting ADC Reference Voltage (V)

Resulting Input Span (Differential V p-p)

External Reference

AVDD

N/A1

2 × external reference

Internal Fixed Reference

GND

VREF2

2 × VREF2

1 N/A means not applicable.

2 VREF is set via Register 0x18. The default VREF is 1.25 V.

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