AD9261 datasheet(16/29 Pages) AD

16 / 29 page

AD9261

Rev. 0 | Page 15 of 28

Input Common Mode

The analog inputs of the AD9261 are not internally dc biased. In

ac-coupled applications, the user must provide this bias externally.

Setting the device such that VCM = AVDD is recommended for

optimum performance. The analog inputs are 500 Ω resistors,

and the internal reference loop aims to develop 0.5 V across

each input resistor (see Figure 36). With 0 V differential input,

the driver sources 1 mA into each analog input.

TO LOOP FILTER

STAGE 2

DAC

AVDD – 0.5V

500

Ω

500

Ω

VIN+

VIN–

FROM QUANTIZER

VCM =AVDD

VIN p-p = 2V

Figure 36. Input Common Mode

Differential Input Configurations

The AD9261 can also be configured for differential inputs. The

ADA4937-1 differential driver provides excellent performance

and a flexible interface to the ADC. The output common-mode

voltage of the ADA4937-1 is easily set by connecting AVDD to

the VOCM pin of the ADA4937-1 (see Figure 37). The noise and

linearity of the ADA4937-1 needs important consideration because

the system performance may be limited by the ADA4937-1.

200

Ω

200

Ω

200

Ω

60.4

Ω

49.9

Ω

SIGNAL

SOURCE

2V p-p

60.4

Ω

VIN–

VIN+

0.1µF

+5V

AVDD

–5V

+1.8V

ADA4937-1

AD9261

VOCM

Figure 37. Differential Input Configuration Using the ADA4937-1

For frequencies offset from dc, where SNR is a key parameter,

differential transformer coupling is the recommended input

configuration. An example is shown in Figure 38. The center

tap of the secondary winding of the transformer is connected to

AVDD to bias the analog input.

The signal characteristics must be considered when selecting a

transformer. Most RF transformers saturate at frequencies

below a couple of megahertz (MHz), and excessive signal power

can cause core saturation, which leads to distortion.

Ω

SIGNAL

SOURCE

2V p-p

1:1

Ω

VIN–

VIN+

0.1µF

AVDD

AD9261

Figure 38. Differential Transformer Configuration

Voltage Reference

A stable and accurate 0.5 V voltage reference is built into the

AD9261. The reference voltage should be decoupled to minimize

the noise bandwidth using a 10 μF capacitor. The reference is

used to generate a bias current into a matched resistor such that,

when used to bias the current in the feedback DAC, a voltage

of AVDD − 0.5 V is developed at the internal side of the input

resistors (see Figure 39). The current bias circuit should also be

decoupled on the CFILT pin with a 10 μF capacitor. For this

reason, the VREF voltage should always be 0.5 V.

AVDD

AVDD – 0.5V

CFILT

AVDD – 0.5V

500

Ω

TO LOOP

FILTER

STAGE 2

500

Ω

VIN+

VCM =AVDD

VIN p-p = 2V

VIN–

500

Ω

10k

Ω

10µF

0.5V

VREF

10µF

REF

Figure 39. Voltage Reference Loop

Internal Reference Connection

To minimize thermal noise, the internal reference on the AD9261

is an unbuffered 0.5 V. It has an internal 10 kΩ series resistor,

which, when externally decoupled with a 10 μF capacitor, limits

the noise (see Figure 40). The unbuffered reference should not

be used to drive any external circuitry. The internal reference is

used by default.

10k

Ω

2.85k

Ω

8.5k

Ω

3.5k

Ω

0.5V

TO CURRENT

GENERATOR

10µF

Figure 40. Internal Reference Configuration

AD9261 Datasheet(PDF) 16 Page - Analog Devices

AD9261 Datasheet(HTML) 16 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Datasheet Download

Link URL