AD9262EBZ datasheet(18/33 Pages) AD

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AD9262

Rev. A | Page 17 of 32

Input Common Mode

The analog inputs of the AD9262 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 44). With 0 V differential input,

the driver sources 1 mA into each analog input.

TO LOOP FILTER

STAGE 2

DAC

AVDD – 0.5V

500Ω

VIN+x

VIN–x

FROM QUANTIZER

VCM =AVDD

VIN p-p = 2V

Figure 44. Input Common Mode

Differential Input Configurations

The AD9262 can also be configured for differential inputs. The

ADA4937-2 differential driver provides excellent performance

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

voltage of the ADA4937-2 is easily set by connecting AVDD to

the VOCM2 pin of the ADA4937-2 (see Figure 45). The noise and

linearity of the ADA4937-2 need important consideration because

the system performance may be limited by the ADA4937-2.

200Ω

60.4Ω

49.9Ω

50Ω

SIGNAL

SOURCE

2V p-p

60.4

VIN–x

VIN+x

0.1µF

+5V

AVDD

–5V

+1.8V

ADA4937-2

AD9262

VOCM2

Figure 45. Differential Input Configuration Using the ADA4937-2

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 46. 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.

50Ω

SIGNAL

SOURCE

2V p-p

1:1

50Ω

VIN–x

VIN+x

0.1µF

AVDD

AD9262

Figure 46. Differential Transformer Configuration

Voltage Reference

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

AD9262. 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 47). 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

500Ω

TO LOOP

FILTER

STAGE 2

500Ω

VIN+x

VCM =AVDD

VIN p-p = 2V

VIN–x

500Ω

10kΩ

10µF

0.5V

VREF

10µF

REF

Figure 47. Voltage Reference Loop

Internal Reference Connection

To minimize thermal noise, the internal reference on the AD9262

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 48). The unbuffered reference should not

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

used by default and when Serial Register 0x18[6] is reset.

10kΩ

2.85kΩ

8.5kΩ

3.5kΩ

0.5V

TO CURRENT

GENERATOR

10µF

Figure 48. Internal Reference Configuration

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