AD9224ARSZ datasheet(16/24 Pages) AD

16 / 24 page

AD9224

–16–

REV. A

REFERENCE CONFIGURATIONS

The figures associated with this section on internal and external

reference operation do not show recommended matching series

resistors for VINA and VINB for the purpose of simplicity.

Please refer to the Driving the Analog Inputs section for a dis-

cussion of this topic. Also, the figures do not show the decou-

pling network associated with the CAPT and CAPB pins.

Please refer to the Reference Operation section for a discussion

of the internal reference circuitry and the recommended decou-

pling network shown in Figure 17.

USING THE INTERNAL REFERENCE

Single-Ended Input with 0 to 2

VREF Range

Figure 26a shows how to connect the AD9224 for a 0 V to 2 V

or 0 V to 4 V input range via pin strapping the SENSE pin. An

intermediate input range of 0 to 2

× VREF can be established

using the resistor programmable configuration in Figure 28.

In either case, both the midscale voltage and input span are

directly dependent on the value of VREF. More specifically, the

midscale voltage is equal to VREF while the input span is equal

to 2

× VREF. Thus, the valid input range extends from 0 to 2 ×

VREF. When VINA is

≤ 0 V, the digital output will be 000 Hex;

when VINA is

≥ 2 × VREF, the digital output will be FFF Hex.

Shorting the VREF pin directly to the SENSE pin places the

internal reference amplifier in unity-gain mode and the resultant

VREF output is 1 V. Therefore, the valid input range is 0 V to

2 V. However, shorting the SENSE pin directly to the REFCOM

pin configures the internal reference amplifier for a gain of 2.0

and the resultant VREF output is 2.0 V. Thus, the valid input

range becomes 0 V to 4 V. The VREF pin should be bypassed to

the REFCOM pin with a 10

µF tantalum capacitor in parallel

with a low-inductance 0.1

µF ceramic capacitor.

10 F

VINA

VREF

AD9224

0.1 F

VINB

VREF

SHORT FOR 0V TO 2V

INPUT SPAN

SENSE

SHORT FOR 0V TO 4V

INPUT SPAN

REFCOM

Figure 26a. Internal Reference—2 V p-p Input Span,

VCM = 1 V, or 4 V p-p Input Span

Figure 26b illustrates the relation between reference voltage and

THD. Note that optimal performance occurs when the refer-

ence voltage is set to 1.5 V (input span = 3 V).

REFERENCE VOLTAGE – V

–60

–65

–80

1.0

1.2

1.4

1.6

1.8

2.0

2.2

–70

–75

–85

–90

Figure 26b. THD vs. Reference Voltage, FS = 40 MHz,

FIN = 10 MHz (Differential)

Figure 27 shows the single-ended configuration that gives good

dynamic performance (SINAD, SFDR). To optimize dynamic

specifications, center the common-mode voltage of the analog

input at approximately by 2.5 V by connecting VINB to a low

impedance 2.5 V source. As described above, shorting the

VREF pin directly to the SENSE pin results in a 1 V reference

voltage and a 2 V p-p input span. The valid range for input

signals is 1.5 V to 3.5 V. The VREF pin should be bypassed to

the REFCOM pin with a 10

µF tantalum capacitor in parallel

with a low-inductance 0.1

µF ceramic capacitor.

This reference configuration could also be used for a differential

input in which VINA and VINB are driven via a transformer as

shown in Figure 24. In this case, the common-mode voltage,

VCM, is set at midsupply by connecting the transformer’s center

tap to CML of the AD9224. VREF can be configured for 1.0 V or

2.0 V by connecting SENSE to either VREF or REFCOM re-

spectively. Note that the valid input range for each of the

differential inputs is one half of the single-ended input and thus

becomes VCM – VREF/2 to VCM + VREF/2.

0.1 F

10 F

VINA

VINB

VREF

SENSE

REFCOM

AD9224

3.5V

1.5V

VCM

Figure 27. Internal Reference—2 V p-p Input Span,

VCM = 2.5 V

AD9224ARSZ датащи(PDF) 16 Page - Analog Devices

AD9224ARSZ датащи(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

датащи скачать

ссылки URL