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AD9225

–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 section “Driving the Analog Inputs, Introduc-

tion” for a discussion of this topic. Also, the figures do not show

the decoupling network associated with the CAPT and CAPB

pins. Please refer to the section “Reference Operation” for a

discussion of the internal reference circuitry and the recom-

mended decoupling network shown in Figure 17.

USING THE INTERNAL REFERENCE

Single-Ended Input with 0 to 2

VREF Range

Figure 26 shows how to connect the AD9225 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

AD9225

0.1 F

VINB

VREF

SHORT FOR 0V TO 2V

INPUT SPAN

SENSE

SHORT FOR 0V TO 4V

INPUT SPAN

REFCOM

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

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

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 REF-

COM 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 AD9225. VREF can be configured

for 1.0 V or 2.0 V by connecting SENSE to either VREF or

REFCOM respectively. 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

AD9225

3.5V

1.5V

VCM

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

VCM = 2.5 V

Resistor Programmable Reference

Figure 28 shows an example of how to generate a reference

voltage other than 1.0 V or 2.0 V with the addition of two exter-

nal resistors and a bypass capacitor. Use the equation,

VREF = 1 V

× (1 + R1/R2)

to determine appropriate values for R1 and R2. These resistors

should be in the 2 k

Ω to 100 kΩ range. For the example shown,

R1 equals 2.5 k

Ω and R2 equals 5 kΩ. From the equation above,

the resultant reference voltage on the VREF pin is 1.5 V. This

sets the input span to be 3 V p-p. To assure stability, place a

0.1

µF ceramic capacitor in parallel with R1.

The midscale voltage can be set to VREF by connecting VINB

to VREF to provide an input span of 0 to 2

× VREF. Alterna-

tively, the midscale voltage can be set to 2.5 V by connecting

VINB to a low impedance 2.5 V source. For the example

shown, the valid input single-ended range for VINA is 1 V to

4 V since VINB is set to an external, low impedance 2.5 V source.

The VREF pin should be bypassed to the REFCOM pin with a

µF tantalum capacitor in parallel with a low inductance

0.1

µF ceramic capacitor.

1.5V

0.1 F

10 F

VINA

VINB

VREF

SENSE

REFCOM

AD9225

2.5V

2.5k

0.1 F

Figure 28. Resistor Programmable Reference +3 V p-p

Input Span, VCM = 2.5 V

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