AD9225ARSZRL datasheet(17/26 Pages) AD

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AD9225

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. Refer

to the Driving the Analog Inputs and Introduction sections for a

discussion of this topic. The figures do not show the decoupling

network associated with the CAPT and CAPB pins. Refer to the

Reference Operation section for a discussion of the internal refer-

ence circuitry and the recommended decoupling network shown

in Figure 16.

USING THE INTERNAL REFERENCE

Single-Ended Input with 0 to 2 3 VREF Range

Figure 16 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 inter-

mediate input range of 0 to 2

¥ VREF can be established using the

resistor programmable configuration in Figure 19.

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 0x000; when VINA is

≥ 2 ¥ VREF, the digital output will be 0xFFF.

Shorting the VREF pin directly to the SENSE pin places the inter-

nal reference amplifier in unity-gain mode and the resulting 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

resulting VREF output is 2.0 V. Therefore, the valid input range

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

REFCOM pin with a 10

mF tantalum capacitor in parallel with a

low inductance 0.1

mF 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 17. Internal Reference—2 V p-p Input Span,

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

Figure 18 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

mF tantalum capacitor in parallel with a low-inductance

0.1

mF 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 13. 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 differ-

ential 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 18. Internal Reference—2 V p-p Input Span,

VCM = 2.5 V

Resistor Programmable Reference

Figure 19 shows an example of how to generate a reference voltage

other than 1.0 V or 2.0 V with the addition of two external 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

W to 100 kW range. For the example shown,

R1 equals 2.5 k

W and R2 equals 5 kW. 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

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. Alternatively,

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 10

mF tantalum capacitor

in parallel with a low inductance 0.1

mF ceramic capacitor.

1.5V

0.1 F

10 F

VINA

VINB

VREF

SENSE

REFCOM

AD9225

2.5V

2.5k

0.1 F

Figure 19. Resistor Programmable Reference

3 V p-p Input Span, VCM = 2.5 V

Rev. C

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