AD9244 datasheet(21/37 Pages) AD

21 / 37 page

AD9244

Rev. C | Page 20 of 36

The circuit in Figure 47 shows a method for applying a differential,

direct-coupled signal to the AD9244. An AD8138 amplifier is used

to derive a differential signal from a single-ended signal.

REFT

REFB

AD9244

VIN+

AVDD

VIN–

20pF

0.1

μF

0.1

μF

0.1

μF

499

AD8138

499

475

1V p-p

μF

0.1

μF

0.1

μF

Figure 47. Direct-Coupled Drive Circuit with AD8138 Differential Op Amp

REFERENCE OPERATION

The AD9244 contains a band gap reference that provides a pin-

strappable option to generate either a 1 V or 2 V output. With

the addition of two external resistors, the user can generate

reference voltages between 1 V and 2 V. Another alternative is

to use an external reference for designs requiring enhanced

accuracy and/or drift performance, as described later in this

section. Figure 48 shows a simplified model of the internal

voltage reference of the AD9244. A reference amplifier buffers a

1 V fixed reference. The output from the reference amplifier,

A1, appears on the VREF pin. As stated earlier, the voltage on

the VREF pin determines the full-scale differential input span

of the ADC.

ADC

REFT

REFB

VREF

SENSE

REFGND

AD9244

2.5V

LOGIC

DISABLE

Figure 48. Equivalent Reference Circuit

The voltage appearing at the VREF pin and the state of the

internal reference amplifier, A1, are determined by the voltage

present at the SENSE pin. The logic circuitry contains compara-

tors that monitor the voltage at the SENSE pin. The various

reference modes are summarized in Table 9 and are described

in the next few sections.

The actual reference voltages used by the internal circuitry of

the AD9244 appear on the REFT and REFB pins. The voltages

on these pins are symmetrical about midsupply or CML. For

proper operation, it is necessary to add a capacitor network to

decouple these pins. Figure 49 shows the recommended

decoupling network. The turn-on time of the reference voltage

appearing between REFT and REFB is approximately 10 ms and

should be taken into consideration in any power-down mode of

operation. The VREF pin should be bypassed to the REFGND

pin with a 10 μF tantalum capacitor in parallel with a low

inductance 0.1 μF ceramic capacitor.

μF

0.1

μF

0.1

μF

0.1

μF

0.1

μF1

μF

VREF

REFT

REFB

AD9244

1LOCATE AS CLOSE AS POSSIBLE TO REFT/REFB PINS.

REFGND

Figure 49. Reference Decoupling

Pin-Programmable Reference

By shorting the VREF pin directly to the SENSE pin, the inter-

nal reference amplifier is placed in a unity gain mode, and the

resulting VREF output is 1 V. By shorting the SENSE pin directly to

the REFGND pin, the internal reference amplifier is configured

for a gain of 2, and the resulting VREF output is 2 V.

Resistor-Programmable Reference

Figure 50 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. Use the equation

VREF = 1 V × (1 + R1/R2)

(5)

to determine the appropriate values for R1 and R2. These resistors

should be in the 2 kΩ to 10 kΩ range. For the example shown, R1

equals 2.5 kΩ and R2 equals 5 kΩ. From the previous equation, the

resulting reference voltage on the VREF pin is 1.5 V. This sets the

differential input span to 1.5 V p-p. The midscale voltage can also

be set to VREF by connecting VIN− to VREF.

2.5k

VREF

SENSE

AD9244

VIN+

VIN–

20pF

1.5V

REFGND

3.25V

1.75V

μF

0.1

μF

2.5V

REFT

REFB

0.1

μF

0.1

μF

0.1

μF

Figure 50. Resistor-Programmable Reference

(1.5 V p-p Input Span, Differential Input with VCM = 2.5 V)

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