AD9244 datasheet(18/36 Pages) AD

18 / 36 page

REV. A

–18–

AD9244

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 9a

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.

REFT

REFB

VREF

SENSE

REFGND

AD9244

ADC

2.5V

LOGIC

DISABLE

Figure 9a. 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 comparators that

monitor the voltage at the SENSE pin. The various reference

modes are summarized in Table II 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 9b 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.

10 F

0.1 F

0.1 F*

10 F

VREF

REFT

REFB

AD9244

*LOCATE AS CLOSE AS POSSIBLE TO REFT/REFB PINS

REFGND

Figure 9b. Reference Decoupling

Pin Programmable Reference

By shorting the VREF pin directly to the SENSE pin, the internal

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 2.0 and the resulting VREF output is 2.0 V.

Resistor Programmable Reference

Figure 10 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

R R

=× +

()

1 2

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

should be in the 2 k

Ω to 10 kΩ range. For the example shown,

R1 equals 2.5 k

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

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.

10 F

0.1 F

10 F

0.1 F

REFT

REFB

VREF

SENSE

REFGND

AD9244

VIN+

VIN–

3.25V

1.75V

2.5V

1.5V

20pF

2.5k

Figure 10. Resistor Programmable Reference (1.5 V p-p

Input Span, Differential Input with VCM = 2.5 V)

Using an External Reference

To use an external reference, the internal reference must be

disabled by connecting the SENSE pin to AVDD. The AD9244

contains an internal reference buffer, A2 (see Figure 9a), that

simplifies the drive requirements of an external reference. The

external reference must be able to drive a 5 k

Ω (±20%) load. The

bandwidth of the reference is deliberately left small to minimize

the reference noise contribution. As a result, it is not possible to

drive VREF externally with high frequencies.

Figure 11 shows an example of an external reference driving both

VIN– and VREF. In this case, both the common-mode voltage

and input span are directly dependent on the value of VREF.

Both the input span and the center of the input span are equal

to the external VREF. Thus the valid input range extends from

(VREF + VREF/2) to (VREF – VREF/2). For example, if the

precision reference part, REF191, a 2.048 V external reference,

is used, the input span is 2.048 V. In this case, 1 LSB of the

AD9244 corresponds to 0.125 mV. It is essential that a minimum

of a 10

µF capacitor, in parallel with a 0.1 µF low inductance

ceramic capacitor, decouple the reference output to AGND.

VIN+

VREF

VIN–

SENSE

20pF

VREF + VREF/2

REFT

REFB

VREF – VREF/2

VREF

0.1 F

AVDD

0.1 F

10 F

AD9244

Figure 11. Using an External Reference

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