AD9241 查看數據表（PDF） - Analog Devices

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AD9241

Complete 14-Bit, 1.25 MSPS Monolithic A/D Converter

Analog Devices 

AD9241

REFERENCE OPERATION

The AD9241 contains an onboard bandgap reference that pro-

vides a pin-strappable option to generate either a 1 V or 2.5 V

output. With the addition of two external resistors, the user can

generate reference voltages other than 1 V and 2.5 V. Another

alternative is to use an external reference for designs requiring

enhanced accuracy and/or drift performance. See Table II for a

summary of the pin-strapping options for the AD9241 reference

configurations.

Figure 26 shows a simplified model of the internal voltage refer-

ence of the AD9241. A pin-strappable reference amplifier buff-

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

A1, appears on the VREF pin. The voltage on the VREF pin

determines the full-scale input span of the A/D. This input span

equals,

Full-Scale Input Span = 2 × VREF

TO

A/D

5kΩ

AD9241

5kΩ

A2

5kΩ

DISABLE

A2

5kΩ

LOGIC

A1

1V

DISABLE

A1

LOGIC

CAPT

CAPB

VREF

7.5kΩ

SENSE

5kΩ

REFCOM

Figure 26. Equivalent Reference Circuit

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

internal reference amplifier, A1, are determined by the voltage

appearing at the SENSE pin. The logic circuitry contains two

comparators that monitor the voltage at the SENSE pin. The

comparator with the lowest set point (approximately 0.3 V)

controls the position of the switch within the feedback path of

A1. If the SENSE pin is tied to REFCOM, the switch is

connected to the internal resistor network thus providing a

VREF of 2.5 V. If the SENSE pin is tied to the VREF pin via a

short or resistor, the switch is connected to the SENSE pin. A

short will provide a VREF of 1.0 V while an external resistor

network will provide an alternative VREF between 1.0 V and

2.5 V. The second comparator controls internal circuitry that

will disable the reference amplifier if the SENSE pin is tied to

AVDD. Disabling the reference amplifier allows the VREF pin

to be driven by an external voltage reference.

The actual reference voltages used by the internal circuitry of

the AD9241 appear on the CAPT and CAPB pins. For proper

operation when using the internal or an external reference, it is

necessary to add a capacitor network to decouple these pins.

Figure 27 shows the recommended decoupling network. This

capacitive network performs the following three functions: (1) in

conjunction with the reference amplifier, A2, it provides a low

source impedance over a large frequency range to drive the A/D

internal circuitry, (2) it provides the necessary compensation for

A2, and (3) it bandlimits the noise contribution from the refer-

ence. The turn-on time of the reference voltage appearing be-

tween CAPT and CAPB is approximately 15 ms and should be

evaluated in any power-down mode of operation.

CAPT

AD9241

0.1µF

0.1µF

10µF

CAPB

0.1µF

Figure 27. Recommended CAPT/CAPB Decoupling Network

The A/D’s input span may be varied dynamically by changing

the differential reference voltage appearing across CAPT and

CAPB symmetrically around 2.5 V (i.e., midsupply). To change

the reference at speeds beyond the capabilities of A2, it will be

necessary to drive CAPT and CAPB with two high speed, low

noise amplifiers. In this case, both internal amplifiers (i.e., A1

and A2) must be disabled by connecting SENSE to AVDD and

VREF to REFCOM, and the capacitive decoupling network

removed. The external voltages applied to CAPT and CAPB

must be 2.5 V + Input Span/4 and 2.5 V – Input Span/4, respec-

tively where the input span can be varied between 2 V and 5 V.

Note that those samples within the pipeline A/D during any

reference transition will be corrupted and should be discarded.

Reference

Operating Mode

INTERNAL

INTERNAL

INTERNAL

EXTERNAL

(NONDYNAMIC)

EXTERNAL

(DYNAMIC)

Table II. Reference Configuration Summary

Input Span (VINA–VINB)

(V p-p)

Required VREF (V)

Connect

2

5

2 ≤ SPAN ≤ 5 AND

SPAN = 2 × VREF

2 ≤ SPAN ≤ 5

2 ≤ SPAN ≤ 5

1

2.5

1 ≤ VREF ≤ 2.5 AND

VREF = (1 + R1/R2)

1 ≤ VREF ≤ 2.5

CAPT and CAPB

Externally Driven

SENSE

SENSE

R1

R2

SENSE

VREF

SENSE

VREF

EXT. REF. 1

EXT. REF. 2

To

VREF

REFCOM

VREF AND SENSE

SENSE AND REFCOM

AVDD

EXT. REF.

AVDD

REFCOM

CAPT

CAPB

REV. 0

–11–

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