AD9225 datasheet(10/24 Pages) AD

10 / 24 page

AD9225

–10–

REV. A

REFERENCE OPERATION

The AD9225 contains an onboard bandgap reference that pro-

vides 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 other than 1 V and 2 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 AD9225 reference

configurations.

Figure 16 shows a simplified model of the internal voltage refer-

ence of the AD9225. A pin strappable reference amplifier

buffers 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

The voltage appearing at the VREF pin as well as the state of

the internal reference amplifier, A1, are determined by the volt-

age appearing at the SENSE pin. The logic circuitry contains

two comparators which 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 AVSS (AGND), the switch is

connected to the internal resistor network thus providing a

VREF of 2.0 V. If the SENSE pin is tied to the VREF pin via a

short or resistor, the switch will connect to the SENSE pin. This

short will provide a VREF of 1.0 V. An external resistor network

will provide an alternative VREF between 1.0 V and 2.0 V. The

other comparator controls internal circuitry which will disable

the reference amplifier if the SENSE pin is tied AVDD. Dis-

abling the reference amplifier allows the VREF pin to be driven

by an external voltage reference.

LOGIC

DISABLE

6.25k

LOGIC

DISABLE

A/D

AD9225

CAPT

CAPB

VREF

SENSE

REFCOM

6.25k

Figure 16. Equivalent Reference Circuit

The actual reference voltages used by the internal circuitry of

the AD9225 appears 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 17 shows the recommended decoupling network. This

capacitive network performs the following three functions: (1)

along with the reference amplifier, A2, it provides a low source

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

nal circuitry, (2) it provides the necessary compensation for A2,

and (3) it bandlimits the noise contribution from the reference.

The turn-on time of the reference voltage appearing between

CAPT and CAPB is approximately 15 ms and should be evalu-

ated in any power-down mode of operation.

0.1 F

10 F

0.1 F

CAPT

CAPB

AD9225

Figure 17. 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,

connecting VREF to AVSS and removing the capacitive decou-

pling network. The external voltages applied to CAPT and

CAPB must be 2.0 V + Input Span/4 and 2.0 V – Input Span/4

respectively in which the input span can be varied between 2 V

and 4 V. Note that those samples within the pipeline A/D dur-

ing any reference transition will be corrupted and should be

discarded.

DRIVING THE ANALOG INPUTS

The AD9225 has a highly flexible input structure allowing it to

interface with single ended or differential input interface cir-

cuitry. The applications shown in sections “Driving the Analog

Inputs” and “Reference Configurations” along with the infor-

mation presented in “Input and Reference Overview” of this

data sheet, give examples of both single-ended and differential

operation. Refer to Tables I and II for a list of the different

possible input and reference configurations and their associated

figures in the data sheet.

The optimum mode of operation, analog input range, and asso-

ciated interface circuitry will be determined by the particular

applications performance requirements as well as power supply

options. For example, a dc coupled single-ended input would be

appropriate for most data acquisition and imaging applications.

Also, many communication applications which require a dc

coupled input for proper demodulation can take advantage of

the excellent single-ended distortion performance of the AD9225.

The input span should be configured such that the system’s

performance objectives and the headroom requirements of the

driving op amp are simultaneously met.

AD9225 Datasheet(PDF) 10 Page - Analog Devices

AD9225 Datasheet(HTML) 10 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL