AD9243EB datasheet(13/25 Pages) AD

13 / 25 page

AD9243

REV. A

–12–

DRIVING THE ANALOG INPUTS

INTRODUCTION

The AD9243 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 may be

appropriate for many 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 AD9243.

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.

Alternatively, the differential mode of operation provides the

best THD and SFDR performance over a wide frequency range.

A transformer coupled differential input should be considered

for the most demanding spectral-based applications which allow

ac coupling (e.g., Direct IF to Digital Conversion). The dc-

coupled differential mode of operation also provides an enhance-

ment in distortion and noise performance at higher input spans.

Furthermore, it allows the AD9243 to be configured for a 5 V

span using op amps specified for +5 V or

±5 V operation.

Single-ended operation requires that VINA be ac or dc coupled

to the input signal source while VINB of the AD9243 be biased

to the appropriate voltage corresponding to a midscale code

transition. Note that signal inversion may be easily accom-

plished by transposing VINA and VINB.

Differential operation requires that VINA and VINB be simulta-

neously driven with two equal signals that are in and out of

phase versions of the input signal. Differential operation of the

AD9243 offers the following benefits: (1) Signal swings are

smaller and therefore linearity requirements placed on the input

signal source may be easier to achieve, (2) Signal swings are

smaller and therefore may allow the use of op amps which may

otherwise have been constrained by headroom limitations, (3)

Differential operation minimizes even-order harmonic products,

and (4) Differential operation offers noise immunity based on

the device’s common-mode rejection as shown in Figure 16.

As is typical of most CMOS devices, exceeding the supply limits

will turn on internal parasitic diodes resulting in transient cur-

rents within the device. Figure 28 shows a simple means of

clamping a dc coupled input with the addition of two series

resistors and two diodes. Note that a larger series resistor could

be used to limit the fault current through D1 and D2 but should be

evaluated since it can cause a degradation in overall performance.

AVDD

RS1

VCC

VEE

1N4148

RS2

AD9243

Figure 28. Simple Clamping Circuit

DIFFERENTIAL MODE OF OPERATION

Since not all applications have a signal preconditioned for

differential operation, there is often a need to perform a single-

ended-to-differential conversion. A single-ended-to-differential

conversion can be realized with an RF transformer or a dual op

amp differential driver. The optimum method depends on

whether the application requires the input signal to be ac or dc

coupled to AD9243.

AC Coupling via an RF Transformer

In applications that do not need to be dc coupled, an RF trans-

former with a center tap is the best method to generate differen-

tial inputs for the AD9243. It provides all the benefits of

operating the A/D in the differential mode without contributing

additional noise or distortion. An RF transformer has the added

benefit of providing electrical isolation between the signal source

and the A/D.

Figure 29 shows the schematic of the suggested transformer

circuit. The circuit uses a Mini-Circuits RF transformer, model

#T4-6T, which has an impedance ratio of four (turns ratio of

2). The schematic assumes that the signal source has a 50

Ω

source impedance. The 1:4 impedance ratio requires the 200

Ω

secondary termination for optimum power transfer and VSWR.

The centertap of the transformer provides a convenient means

of level shifting the input signal to a desired common-mode

voltage. Optimum performance can be realized when the centertap

is tied to CML of the AD9243 which is the common-mode bias

level of the internal SHA.

VINA

CML

VINB

AD9243

0.1 F

200

MINI-CIRCUITS

T4-6T

Figure 29. Transformer Coupled Input

Transformers with other turns ratios may also be selected to

optimize the performance of a given application. For example, a

given input signal source or amplifier may realize an improve-

ment in distortion performance at reduced output power levels

and signal swings. Hence, selecting a transformer with a higher

impedance ratio (i.e., Mini-Circuits T16-6T with a 1:16 imped-

ance ratio) effectively “steps up” the signal level, further reduc-

ing the driving requirements of the signal source.

AD9243EB 数据表(PDF) 13 Page - Analog Devices

AD9243EB 数据表(HTML) 13 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 25

数据表下载

链接网址

AD9243EB 数据表(PDF) 13 Page - Analog Devices

AD9243EB 数据表(HTML) 13 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 25

数据表 下载

链接网址

数据表下载