AD9240

REV. A

–13–

DRIVING THE ANALOG INPUTS

INTRODUCTION

The AD9240 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 informa-

tion presented in the Input and Reference Overview section of

this data sheet, give examples of both single-ended and differen-

tial 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 AD9240.

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 AD9240 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 AD9240 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

AD9240 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 prod-

ucts 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 31 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

30

D2

1N4148

D1

1N4148

20

AD9240

Figure 31. 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 AD9240.

AC Coupling via an RF Transformer

An RF transformer with a center tap can be used to generate

differential inputs for the AD9240. It provides all of the benefits

of operating the ADC in the differential mode while contribut-

ing no additional noise and minimal distortion. As a result, an

RF transformer is recommended in high frequency applica-

tions, especially undersampling, in which the performance of

a dual op amp differential driver may not be adequate. An RF

transformer has the added benefit of providing electrical isola-

tion between the signal source and the ADC. However, since the

lower cutoff frequency of most RF transformers is nominally a

few 100 kHz, a dual op amp differential driver may be more suit-

able in ac-coupling applications, where the spectral content of the

input signal falls below the cutoff frequency of a suitable RF

transformer.

Figure 32 is a suggested transformer circuit using a Mini-

Circuits RF transformer, model #T4-6T, which has an imped-

ance ratio of four (turns ratio of 2). 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 de-

sired common-mode voltage. Optimum performance can be

realized when the centertap is tied to CML of the AD9240

which is the common-mode bias level of the internal SHA.

VINA

CML

VINB

AD9240

0.1 F

200

MINI-CIRCUITS

T4-6T

50

Figure 32. 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.