REV. 0

AD9203

REV. 0

–13–

DIFFERENTIAL MODE OF OPERATION

Since not all applications have a signal preconditioned for differ-

ential operation, there is often a need to perform a single-ended-

to-differential conversion. In systems that do not need a dc

input, an RF transformer with a center tap is a method to gener-

ate differential inputs beyond 20 MHz for the AD9203. This

provides all the benefits of operating the A/D in the differential

mode without contributing additional noise or distortion. An RF

transformer also has the added benefit of providing electrical isola-

tion between the signal source and the A/D.

An improvement in THD and SFDR performance can be real-

ized by operating the AD9203 in differential mode. The perfor-

mance enhancement between the differential and single-ended

mode is most considerable as the input frequency approaches

49.9

0.1 F

10k

523

499

10k

20pF

49.9

20pF

499

499

0.1 F

AINP

+3V

DRVDD

AVDD

AVSS DRVSS

AINN

0.1 F

0.1 F

DIGITAL

OUTPUTS

+3V

AD9203

AD8138

49.9

10 F10 F

Figure 26. AD8138 Driving an AD9203, a 10-Bit, 40 MSPS

A/D Converter

The AD8138 provides a convenient method of converting a

single-ended signal to a differential signal. This is an ideal

method for generating a direct coupled signal to the AD9203.

The AD8138 will accept a signal and shift it to an externally

provided common-mode level. The AD8138 configuration is

shown in Figure 26.

Figure 27 shows the schematic of a suggested transformer

circuit. The circuit uses a Minicircuits RF transformer, model

number T4–1T, which has an impedance ratio of four (turns

ratio of 2).

The center tap of the transformer provides a convenient means

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

voltage. Figure 28 illustrates the performance of the AD9203

over a wide range of common-mode levels.

AINP

AINN

VREF

REFSENSE

2V

1V

AD9203

0.1 F

10 F

Figure 27. Transformer Coupled Input

Transformers with other turns ratios may also be selected to

optimize the performance of a given application. For example,

selecting a transformer with a higher impedance ratio (e.g.,

Minicircuits T16–6T with a impedance ratio of 16) effectively

“steps up” the signal amplitude, thus further reducing the driv-

ing requirements of the signal source.

The AD9203 can be easily configured for either a 1 V p-p input

span or 2 V p-p input span by setting the internal reference.

Other input spans can be realized with two external gain setting

resistors as shown in Figure 19 of this data sheet. Figures 32

and 33 demonstrate the SNR and SFDR performance over a

wide range of amplitudes required by most communication

applications.

–80

0

–70

–60

–50

–40

–30

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1.0V REF

0.5V REF

COMMON-MODE VOLTAGE – Volts

Figure 28. THD vs. Common-Mode Voltage vs. THD (AIN =

–90

40.0

–80

–70

–60

–50

–40

42.5

45.0

47.5

50.0

52.5

55.0

57.5

60.0

THD

SNR

DUTY CYCLE – %

Differential, Clock = 40 MSPS)