AD9203ARURL7 datasheet(16/29 Pages) AD

16 / 29 page

AD9203

Rev. B | Page 15 of 28

AD8051:

f–3 dB = 110 MHz. Low cost. Best used for driving

single-ended ac-coupled configuration. Operates on a 3 V

power rail.

AD8052:

Dual Version of above amp.

AD8138

is a higher performance version of AD8131. Its gain is

programmable and provides 14-bit performance.

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. In systems that do not need a

dc input, an RF transformer with a center tap is one method to

generate 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 benefit of providing

electrical isolation between the signal source and the A/D.

An improvement in THD and SFDR performance can be

realized by operating the AD9203 in differential mode. The

performance enhancement between the differential and single-

ended mode is greatest as the input frequency approaches and

goes beyond the Nyquist frequency (i.e., fIN > FS/2).

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 28.

49.9

Ω

10k

Ω

523

Ω

499

Ω

10k

Ω

20pF

49.9

Ω

20pF

499

Ω

499

Ω

AINP

DRVDD

AVDD

AVSS DRVSS

AINN

DIGITAL

OUTPUTS

AD9203

49.9

Ω

AD8138

µF

0.1

µF

0.1

µF

0.1

µF

0.1

µF

Figure 28. AD8138 Driving an AD9203, a 10-Bit, 40 MSPS A/D Converter

Figure 29 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).

AINP

AINN

VREF

REFSENSE

AD9203

0.1

µF

Figure 29. Transformer Coupled Input

The center tap of the transformer provides a convenient means

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

voltage. Figure 30 illustrates the performance of the AD9203

over a wide range of common-mode levels.

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, such as

minicircuits T16–6T with an impedance ratio of 16, effectively

steps up the signal amplitude, thus further reducing the driving

requirements of the signal source.

The AD9203 can be easily configured for either a 1 V p-p 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 21 of this data sheet. Figure 34 and Figure 35

demonstrate the SNR and SFDR performance over a wide range

of amplitudes required by most communication applications.

–30

–40

–50

–60

–70

–80

0.5

1.0

1.5

2.0

2.5

3.0

3.5

COMMON-MODE VOLTAGE (V)

1.0V REF

0.5V REF

Figure 30. THD vs. Common-Mode Voltage vs. THD

(AIN = 2 V Differential) (fIN = 5 MHz, fS = 40 MSPS)

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