AD9225ARS datasheet(15/24 Pages) AD

15 / 24 page

AD9225

–15–

REV. A

Figure 24 shows the schematic of the suggested transformer

circuit. The circuit uses a minicircuits RF transformer, model

#T4-1T, 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 center tap of the transformer provides a convenient means

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

voltage.

VINA

VINB

AD9225

200

49.9

CML

MINICIRCUITS

T4-1T

0.1 F

Figure 24. Transformer Coupled Input

This (Figure 24) configuration was used to gather the differen-

tial data on pages 2 and 3.

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. For example, selecting a transformer with a

higher impedance ratio (e.g., Minicircuits T16-6T with a 1:16

impedance ratio) effectively “steps up” the signal level thus

further reducing the driving requirements of the signal source.

Referring to Figure 24, a series resistors, RS, and shunt capaci-

tor, CS, were inserted between the AD9225 and the secondary

of the transformer. The value of 33

Ω was selected to specifi-

cally optimize both the THD and SNR performance of the A/D.

RS and CS help provide a low-pass filter to block high frequency

noise.

The AD9225 can be easily configured for either a 2 V p-p input

span or 4.0 V p-p input span by setting the internal reference

(see Table II). Other input spans can be realized with two exter-

nal gain setting resistors as shown in Figure 28 of this data

sheet. Figures 25a and 25b demonstrate how both spans of the

AD9225 achieve the high degree of linearity and SFDR over a

wide range of amplitudes required by the most demanding com-

munication applications.

Figures 25a and 25b demonstrate the flexibility of common-

mode voltage (transformer center tap) with respect to THD.

COMMON-MODE VOLTAGE – V

–76

–78

–86

–80

–82

–84

234

FIN = 10MHz

FIN = 2.5MHz

Figure 25a. Common-Mode Voltage vs. THD

(AIN = 2 V Differential)

COMMON-MODE VOLTAGE – V

–76

–78

–86

–80

–82

–84

0.5

1.0

1.5

FIN = 10MHz

FIN = 2.5MHz

2.0

2.5

3.0

3.5

4.0

4.5

Figure 25b. Common-Mode Voltage vs. THD

(AIN = 4 V Differential)

FUND

2ND

3RD

4TH

–119.7

–110.0

–100.0

–90.0

–80.0

–70.0

–60.0

–50.0

–40.0

–30.0

–20.0

–10.0

0.0

2.0E+6

4.0E+6

6.0E+6

8.0E+6

10.0E+6

12.5E+6

FIN = 2.5MHz

FS = 25MHz

Figure 25c. Single Tone Frequency Domain Plot

Common-Mode Voltage = 2.5 V (AIN = 4 V

Differential)

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