AD9225ARSZRL datasheet(16/26 Pages) AD

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AD9225

–15–

Figure 13 shows the schematic of the suggested transformer circuit.

The circuit uses a minicircuits RF transformer, model #T4-1T,

which has an impedance ratio of 4 (turns ratio of 2). The sche-

matic assumes that the signal source has a 50

W source impedance.

The 1:4 impedance ratio requires the 200

W 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 13. Transformer Coupled Input

The configuration in Figure 13 was used to gather the differential

data on the Specifications tables.

Transformers with other turns ratios may also be selected to opti-

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

input signal source or amplifier may realize an improvement in

distortion performance at reduced output power levels and signal

swings. For example, selecting a transformer with a higher imped-

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

effectively steps up the signal level further reducing the driving

requirements of the signal source.

Referring to Figure 13, a series resistors, RS, and shunt capacitor,

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

transformer. The value of 33

W was selected to specifically opti-

mize both the THD and SNR performance of the ADC. 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 a 4.0 V p-p input span by setting the internal reference (see

Table II). Other input spans can be realized with two external gain

setting resistors as shown in Figure 19. Figures 14 and 15 demon-

strate 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 communication applications.

Figures 14 and 15 demonstrate the flexibility of common-mode

voltage (transformer center tap) with respect to THD.

COMMON-MODE VOLTAGE (V)

–76

–78

–86

–80

–82

–84

fIN = 10MHz

fIN = 2.5MHz

Figure 14. 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 15. 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 16. Single-Tone Frequency Domain Plot

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

Differential)

Rev. C

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