ADL5372ACPZ-R71 datasheet(15/24 Pages) AD

15 / 24 page

ADL5372

Rev. 0 | Page 15 of 24

An example is shown in Figure 32 with a third-order, elliptical,

low-pass filter with a 3 dB frequency of 3 MHz. Matching input

and output impedances makes the filter design easier, so the

shunt resistor chosen is 100 Ω, producing an ac swing of

1 V p-p differential.

RBIP

50Ω

RBIN

50Ω

IBBN

IBBP

AD9779

F-MOD

RBQN

50Ω

RBQP

50Ω

RSLI

100Ω

RSLQ

100Ω

OUT1_N

OUT1_P

OUT2_P

OUT2_N

QBBP

QBBN

LPI

2.7nH

LNI

2.7nH

1.1nF

C1I

1.1nF

C2I

LNQ

2.7nH

LPQ

2.7nH

1.1nF

C1Q

1.1nF

C2Q

Figure 32. DAC Modulator Interface with

3 MHz Third-Order, Elliptical Low-Pass Filter

USING THE AD9779 AUXILIARY DAC FOR CARRIER

FEEDTHROUGH NULLING

The AD9779 features an auxiliary DAC that can be used to

inject small currents into the differential outputs for each main

DAC channel. This feature can be used to produce the small

offset voltages necessary to null out the carrier feedthrough

from the modulator. Figure 33 shows the interface required

to use the auxiliary DACs. This adds four resistors to the

interface.

RBIP

50Ω

RBIN

50Ω

IBBN

IBBP

AD9779

F-MOD

RBQN

50Ω

RBQP

50Ω

RSLI

100Ω

RSLQ

100Ω

OUT1_N

AUX1_N

AUX2_N

OUT1_P

AUX1_P

OUT2_P

AUX2_P

OUT2_N

QBBP

QBBN

LPI

2.7nH

LNI

2.7nH

1.1nF

C1I

1.1nF

C2I

LNQ

2.7nH

LPQ

2.7nH

1.1nF

C1Q

1.1nF

C2Q

250Ω

500Ω

250Ω

Figure 33. DAC Modulator Interface with Auxiliary DAC Resistors

GSM OPERATION

Figure 34 shows the GSM EVM, spectral mask, and noise vs. the

output power for the ADL5372 at 1960 MHz. For a given LO

amplitude, the performance is independent of output power.

OUTPUT POWER (dBm)

–110

–100

–90

–80

–70

–60

–50

–40

–30

–6

–4

–2

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

6 MHz NOISE FLOOR

1.2MHz

PEAK EVM

600kHz

400kHz

250kHz

RMS EVM

Figure 34. GSM EVM and Spectral Performance vs. Channel Power at

1960 MHz vs. Output Power; LO Power = 0 dBm

Figure 35 shows the GSM EVM and noise performance vs. the

LO amplitude at 1960 MHz with an output power of 5 dBm.

Increasing the LO drive level improves the noise performance

but degrades EVM performance.

LO DRIVE (dBm)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

–6

–4

–2

–120

–115

–110

–105

–100

–95

–90

–85

–80

6MHz NOISE FLOOR

RMS EVM

PEAK EVM

Figure 35. GSM EVM and 6 MHz Noise Floor vs. LO Power at 1960 MHz;

Output Power = 5 dBm

Figure 35 illustrates that an LO amplitude of 3 dBm provides

the ideal operating point for noise and EVM for a GSM signal

at 1960 MHz.

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