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AD8340

Data Sheet

INTERFACING TO HIGH SPEED DACs

The AD977x family of dual DACs is well suited for driving the

I and Q vector controls of the AD8340. While these inputs can

in general be driven by any DAC, the differential outputs and

bias level of the Analog Devices TxDAC® family allow for a

direct connection between DAC and modulator.

The AD977x family of dual DACs has differential current

outputs. The full-scale current is user programmable and is

usually set to 20 mA, that is, each output swings from 0 mA

to 20 mA.

The basic interface between the AD9777 DAC outputs and the

AD8340 I and Q inputs is shown in Figure 33. The resistors R1

and R2 set the dc bias level according to the equation:

Bias Level = Average Output Current × R1

For example, if the full-scale current from each output is 20 mA,

each output will have an average current of 10 mA. Therefore,

to set the bias level to the recommended 0.5 V, R1 and R2

should be set to 50 Ω each. R1 and R2 should always be equal.

If R3 is omitted, this results in an available swing from the DAC

of 2 V p-p differential, which is twice the maximum voltage

range required by the AD8340. DAC resolution can be

maximized by adding R3, which scales down this voltage

according to the following equation:

(

)

(

)













−

Swing

Scale

Full

MAX

OPTIONAL

LOW-PASS

FILTER

IOUTB2

IOUTA2

QBBM

QBBP

IOUTB1

IOUTA1

IBBM

IBBP

AD9777

AD8340

OPTIONAL

LOW-PASS

FILTER

Figure 33. Basic AD9777 to AD8340 Interface

130

50 55 60 65 70 75 80 85 90

100 105

115 120

110

125

1.15

1.08

1.10

1.13

1.00

1.02

1.05

0.95

0.97

0.88

0.90

0.92

0.77

0.80

0.82

0.85

0.70

0.75

0.72

Figure 34. Peak-to-Peak DAC Output Swing vs.

Swing Scaling Resistor R3 (R1 = R2 = 50 Ω)

Figure 34 shows the relationship between the value of R3 and

the peak baseband voltage with R1 and R2 equal to 50 Ω. Figure 34

shows that a value of 100 Ω for R3 provides a peak-to-peak

swing of 1 V p-p differential into the AD8340 I and Q inputs.

When using a DAC, low-pass image reject filters are typically

used to eliminate the Nyquist images produced by the DAC.

They also provide the added benefit of eliminating broadband

noise that might feed into the modulator from the DAC.

CDMA2000 APPLICATION

To test the compliance to the CDMA2000 base station standard,

a single-carrier CDMA2000 test model signal (forward pilot, sync,

paging, and six traffic as per 3GPP2 C.S0010-B, Table 6.5.2.1)

was applied to the AD8340. A cavity-tuned filter was used to

reduce noise from the signal source being applied to the device.

The 4.6 MHz pass band of this filter is apparent in the

subsequent spectral plots (see Figure 35 to Figure 38).

Figure 35 shows a plot of the spectrum of the output signal

under nominal conditions. POUT is equal to −5 dBm and VI = VQ

= 0.353 V, that is, VIBBP − VIBBM = VQBBP − VQBBM =

0.353 V. Adjacent channel power is measured in 30 kHz

resolution bandwidth at 750 kHz and 1.98 MHz carrier offset.

Noise floor is measured at ±4 MHz carrier offset.

Rev. C | Page 14 of 20

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