AD9737ABBCZ datasheet(58/64 Pages) AD

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AD9737A/AD9739A

Data Sheet

Rev. | Page 58 of 64

The code-dependent current measured at the IOUTP and

IOUTN outputs is as follows:

IOUTP = 17/32 × IOUTFS + 15/32 × IOUTFS × F(Code)

(4)

IOUTN = 17/32 × IOUTFS − 15/32 × IOUTFS × F(Code)

(5)

Figure 179 shows the IOUTP vs. DACCODE transfer function

when IOUTFS is set to 19.65 mA.

4096

8192

12,288

DAC CODE

16,384

Figure 179. Gain Curve for FSC[9:0] = 512, DAC OFFSET = 1.228 mA

Peak DAC Output Power Capability

The maximum peak power capability of a differential current

output DAC is dependent on its peak differential ac current, IPEAK,

and the equivalent load resistance it sees. Because the AD9737A/

AD9739A include a differential 70 Ω resistance, it is best to use

a doubly terminated external output network similar to what is

shown in Figure 181. In this case, the equivalent load seen by

the ac current source of the DAC is 25 Ω.

If the AD9737A/AD9739A are programmed for IOUTFS = 20 mA,

the peak ac current is 9.375 mA and the peak power delivered to

the equivalent load is 2.2 mW (that is, P = I2R). Because the source

and load resistance seen by the 1:1 balun are equal, this power is

shared equally; therefore, the output load receives 1.1 mW or

0.4 dBm.

To calculate the rms power delivered to the load, the following

must be considered:

•

Peak-to-rms of the digital waveform

•

Any digital backoff from digital full scale

•

The DAC’s sinc response and nonideal losses in external

network

For example, a reconstructed sine wave with no digital backoff

ideally measures −2.6 dBm because it has a peak-to-rms ratio of

3 dB. If a typical balun loss of 0.4 dBm is included, −3 dBm of

actual power can be expected in the region where the sinc response

of the DAC has negligible influence. Increasing the output

power is best accomplished by increasing IOUTFS, although any

degradation in linearity performance must be considered

acceptable for the target application.

IPEAK =

15/32 × IOUTFS

Ω

IOUTFS = 8.6 – 31.2mA

180

Ω

RLOAD

= 50

Ω

RSOURCE

= 50Ω

LOSSLESS

BALUN

1:1

Figure 180. Equivalent Circuit for Determining Maximum Peak Power

to a 50 Ω Load

AD9737ABBCZ Datasheet(PDF) 58 Page - Analog Devices

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