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Data Sheet

AD9739

Rev. E | Page 43 of 50

Figure 63 shows the IOUTP vs. DACCODE transfer function

when I

OUTFS is set to 19.65 mA.

4096

8192

12,288

DAC CODE

16,384

Figure 63. 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, I

PEAK,

and the equivalent load resistance it sees. Because the AD9739

includes a differential 70 Ω resistance, it is best to use a doubly

terminated external output network similar to what is shown in

Figure 64. In this case, the equivalent load seen by the ac current

source of the DAC is 25 Ω.

If the AD9739 is programmed for I

OUTFS = 20 mA, its peak ac

current is 9.375 mA and its peak power delivered to the equivalent

load is 2.2 mW (that is, P = I

2R). 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 I

OUTFS, although any degradation in

linearity performance must be considered acceptable for the

target application.

IPEAK =

15/32 × IOUTFS

70Ω

IOUTFS = 8.6 – 31.2mA

180Ω

RLOAD

= 50Ω

RSOURCE

= 50Ω

LOSSLESS

BALUN

1:1

Figure 64. Equivalent Circuit for Determining Maximum Peak Power to a 50 Ω Load

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