AD9737A Datasheet, PDF (58/64 Pages) Analog Devices – 11-/14-Bit, 2.5 GSPS

AD9737A/AD9739A
Data Sheet
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.
20
18
16
14
12
10
8
6
4
2
0
0
4096
8192
12,288
16,384
DAC CODE
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.
I_OUTFS = 8.6 – 31.2mA
I_PEAK =
15/32 × I_OUTFS
AC
70Ω
R_SOURCE
= 50Ω
180Ω
LOSSLESS
BALUN
1:1
R_LOAD
= 50Ω
Figure 180. Equivalent Circuit for Determining Maximum Peak Power
to a 50 Ω Load
Rev.C | Page 58 of 64