DAC5687IPZP datasheet(54/79 Pages) TI

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IOUT1

4:1

IOUT2

100

Ω

100

Ω

RLOAD

Ω

AVDD (3.3 V)

S0033-02

Combined Output Termination

S0069-01

IOUTA1

1:1

RLOAD

Ω

IOUTA2

AVDD (3.3 V)

IOUTB1

IOUTB2

DAC5687

SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006

Figure 61. Driving a Doubly Terminated 50-

Ω Cable Using a 4:1 Impedance Ratio Transformer

The DAC5687 DAC A and DAC B outputs can be summed together as shown in Figure 62 to provide a 40-mA

full-scale output for increased output power.

Figure 62. Combined Output Termination Using a 1:1 Impedance Ratio Transformer Into 50-

Ω Load

For the case where the digital codes for the two DACs are identical, the termination results in a full-scale swing

of 2 VPP into the 50-Ω load, or 10 dBm. This is 6 dB higher than the 4:1 output termination recommended for a

single DAC output.

There are two methods to produce identical DAC codes. In modes where there is mixing between digital

channels A and B, i.e., when channels A and B are isolated, the identical data can be sent to both input ports to

produce identical DAC codes. Channels A and B are isolated when FMIX is disabled, the QMC is disabled or

enabled with QMC phase register set to 0, and CMIX is disabled or set to fDAC/2. Note that frequency

upconversion is still possible using the high-pass filter setting and CMIX fDAC/2.

Alternatively, by applying the input data on one input port only and setting the other input port to midscale (zero),

the NCO can be used to duplicate the output of the active input channel in the other channel by setting the

frequency to zero, phase to 8192 and NCO_GAIN = 1 and QMC gain = 1446. Assuming I(t) is the wanted signal

and Q(t) = 0, this is demonstrated by the simplification of the NCO equations in the Fine Mixer (FMIX) section:

IOUT(t) = (IIN(t)cos(2π × 0 × t + π/4) – 0 × sin(2π × 0 × t + π/4) × 2

(1 – 1) = I

IN(t)cos(π/4) = IIN(t)/2

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