DAC39J84IAAVR datasheet(59/146 Pages) TI

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IOUTP

4 : 1

50 :

RLOAD

100 :

IOUTN

AGND

IOUTP

1 : 1

IOUTN

50 :

RLOAD

100 :

50 :

AGND

DAC39J84

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SLASE48A – NOVEMBER 2014 – REVISED JANUARY 2015

7.3.29 DAC Transfer Function

The DAC39J84 can be easily configured to drive a doubly terminated 50

Ω cable using a properly selected RF

transformer. Figure 77 and Figure 78 show the 50

Ω doubly terminated transformer configuration with 1:1 and 4:1

impedance ratio, respectively. Note that the center tap of the primary input of the transformer has to be grounded

to enable a DC current flow. Applying a 20 mA full-scale output current would lead to a 0.5 Vpp for a 1:1

transformer and a 1 Vpp output for a 4:1 transformer. The low dc-impedance between IOUTP or IOUTN and the

transformer center tap sets the center of the ac-signal to GND, so the 1 Vpp output for the 4:1 transformer

results in an output between –0.5 V and +0.5 V.

Figure 77. Driving a Doubly Terminated 50

Ω Cable Using a 1:1 Impedance Ratio Transformer

Figure 78. Driving a Doubly Terminated 50

Ω Cable Using a 4:1 Impedance Ratio Transformer

7.4 Device Functional Modes

7.4.1 Clocking Modes

The DAC39J84 has a single differential clock DACCLKN/P to clock the DAC cores and internal digital logic. The

DAC39J84 DACCLK can be sourced directly or generated through an on-chip low-jitter phase-locked loop (PLL).

In those applications requiring extremely low noise it is recommended to bypass the PLL and source the DAC

clock directly from a high-quality external clock to the DACCLK input. In most applications system clocking can

be simplified by using the on-chip PLL to generate the DAC core clock while still satisfying performance

requirements. In this case the DACCLK pins are used as the reference frequency input to the PLL.

7.4.1.1 PLL Bypass Mode

In PLL bypass mode a high quality clock is sourced to the DACCLK inputs. This clock is used to directly clock

the DAC39J84 DAC cores. This mode gives the device best performance and is recommended for extremely

demanding applications.

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