DAC38RF80 datasheet(74/155 Pages) TI1

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VOUT1/2+

2 : 1

50 :

RLOAD

VOUT1/2-

VDDADAC18

(1.8V)

100 :

IOUT

VOUT

IOUT

1/ 2

200

IOUT+ IOUT

IOUT

IOUT- IOUT

IOUT+

VOUT

1 / 2

IOUT -

Zext -

IOUT + Zext +

VOUT

Zext +

Zext -

Zext + Zext -

100

1/ 2

100

IOUT +

Zext +

IOUT - Zext -

VOUT

Zext +

Zext + Zext -

100

1/ 2

100

IOUT

CODE

IOUT-

16383

16384

IOUT

CODE

IOUT+

16384

DAC38RF80, DAC38RF83, DAC38RF84

DAC38RF85, DAC38RF90, DAC38RF93

SLASEA3C – DECEMBER 2016 – REVISED JULY 2017

www.ti.com

DAC38RF83 DAC38RF93

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(12)

(13)

where CODE is the decimal representation of the 14-bit DAC core data input word. Note the signal path up to the

DAC is 16-bits and the 2 LSBs are truncated for the DAC core data input word.

8.3.29 DAC Transfer Function for DAC38RF83, 93, 85

The DAC38RF83/93/85 has a differential output and is terminated internally with a differential 100-Ω load. The

DAC38RF83/93/85 output compliance range is 1.3 to 2.3 V. Note that care should be taken not to exceed the

compliance voltages at node VOUT1/2+ and VOUT1/2-, which would lead to increased signal distortion.

Referring again toFigure 71, denote the external impedance as seen by VOUT1/2+ as Zext+ and by VOUT1/2-

as Zext-. Note that Zext+ and Zext- should terminate to VDDOUT18 to supply the output current for the DAC.

Also, Zext+ and Zext- are ideally identical to maintain the differential balance of the output. The voltage at nodes

VOUT1/2+ and VOUT1/2- generated by the current flow through the impedance is

(14)

(15)

The DAC38RF83/93/85 can be easily configured to drive a doubly terminated 50-Ω cable using a properly

selected 2:1 RF transformer (Figure 72). Note that the center tap of the primary input of the transformer has to

be connected to the VDDOUT18 supply (nominally 1.8 V) to enable a DC current flow into the DAC. The AC load

impedance as seen through 2:1 transformer is 100 Ω, which is split equally into Zext+ = Zext- = 50 Ω. The DC

impedance for the transformer is a short to the center tap of the transformer, which drives the common mode of

VOUT1/2+ and VOUT1/2- to 1.8V. To calculate the peak to peak output swing VOUT1/2PP at each node, the

equations above simplify to:

(16)

(17)

With IOUTFS = 40 mA, the swing becomes 1 VPP at each node. With the common mode at 1.8 V due to the

center tap, the voltage at VOUT1/2+ and VOUT1/2- varies between 1.3 and 2.3 V, which is the compliance range

of the DAC.

The differential output swing is 2x VOUT1/2PP, or 2 VPPDIFF. On the load side of the transformer, this reduces to

1.414 VPP, for a transferred load power of 7 dBm (assuming no loss).

Figure 72. Driving a 50-Ω Load Using a 2:1 Impedance Ratio Transformer (DAC38RF83/93/85)

The DAC38RF83/93/85 can also be DC coupled. In this case, the termination voltage can be raised above 1.8 V

(for example 2.3 V) so that the common mode for the output pin is nominally 1.8 V.

DAC38RF80 Datasheet(PDF) 74 Page - Texas Instruments

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