DAC38RF80IAAV datasheet(72/160 Pages) TI2

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IOUT

IOUT +

IOUT-

(11)

Since the output stage is a current sinking architecture, we will denote current into the DAC as + current, and the

current flows IOUT+ and IOUT- into terminals VOUT1/2+ and VOUT1/2- respectively. IOUT+ and IOUT- can be

expressed as:

IOUT

CODE

IOUT+

16384

(12)

IOUT

CODE

IOUT-

16383

16384

(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.

7.3.28 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 7-27, 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

IOUT +

Zext +

IOUT - Zext -

VOUT

Zext +

Zext -

Zext +

Zext -

100

1 / 2

100

(14)

IOUT -

Zext -

IOUT + Zext +

VOUT

Zext +

Zext -

Zext +

Zext -

Zext +

Zext -

100

1/ 2

100

(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 7-28). 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:

IOUT+ IOUT

IOUT

IOUT- IOUT

IOUT+

VOUT

1 / 2

(16)

IOUT

VOUT

IOUT

1 / 2

200

(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).

DAC38RF80, DAC38RF83, DAC38RF84

DAC38RF85, DAC38RF90, DAC38RF93

SLASEA3D – DECEMBER 2016 – REVISED DECEMBER 2023

www.ti.com

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Product Folder Links: DAC38RF80 DAC38RF83 DAC38RF84 DAC38RF85 DAC38RF90 DAC38RF93

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