Datasheet

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AVDD

S(1)

S(1)C

S(2)

S(2)C

S(N)

S(N)C

...

IOUT1 IOUT2

DAC5681Z

www.ti.com

SLLS865F –AUGUST 2007–REVISED AUGUST 2012

ANALOG CURRENT OUTPUTS

Figure 44 shows a simplified schematic of the current source array output with corresponding switches in a

current sink configuration. Differential switches direct the current into either the positive output node, IOUT1, or

its complement, IOUT2, then through the individual NMOS current sources. The output impedance is determined

by the stack of the current sources and differential switches, and is typically >300 kΩ in parallel with an output

capacitance of 5 pF.

The external output resistors are referenced to an external ground. The minimum output compliance at nodes

IOUT1 and IOUT2 is limited to AVDD – 0.5 V, determined by the CMOS process. Beyond this value, transistor

breakdown may occur resulting in reduced reliability of the DAC5681Z device. The maximum output compliance

voltage at nodes IOUT1 and IOUT2 equals AVDD + 0.5 V. Exceeding the minimum output compliance voltage

adversely affects distortion performance and integral non-linearity. The optimum distortion performance for a

single-ended or differential output is achieved when the maximum full-scale signal at IOUT1 and IOUT2 does not

exceed 0.5 V.

Figure 44. Equivalent Analog Current Output

The DAC5681Z can be easily configured to drive a doubly terminated 50Ω cable using a properly selected RF

transformer. Figure 45 and Figure 46 show the 50Ω 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

connected to AVDD to enable a dc current flow. Applying a 20 mA full-scale output current would lead to a 0.5

for a 1:1 transformer, and a 1 V

output for a 4:1 transformer. The low dc-impedance between IOUT1 or

IOUT2 and the transformer center tap sets the center of the ac-signal at AVDD, so the 1 V

output for the 4:1

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

Product Folder Links: DAC5681Z