Datasheet

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AVDD

S(1)

S(1)C

S(2)

S(2)C

S(N)

S(N)C

...

IOUT1 IOUT2

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DAC5688

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SLLS880C –DECEMBER 2007–REVISED AUGUST 2010

ANALOG CURRENT OUTPUTS

Figure 45 shows a simplified schematic of the current source array output with corresponding switches.

Differential switches direct the current of each individual NMOS current source to either the positive output node

IOUT1 or its complementary negative output node IOUT2. 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 referred 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 DAC5688 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 45. Equivalent Analog Current Output

The DAC5688 can be easily configured to drive a doubly terminated 50Ω cable using a properly selected RF

transformer. Figure 46 and Figure 47 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 cd current flow. Applying a 20mA 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 Link(s): DAC5688