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DAC Transfer Function

CODE

65536

(65536 * CODE)

65536

VOUT1 + IOUT1

R

CODE

65536

IOUT

FS

R

L

VOUT2 + IOUT2

R

(65536 * CODE)

65536

IOUT

FS

R

L

VOUT

(2CODE * 65536)

65536

IOUT

FS

R

L

Reference Operation

IOUT

16

V

EXTIO

R

BIAS

DAC5686

SLWS147B – APRIL 2003 – REVISED AUGUST 2004

CLOCK INPUT AND TIMING (continued)

The CMOS DACs consist of a segmented array of NMOS current sources, capable of delivering a full-scale

output current up to 20 mA. Differential current switches direct the current of each current source to either one of

the complementary output nodes IOUT1 or IOUT2. Complementary output currents enable differential operation,

thus canceling out common-mode noise sources (digital feed-through, on-chip, and PCB noise), dc offsets,

even-order distortion components, and increasing signal output power by a factor of two.

mA.

The DAC5686 delivers complementary output currents IOUT1 and IOUT2. Output current IOUT1 equals the

approximate full-scale output current when all bits (after the digital processing) are high. Full-scale output current

flows through terminal IOUT2 when all input bits are low. The relation between IOUT1 and IOUT2 can thus be

expressed as:

where CODE is the decimal representation of the DAC data input word. Output currents IOUT1 and IOUT2 drive

voltages VOUT1 and VOUT2 at terminals IOUT1 and IOUT2, respectively, of:

The latter equation shows that applying the differential output results in doubling of the signal power delivered to

the load. Because the output currents IOUT1 and IOUT2 are complementary, they become additive when

processed differentially. Note that care should be taken not to exceed the compliance voltages at nodes IOUT1

and IOUT2, which would lead to increased signal distortion.

The DAC5686 comprises a band-gap reference and control amplifier for biasing the full-scale output current. The

is defined by the on-chip band-gap reference voltage and control amplifier. The full-scale output current equals

gain = 0):

,

1.2 V. This reference is active when terminal EXTLO is connected to AGND. An external decoupling capacitor

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