DAC

AS5181

AS5181

Rev. 0.4 01/10

Micross Components reserves the right to change products or specifications without notice.

11

Digital Feedthrough

Digital feedthrough is the noise generated on a DAC’s

output when any digital input transitions. Proper board layout

and grounding will significantly reduce this noise, but there

will always be some feedthrough caused by the DAC itself.

Total Harmonic Distortion

Total harmonic distortion (THD) is the ratio of the RMS sum

of the input signal’s first four harmonics to the fundamental

itself. This is expressed as:

where V1 is the fundamental amplitude, and V2 through V5 are

the amplitudes of the 2nd- through 5th-order harmonics.

Spurious-Free Dynamic Range

Spurious-free dynamic range (SFDR) is the ratio of RMS

amplitude of the fundamental (maximum signal component)

to the RMS value of the next-largest distortion component.

Differential to Single-Ended Conversion

A low-distortion, high-input bandwidth amplifier may be

used to generate a voltage from the array current output of the

AS5181. The differential voltage across OUTP and OUTN

is converted into a single-ended voltage by designing an

appropriate operational amplifier configuration (Figure 6).

I/Q Reconstruction in a QAM Application

The low-distortion performance of two AS5181s

supports analog reconstruction of in-phase (I) and quadrature

(Q) carrier components typically used in quadrature amplitude

modulation (QAM) architectures where two separate buses

carry the I and Q data. A QAM signal is both amplitude (AM)

and phase modulated, created by summing two independently

modulated carriers of identical frequency but different phase

(90° phase difference).

In a typical QAM application (Figure 7), the modulation

occurs in the digital domain, and two DACs such as the AS5181

may be used to reconstruct the analog I and Q components.

The I/Q reconstruction system is completed by a

quadrature modulator that combines the reconstructed

components with in-phase and quadrature carrier frequencies

and then sums both outputs to provide the QAM signal.

FIGURE 4: TIMING DIAGRAM