MT-017 Datasheet, PDF (4/7 Pages) Analog Devices – Oversampling Interpolating DACs

MT-017
an input update rate of 160 MHz, and an oversampling ratio of 2×, the image frequency occurs at
320 MHz – 50 MHz = 270 MHz. The transition band for the analog filter is therefore 50 MHz to
270 MHz. Without 2× oversampling, the image frequency occurs at 160 MHz – 50 MHz = 110
MHz, and the filter transition band is 50 MHz to 110 MHz.
N
LATCH ^N
DIGITAL
N
INTERPOLATION
FILTER
N
LATCH
DAC
fc
PLL
K•fc
LPF
TYPICAL APPLICATION: fc = 160MSPS
fo
fo = 50MHz
K=2
Image Frequency = 320 – 50 = 270MHz
Figure 4: Oversampling Interpolating TxDAC^® Simplified Block Diagram
Notice also that an oversampling interpolating DAC allows both a lower frequency input clock
and input data rate, which are much less likely to generate noise within the system.
SIGMA-DELTA DACS
Sigma-delta DACs operate very similarly to sigma-delta ADCs, however in a sigma-delta DAC,
the noise shaping function is accomplished with a digital modulator rather than an analog one.
A Σ-Δ DAC, unlike the Σ-Δ ADC, is mostly digital (see Figure 5A). It consists of an
"interpolation filter" (a digital circuit which accepts data at a low rate, inserts zeros at a high rate,
and then applies a digital filter algorithm and outputs data at a high rate), a Σ-Δ modulator
(which effectively acts as a low pass filter to the signal but as a high pass filter to the
quantization noise, and converts the resulting data to a high speed bit stream), and a 1-bit DAC
whose output switches between equal positive and negative reference voltages. The output is
filtered in an external analog LPF. Because of the high oversampling frequency, the complexity
of the LPF is much less than the case of traditional Nyquist operation.
Page 4 of 7