AD9786 Analog Devices, AD9786 Datasheet - Page 31

• Manufacturer Part Number: AD9786
• Manufacturer: Analog Devices

Specifications of AD9786

• Resolution (bits): 16bit
• Dac Update Rate: 500MSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.5V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: Par

Interpolation Modes
Table 28. Interpolation Modes
INTERP[1]
0
0
1
1
Interpolation is the process of increasing the number of points
in a time domain waveform by approximating points between
the input data points on a uniform time grid. This produces
a higher output data rate. Applied to an interpolation DAC,
a digital interpolation filter is used to approximate the interpolated
points, having an output data rate increased by the interpolation
factor. Interpolation filter responses are achieved by cascading
individual digital filter banks, whose filter coefficients are given in
Table 23, Table 24, and Table 25. Filter responses are shown in
Figure 57, which shows the interpolation filters of the AD9786
under different interpolation rates, normalized to the input data
rate, f
The digital filter’s frequency domain response exhibits symmetry
about half the output data rate and dc. It causes images of the
input data to be shaped by the interpolation filter’s frequency
response. This has the advantage of causing input data images
that fall in the stop band of the digital filter to be rejected by
the stop-band attenuation of the interpolation filter, while input
SIN
.
INTERP[0]
0
1
0
1
–100
–150
–100
–150
–100
–150
–100
–150
–50
–50
–50
–50
0
0
0
0
–8
–8
–8
–8
–6
–6
–6
–6
Mode
No interpolation
×2 interpolation
×4 interpolation
×8 interpolation
–4
–4
–4
–4
–2
–2
–2
–2
Figure 57. Interpolation Modes
Rev. B | Page 31 of 56
–0
0
0
0
SINC RESPONSE
2
2
2
2
data images falling in the interpolation filter pass band are passed.
In band-limited applications, the images at the output |of the
DAC must be limited by an analog reconstruction filter. The
complexity of the analog reconstruction filter is determined by
the proximity of the closest image to the required signal band.
Higher interpolation rates yield larger stop-band regions,
suppressing more input images and resulting in a much relaxed
analog reconstruction filter.
A DAC shapes its output with a sinc function, having a null at
the sampling frequency of the DAC. The higher the DAC sam-
pling rate compared to the input signal bandwidth, the less the
DAC sinc function shapes the output. The higher the
interpolation rate, the more input data images fall in the
interpolation filter stop band and are rejected; the bandwidth
between passed images is larger with higher interpolation
factors. The sinc function shaping is also reduced with a higher
interpolation factor.
Table 29. Sinc Shaping at Band Edge of Interpolation Filters
Mode
No interpolation
×2 interpolation
×4 interpolation
×8 interpolation
4
4
4
4
NO INTERPOLATION
×2 INTERPOLATION
×4 INTERPOLATION
×8 INTERPOLATION
6
6
6
6
Sinc Shaping
@ 0.43 f
–2.8241
–0.6708
–0.1657
–0.0413
SIN
8
8
8
8
(dB)
f
f
f
f
INTERP[1] = 0
INTERP[0] = 0
INTERP[1] = 0
INTERP[0] = 1
INTERP[1] = 1
INTERP[0] = 0
INTERP[1] = 1
INTERP[0] = 1
SIN
SIN
SIN
SIN
Bandwidth to First Image
f
2 f
4 f
8 f
SIN
SIN
SIN
SIN
AD9786