DS588 Datasheet, PDF (8/24 Pages) Xilinx, Inc – Programmable interrupt generation

XPS Delta-Sigma Digital to Analog Converter (DAC) (v1.01a)
Output Attenuation
By convention, the cutoff frequency of a low pass filter is defined as the half-power point. The cutoff
frequency of the simple RC filter may be expressed as:
f_C = 1/(2πτ)
where:
f_C is the filter cutoff frequency
τ is the filter time constant, RC.
The above equation was used to create Figure 6.
Figure 6 may be used in conjunction with Figure 3, Figure 4, or Figure 6 to choose the RC time constant
that is optimum for a particular application. All figures cover the same RC range.
Figure 4 shows an RC value of 13.0 x 10^-6 results in a peak-to-peak noise voltage of 0.25 V when a DAC
clock frequency of 80 MHz is used on a 8-bit DAC. From Figure 6, it can be determined that the filter
cutoff frequency for this RC value is about 12 KHz. If the expected output is essentially a DC level, e.g.,
a programmable voltage generator, then RC may be increased to reduce the clock noise. On the other
hand, if the fundamental frequency of the analog output is high, or it has sharp edges, then a lower RC
may be needed. When determining the actual component values, remember that R should be at least
2500 Ω.
The user may implement a more sophisticated filter if the simple RC filter has inadequate cutoff or
drive characteristics for the application.
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DS588 December 2, 2009
Product Specification