DAC5688_15 Datasheet, PDF (40/56 Pages) Texas Instruments – DUAL-CHANNEL, 16-BIT, 800 MSPS, 2x–8x INTERPOLATING DIGITAL-TO-ANALOG CONVERTER (DAC)

DAC5688
SLLS880C – DECEMBER 2007 – REVISED AUGUST 2010
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The full-scale output current is set using external resistor RBIAS in combination with an on-chip bandgap voltage
reference source (+1.2 V) and control amplifier. Current I_BIAS through resistor R_BIAS is mirrored internally to
provide a maximum full-scale output current equal to 16 times I_BIAS.
The relation between IOUT1 and IOUT2 can be expressed as:
IOUT1 = – IOUT_FS – IOUT2
We will denote current flowing into a node as – current and current flowing out of a node as + current. Since the
output stage is a current sink the current can only flow from AVDD into the IOUT1 and IOUT2 pins. The output
current flow in each pin driving a resistive load can be expressed as:
IOUT1 = IOUT_FS × (65536 – CODE) / 65536
IOUT2 = IOUT_FS × CODE / 65536
where CODE is the decimal representation of the DAC data input word.
For the case where IOUT1 and IOUT2 drive resistor loads R_L directly, this translates into single ended voltages
at IOUT1 and IOUT2:
VOUT1 = AVDD – | IOUT1 | × R_L
VOUT2 = AVDD – | IOUT2 | × R_L
Assuming that the data is full scale (65536 in offset binary notation) and the R_L is 25 Ω, the differential voltage
between pins IOUT1 and IOUT2 can be expressed as:
VOUT1 = AVDD – | –0mA | × 25 Ω = 3.3 V
VOUT2 = AVDD – | –20mA | × 25 Ω = 2.8 V
VDIFF = VOUT1 – VOUT2 = 0.5V
Note that care should be taken not to exceed the compliance voltages at node IOUT1 and IOUT2, which would
lead to increased signal distortion.
DAC OUTPUT SINC RESPONSE
Due to sampled nature of a high-speed DAC’s, the well known sin(x)/x (or SINC) response can significantly
attenuate higher frequency output signals. Refer to Figure 44 which shows the unitized SINC attenuation roll-off
with respect to the final DAC sample rate in 4 Nyquist zones. For example, if the final DAC sample rate F_S = 1.0
GSPS, then a tone at 440MHz will be attenuated by 3.0dB. Although the SINC response can create challenges
in frequency planning, one side benefit is the natural attenuation of Nyquist images. The increased over-sampling
ratio of the input data provided by the DAC5688’s 2x, 4x and 8x digital interpolation modes improve the SINC
roll-off (droop) within the original signal’s band of interest.
Figure 44. Unitized DAC sin(x)/x (SINC) Response
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