DAC5682Z_09 Datasheet, PDF (44/59 Pages) Texas Instruments – 16-BIT, 1.0 GSPS 2x-4x INTERPOLATING DUAL-CHANNEL DIGITAL-TO-ANALOG CONVERTER (DAC)

DAC5682Z
SLLS853C – AUGUST 2007 – REVISED JUNE 2009 ........................................................................................................................................................ www.ti.com
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 – | –0 mA | × 25 Ω = 3.3 V
VOUT2 = AVDD – | –20 mA | × 25 Ω = 2.8 V
VDIFF = VOUT1 – VOUT2 = 0.5 V
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. See the 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 is 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 DAC5682Z’s 2x and 4x 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
44
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