Datasheet

Differential Output = 7.5Current ·
V
R
REF
REF
(TDATA 50%)
25%
-
· ·Gain
DAC1280
Bitstream
Source
Voltage
Output
256kHz
Clock
5V
Reference
VREF
4.096MHz
TDATA
CLK
IOUTN
IOUTP
I/VI/V
V
COM
(1)
V
N
V
P
V
R
REF
REF
V
COM
1.25V Gain- ·
V
COM
1.25V Gain- ·
V
COM
+ 1.25V Gain·
V
COM
+ 1.25V Gain·
I/V
V
COM
V
COM
V
N
V
P
Differential output voltage = = 2.5-V V V Gain
P N
±
·
V = 0V (dual analog supply), +2.5V (single +5V analog supply)
COM
DAC1280
SBAS432A –APRIL 2010–REVISED SEPTEMBER 2010 www.ti.com
DAC1280 Basics Output Scaling
The basic requirements of DAC1280 operation are a The full-scale output of the DAC1280 is set by the
clock, a bitstream input, an external current-to-voltage reference voltage and an external reference resistor.
converter, and a reference voltage. The bitstream The GAIN[2:0] control pins select one of seven output
originates either from an FPGA-based digital ranges. In operation, the reference voltage and
modulator or playback from a ROM device holding a reference resistor are usually fixed, and the DAC
stored bitstream file. The external reference voltage is output range is selected by the gain pins.
+5V and should be precision (low drift and low noise).
The DAC1280 differential output current is
The current-to-voltage converter is an active circuit.
determined by Equation 1:
The amplifiers used for the current-to-voltage
converter should have good dynamic characteristics
(low THD) with low noise. Figure 27 illustrates the
system block diagram.
Where:
• V
REF
= 5V (nominal)
• R
REF
= 30kΩ (nominal)
• Gain = 1/1 to 1/64
• TDATA = TDATA 1s density, ranging 25% to
75% (1)
The DAC full-scale output can be fine-trimmed, if
desired, by changing the ratio from the nominal
values of V
REF
= 5V and R
REF
= 30kΩ. See the
(1) V
COM
= midsupply voltage.
Electrical Characteristics for the adjustment range.
Figure 27. DAC1280 System Block Diagram
The external current-to-voltage converter scales the
DAC output current into an output voltage. Table 1
Output Voltage
shows the DAC1280 gain (differential output current
to bitstream density) and the resulting output voltage
As shown in the system block diagram, the external
of the external current-to-voltage converter.
I/V circuit converts the current output of the DAC to a
voltage output. The voltage output of the I/V circuit is
differential, as shown in Figure 28. The
common-mode output voltage (V
COM
) at the I/V circuit
is normally set to the midsupply point of the DAC.
The differential output voltage is ±2.5V/gain.
Figure 28. I/V Converter Output Voltage
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