DAC900-Q1 Datasheet, PDF (14/26 Pages) Texas Instruments – 10-BIT 165-MSPS DIGITAL-TO-ANALOG CONVERTER

DAC900-Q1
SBAS505 – JUNE 2010
www.ti.com
DAC Transfer Function
The total output current, I_OUTFS, of the DAC900 is the summation of the two complementary output currents:
I_OUTFS
=
I_OUT
+
I
OUT
(1)
The individual output currents depend on the DAC code and can be expressed as:
Code
I_OUT = I_OUTFS × ₁₀₂₄
(2)
æ
Code ö
I
OUT
= I_OUTFS × _çè1023 –
1024 ^÷ø
(3)
Where,
Code is the decimal representation of the DAC data input word.
Additionally, I_OUTFS is a function of the reference current I_REF, which is determined by the reference voltage and
the external setting resistor, R_SET.
I_OUTFS
= 32 × I_REF
= 32 ×
V_REF
R_SET
(4)
In most cases the complementary outputs will drive resistive loads or a terminated transformer. A signal voltage
will develop at each output according to:
V_OUT = I_OUT × R_LOAD
(5)
V
OUT
=
I
OUT
×
R_LOAD
(6)
The value of the load resistance is limited by the output compliance specification of the DAC900. To maintain
specified linearity performance, the voltage for I_OUT and I_OUT should not exceed the maximum allowable
compliance range. The two single-ended output voltages can be combined to find the total differential output
swing:
2 × Code – 1023
V_OUTDIFF
=
V_OUT
–
V
OUT
=
1024
× I_OUTFS × R_LOAD
(7)
Analog Outputs
The DAC900 provides two complementary current outputs, I_OUT and I_OUT. The simplified circuit of the analog
output stage representing the differential topology is shown in Figure 2. The output impedance of 200kΩ || 12pF
for I_OUT and I_OUT results from the parallel combination of the differential switches, along with the current sources
and associated parasitic capacitances.
+V_A
DAC900
I_OUT
R_L
I_OUT
R_L
Figure 2. Equivalent Analog Output
The signal voltage swing that may develop at the two outputs, I_OUT and I_OUT, is limited by a negative and positive
compliance. The negative limit of –1V is given by the breakdown voltage of the CMOS process, and exceeding it
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