DAC5675A-SP Datasheet, PDF (18/23 Pages) Texas Instruments – CLASS V, 14-BIT, 400-MSPS DIGITAL-TO-ANALOG CONVERTER

DAC5675A-SP
SGLS387D – JULY 2007 – REVISED OCTOBER 2009 ..................................................................................................................................................... www.ti.com
(2)
(3)
where CODE is the decimal representation of the DAC input word. This would translate into single-ended
voltages at IOUT1 and IOUT2, as shown in Equation 4 and Equation 5:
(4)
(5)
Assuming that D[13:0] = 1 and the R_L is 50Ω, the differential voltage between pins IOUT1 and IOUT2 can be
expressed as shown in Equation 6 through Equation 8:
(6)
(7)
(8)
If D[13:0] = 0, then IOUT2 = 0mA and IOUT1 = 20mA and the differential voltage VDIFF = –1V.
The output currents and voltages in IOUT1 and IOUT2 are complementary. The voltage, when measured
differentially, is doubled compared to measuring each output individually. Care must be taken not to exceed the
compliance voltages at the IOUT1 and IOUT2 pins to keep signal distortion low.
Reference Operation
The DAC5675A has a bandgap reference and control amplifier for biasing the full-scale output current. The
full-scale output current is set by applying an external resistor R_BIAS. The bias current I_BIAS through resistor R_BIAS
is defined by the on-chip bandgap reference voltage and control amplifier. The full-scale output current equals
16 times this bias current. The full-scale output current IO_(FS) is thus expressed as Equation 9:
I_O(FS) + 16
I _BIAS
+
16
V _EXTIO
R_BIAS
(9)
where V_EXTIO is the voltage at terminal EXTIO. The bandgap reference voltage delivers a stable voltage of 1.2 V.
This reference can be overridden by applying an external voltage to terminal EXTIO. The bandgap reference can
additionally be used for external reference operation. In such a case, an external buffer amplifier with high
impedance input should be selected to limit the bandgap load current to less than 100 nA. The capacitor C_EXT
may be omitted. Terminal EXTIO serves as either an input or output node. The full-scale output current is
adjustable from 20 mA down to 2 mA by varying resistor R_BIAS.
Analog Current Outputs
Figure 23 shows a simplified schematic of the current source array output with corresponding switches.
Differential NPN switches direct the current of each individual NPN current source to either the positive output
node IOUT1 or its complementary negative output node IOUT2. The output impedance is determined by the
stack of the current sources and differential switches and is >300 kΩ in parallel with an output capacitance
of 5 pF.
The external output resistors are referred to the positive supply AV_DD.
18
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