DAC34H84_1109 Datasheet, PDF (74/80 Pages) Texas Instruments – Quad-Channel, 16-Bit, 1.25 GSPS Digital-to-Analog Converter (DAC)

DAC34H84
SLAS751B – MARCH 2011 – REVISED SEPTEMBER 2011
www.ti.com
CMOS DIGITAL INPUTS
Figure 85 shows a schematic of the equivalent CMOS digital inputs of the DAC34H84. SDIO, SCLK, SLEEP and
TXENA have pull-down resistors while SDENB and RESETB have pull-up resistors internal to the DAC34H84. All
the CMOS digital inputs and outputs are referred to the IOVDD2 supply, which can vary from 1.8V to 3.3V. This
facilitates the I/O interface and eliminates the need of level translation. See the specification table for logic
thresholds. The pull-up and pull-down circuitry is approximately equivalent to 100kΩ.
IOVDD2
IOVDD2
SDIO
SCLK
SLEEP
TXENA
100 kΩ
400 Ω
Internal
Digital In
100 kΩ
SDENB
RESETB
400 Ω
Internal
Digital In
GND
Figure 85. CMOS Digital Equivalent Input
GND
S0027-04
REFERENCE OPERATION
The DAC34H84 uses 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 to pin BIASJ. The bias current I_BIAS through
resistor R_BIAS is defined by the on-chip bandgap reference voltage and control amplifier. The default full-scale
output current equals 64 times this bias current and can thus be expressed as:
IOUT_FS = 64 x I_BIAS = 64 x (V_EXTIO / R_BIAS ) / 2
The DAC34H84 has a 4-bit coarse gain control coarse_dac(3:0) in the config3 register. Using gain control, the
IOUT_FS can be expressed as:
IOUT_FS = (coarse_dac + 1)/16 x I_BIAS x 64 = (coarse_dac + 1)/16 x (VEXTIO / RBIAS) / 2 x 64
where V_EXTIO is the voltage at terminal EXTIO. The bandgap reference voltage delivers an accurate voltage of
1.2V. This reference is active when extref_ena = ‘0’ in config27. An external decoupling capacitor C_EXT of 0.1 µF
should be connected externally to terminal EXTIO for compensation. The bandgap reference can additionally be
used for external reference operation. In that case, an external buffer with high impedance input should be
applied in order to limit the bandgap load current to a maximum of 100 nA. The internal reference can be
disabled and overridden by an external reference by setting the extref_ena control bit. Capacitor C_EXT may hence
be omitted. Terminal EXTIO thus serves as either input or output node.
The full-scale output current can be adjusted from 30 mA down to 10 mA by varying resistor R_BIAS, programming
coarse_dac(3:0), or changing the externally applied reference voltage.
NOTE
With internal reference, the minimum Rbias resistor value is 1.28kΩ. Resistor value below
1.28kΩ is not recommended sice it will program the full-scale current to go above 30mA
and potentially damages the device.
DAC TRANSFER FUNCTION
The CMOS DACs consist of a segmented array of PMOS current sources, capable of sourcing a full-scale output
current up to 30 mA. Differential current switches direct the current to either one of the complementary output
nodes IOUTP or IOUTN. Complementary output currents enable differential operation, thus canceling out
common mode noise sources (digital feed-through, on-chip and PCB noise), dc offsets, even order distortion
components, and increasing signal output power by a factor of two.
74
Submit Documentation Feedback
Product Folder Link(s): DAC34H84
Copyright © 2011, Texas Instruments Incorporated