DAC8812 Datasheet, PDF (11/18 Pages) Texas Instruments – Dual, Serial Input 16-Bit Multiplying Digital-to-Analog Converter

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DAC8812
SBAS349A – AUGUST 2005 – REVISED DECEMBER 2005
THEORY OF OPERATION
CIRCUIT OPERATION
The DAC8812 contains two 16-bit, current-output, digital-to-analog converters (DACs). Each DAC has its own
independent multiplying reference input. The DAC8812 uses a 3-wire, SPI-compatible serial data interface, with a
configurable asynchronous RS pin for half-scale (MSB = 1) or zero-scale (MSB = 0) preset. In addition, an LDAC
strobe enables two channel simultaneous updates for hardware synchronized output voltage changes.
Digital-to-Analog Converters
The DAC8812 contains two current-steering R-2R ladder DACs. Figure 30 shows a typical equivalent DAC. Each
DAC contains a matching feedback resistor for use with an external I-to-V converter amplifier. The R_FBX pin is
connected to the output of the external amplifier. The I_OUTX terminal is connected to the inverting input of the
external amplifier. The A_GNDX pin should be Kelvin-connected to the load point in the circuit requiring the full
16-bit accuracy.
V_REFX
2R
R
2R
R
2R
R
R
5 kW
V_DD
R_FBX
S2
S1
I_OUTX
A_GNDX
DGND
Digital interface connections omitted for clarity.
Switches S1 and S2 are closed, V_DD must be powered.
Figure 30. Typical Equivalent DAC Channel
The DAC is designed to operate with both negative or positive reference voltages. The V_DD power pin is only
used by the logic to drive the DAC switches on and off. Note that a matching switch is used in series with the
internal 5 kΩ feedback resistor. If users are attempting to measure the value of R_FB, power must be applied to
V_DD in order to achieve continuity. The DAC output voltage is determined by V_REF and the digital data (D)
according to Equation 1:
V_OUT + *V_REF
D
65536
(1)
Note that the output polarity is opposite of the V_REF polarity for dc reference voltages.
The DAC is also designed to accommodate ac reference input signals. The DAC8812 accommodates input
reference voltages in the range of –15 V to +15 V. The reference voltage inputs exhibit a constant nominal input
resistance of 5 kΩ, ±20%. On the other hand, DAC outputs I_OUTA and B are code-dependent and produce
various output resistances and capacitances.
The choice of external amplifier should take into account the variation in impedance generated by the DAC8812
on the amplifiers' inverting input node. The feedback resistance, in parallel with the DAC ladder resistance,
dominates output voltage noise. For multiplying mode applications, an external feedback compensation capacitor
(C_FB) may be needed to provide a critically damped output response for step changes in reference input
voltages.
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