Datasheet

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THEORY OF OPERATION
CIRCUIT OPERATION
Digital-to-Analog Converters
R R R
2R 2R 2R R
5 k
Ω
S2 S1
DGND
V
REF
X
V
DD
R
FB
X
I
OUT
X
A
GND
X
Digitalinterface onnectionsareomittedforclarity.
SwitchesS1andS2areclosed;V mustbepowered.
DD
c
V
OUT
+ *V
REF
D
16384
(1)
DAC8802
SBAS351B – AUGUST 2005 – REVISED FEBRUARY 2007
The DAC8802 contains two 14-bit, current-output, digital-to-analog converters (DACs). Each DAC has its own
independent multiplying reference input. The DAC8802 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.
The DAC8802 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
FB
X
pin is connected to the output of the external amplifier. The I
OUT
X terminal is connected to the inverting input of
the external amplifier. The A
GND
X pin should be Kelvin-connected to the load point in the circuit requiring the full
14-bit accuracy.
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 k Ω 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 :
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 DAC8802 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 k Ω , ±20%. On the other hand, DAC outputs I
OUT
A 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 DAC8802
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
(4 pF to 20 pF typical), may be needed to provide a critically damped output response for step
changes in reference input voltages.
11
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