DAC8143_15 Datasheet, PDF (6/12 Pages) Analog Devices – 12-Bit Serial Daisy-Chain CMOS D/A Converter

DAC8143
DEFINITION OF SPECIFICATIONS
RESOLUTION
The resolution of a DAC is the number of states (2ⁿ) into which
the full-scale range (FSR) is divided (or resolved), where “n” is
equal to the number of bits.
SETTLING TIME
Time required for the analog output of the DAC to settle to
within 1/2 LSB of its final value for a given digital input stimu-
lus; i.e., zero to full-scale.
GAIN
Ratio of the DAC’s external operational amplifier output voltage
to the V_REF input voltage when all digital inputs are HIGH.
FEEDTHROUGH ERROR
Error caused by capacitive coupling from V_REF to output.
Feedthrough error limits are specified with all switches off.
OUTPUT CAPACITANCE
Capacitance from I_OUT1 to ground.
OUTPUT LEAKAGE CURRENT
Current appearing at I_OUT1 when all digital inputs are LOW, or
at I_OUT2 terminal when all inputs are HIGH.
GENERAL CIRCUIT INFORMATION
The DAC8143 is a 12-bit serial-input, buffered serial-output,
multiplying CMOS D/A converter. It has an R-2R resistor lad-
der network, a 12-bit input shift register, 12-bit DAC register,
control logic circuitry, and a buffered digital output stage.
The control logic forms an interface in which serial data is
loaded, under microprocessor control, into the input shift regis-
ter and then transferred, in parallel, to the DAC register. In
addition, buffered serial output data is present at the SRO pin
when input data is loaded into the input register. This buffered
data follows the digital input data (SRI) by 12 clock cycles and
is available for daisy-chaining additional DACs.
An asynchronous CLEAR function allows resetting the DAC
register to a zero code (0000 0000 0000) without altering data
stored in the registers.
A simplified circuit of the DAC8143 is shown in Figure 10. An
inversed R-2R ladder network consisting of silicon-chrome,
thin-film resistors, and twelve pairs of NMOS current-steering
switches. These switches steer binarily weighted currents into
either I_OUT1 or I_OUT2. Switching current to I_OUT1 or I_OUT2 yields
a constant current in each ladder leg, regardless of digital input
code. This constant current results in a constant input resis-
tance at V_REF equal to R (typically 11 kΩ). The V_REF input may
be driven by any reference voltage or current, ac or dc, that is
within the limits stated in the Absolute Maximum Ratings chart.
The twelve output current-steering switches are in series with
the R-2R resistor ladder, and therefore, can introduce bit errors.
It was essential to design these switches such that the switch
“ON” resistance be binarily scaled so that the voltage drop
across each switch remains constant. If, for example, Switch 1
of Figure 10 was designed with an “ON” resistance of 10 Ω,
Switch 2 for 20 Ω, etc., a constant 5 mV drop would then be
maintained across each switch.
To further ensure accuracy across the full temperature range,
permanently “ON” MOS switches were included in series with
the feedback resistor and the R-2R ladder’s terminating resistor.
The Simplified DAC Circuit, Figure 10, shows the location of
these switches. These series switches are equivalently scaled to
two times Switch 1 (MSB) and top Switch 12 (LSB) to main-
tain constant relative voltage drops with varying temperature.
During any testing of the resistor ladder or R_FEEDBACK (such as
incoming inspection), V_DD must be present to turn “ON” these
series switches.
V_REF
10k⍀
10k⍀
10k⍀
20k⍀
S₁
20k⍀
S₂
20k⍀
S₃
20k⍀
S₁₂
20k⍀
*
BIT 1 (MSB) BIT 2
BIT 3
BIT 12 (LSB)
DIGITAL INPUTS
(SWITCHES SHOWN FOR DIGITAL INPUTS "HIGH")
I_OUT2
10k⍀
I_OUT1
*
R_FEEDBACK
*THESE SWITCHES
PERMANENTLY "ON"
Figure 10. Simplified DAC Circuit
–6–
REV. C