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(Datasheet) AD5545

16-/14-Bit DACs

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Data Sheet
THEORY OF OPERATION
The AD5545/AD5555 contain a 16-/14-bit, current-output,
digital-to-analog converter, a serial-input register, and a DAC
register. Both parts require a minimum of a 3-wire serial data
interface with an additional LDAC for dual channel simultaneous
update.
DIGITAL-TO-ANALOG CONVERTER
The DAC architecture uses a current-steering R-2R ladder
design. Figure 17 shows the typical equivalent DAC. The DAC
contains a matching feedback resistor for use with an external
I-to-V converter amplifier. The RFB pin is connected to the
output of the external amplifier. The IOUT terminal is connected
to the inverting input of the external amplifier. These DACs are
designed to operate with either negative or positive reference
voltages. The VDD power pin is used only 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
attempt to measure the RFB value, power must be applied to VDD
to achieve continuity. The VREF input voltage and the digital data
(D) loaded into the corresponding DAC register, according to
Equation 1 and Equation 2, determine the DAC output voltage.
VOUT = VREF × D /65,536
(1)
VOUT = VREF × D /16,384
(2)
Note that the output full-scale polarity is the opposite of the
VREF polarity for dc reference voltages.
VREF
2R
R
2R
RR
2R R
5k
VDD
RFB
S2 S1
IOUT
GND
DIGITAL INTERFACE CONNECTIONS OMITTED FOR CLARITY:
SWITCHES S1 AND S2 ARE CLOSED, VDD MUST BE POWERED
02918-0-005
Figure 17. Equivalent R-2R DAC Circuit
AD5545/AD5555
These DACs are also designed to accommodate ac reference input
signals. The AD5545/AD5555 accommodate input reference
voltages in the range of –12 V to +12 V. The reference voltage
inputs exhibit a constant nominal input-resistance value of
5 kΩ, ±30%. The DAC output (IOUT) is code dependent, pro-
ducing various output resistances and capacitances. When
choosing an external amplifier, the user should take into
account the variation in impedance generated by the AD5545/
AD5555 on the amplifiers inverting input node. The feedback
resistance in parallel with the DAC ladder resistance dominates
output voltage noise.
2.500V
VIN
VOUT
ADR03
5V
GND
VREFA 2R
R
2R
RR
VDD
RFBA
2R R 5k
+3V
S2 S1 IOUTA
VCC
VOUT
AD8628
AD5545/AD5555
AGNDA
VEE
LOAD
–3V
DIGITAL INTERFACE CONNECTIONS OMITTED FOR CLARITY:
SWITCHES S1 AND S2 ARE CLOSED, VDD MUST BE POWERED
Figure 18. Recommended System Connections
02918-0-006
Rev. I | Page 9 of 23
Analog Devices
Analog Devices


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