Datasheet

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THEORY OF OPERATION
D/A SECTION
REF (+)
REF (-)
Resistor
String
Output
Amplifier
V
OUT
GND
V
DD
DAC Register
V
OUT
 




RESISTOR STRING
V
DD
To Output
Amplifier
R
R R
R
GND
OUTPUT AMPLIFIER
I
2
C Interface
DAC5571
SLAS405A – DECEMBER 2003 – REVISED AUGUST 2005
The architecture of the DAC5571 consists of a string DAC followed by an output buffer amplifier. Figure 39
shows a block diagram of the DAC architecture.
Figure 39. R-String DAC Architecture
The input coding to the DAC5571 is unsigned binary, which gives the ideal output voltage as:
where D = decimal equivalent of the binary code that is loaded to the DAC register; it can range from 0 to 255.
The resistor string section is shown in Figure 40 . It is basically a divide-by-2 resistor, followed by a string of
resistors, each of value R. The code loaded into the DAC register determines at which node on the string the
voltage is tapped off to be fed into the output amplifier by closing one of the switches connecting the string to the
amplifier. Because the architecture consists of a string of resistors, it is specified monotonic.
Figure 40. Resistor String
The output buffer amplifier is a gain-of-2 amplifier, capable of generating rail-to-rail voltages on its output, which
gives an output range of 0 V to V
DD
. It is capable of driving a load of 2 k Ω in parallel with 1000 pF to GND. The
source and sink capabilities of the output amplifier can be seen in the typical characteristics curves. The slew
rate is 1 V/µs with a half-scale settling time of 7 µs with the output unloaded.
I
2
C is a two-wire serial interface developed by Philips Semiconductor (see I
2
C-Bus Specification, Version 2.1,
January 2000). The bus consists of a data line (SDA) and a clock line (SCL) with pullup structures. When the bus
is idle, both SDA and SCL lines are pulled high. All the I
2
C compatible devices connect to the I
2
C bus through
open drain I/O pins, SDA and SCL. A master device, usually a microcontroller or a digital signal processor,
controls the bus. The master is responsible for generating the SCL signal and device addresses. The master also
generates specific conditions that indicate the START and STOP of data transfer. A slave device receives and/or
transmits data on the bus under control of the master device.
The DAC5571 works as a slave and supports the following data transfer modes, as defined in the I
2
C-Bus
12