Datasheet

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THEORY OF OPERATION
D/A SECTION OUTPUT BUFFER AMPLIFIERS
DAC External Reference Input
_
+
Resistor String
Ref +
Ref −
DAC Register
V
OUT
V
REF
GND
V
FB
100 k100 k
50 k
Amplifier Sense Input
V
REF
To Output
Amplifier
R
R R
R
GND
RESISTOR STRING
Power-On Reset
DAC7558
SLAS435A – MAY 2005 – REVISED DECEMBER 2005
The architecture of the DAC7558 consists of a string The output buffer amplifier is capable of generating
DAC followed by an output buffer amplifier. Figure 43 rail-to-rail voltages on its output, which gives an
shows a generalized block diagram of the DAC output range of 0 V to V
DD
. It is capable of driving a
architecture. load of 2 k Ω in parallel with up to 1000 pF to GND.
The source and sink capabilities of the output
amplifier can be seen in the typical curves. The slew
rate is 1 V/µs with a half-scale settling time of 3 µs
with the output unloaded.
Four separate reference pins are provided for eight
DACs, providing maximum flexibility. VREF1 serves
DAC A and DAC B, VREF2 serves DAC C and DAC
Figure 43. Typical DAC Architecture
D, VREF3 serves DAC E and DAC F, and VREF4
serves DAC G and DAC H. VREF1 through VREF4
can be externally shorted together for simplicity.
The input coding to the DAC7558 is unsigned binary,
which gives the ideal output voltage as:
It is recommended to use a buffered reference in the
V
OUT
= V
REF
× D/4096
external circuit (e.g., REF3140). The input impedance
is typically 50 k Ω for each reference input pin.
Where D = decimal equivalent of the binary code that
is loaded to the DAC register which can range from 0
to 4095.
The DAC7558 contains eight amplifier feedback input
pins, VFBA ... VFBH. For voltage output operation,
VFBA ... VFBH must externally connect to VOUTA ...
VOUTH respectively. For better DC accuracy, these
connections should be made at load points. The
VFBA ... VFBH pins are also useful for a variety of
applications, including digitally controlled current
sources. Each feedback input pin is internally
connected to the DAC amplifier's negative input
Figure 44. Typical Resistor String
terminal through a 100-k Ω resistor; and, the
amplifier's negative input terminal internally connects
to ground through another 100-k Ω resistor (See
Figure 43 ). This forms a gain-of-two, non-inverting
The resistor string section is shown in Figure 44 . It is
amplifier configuration. Overall gain remains one
simply a string of resistors, each of value R. The
because the resistor string has a divide-by-two
DAC7558 uses eight separate resistor strings. Each
configuration. The resistance seen at each VFBx pin
VREFx input pin provides the external reference
is approximately 200 k Ω to ground.
voltage for two resistor strings. A resistor string has
100 k Ω total resistance to ground, including a 50 k Ω
divide-by-two resistor. Since each VREFx pin
On power up, all internal registers are cleared and all
connects to two resistor strings, the resistance seen
channels are updated with zero-scale voltages. Until
by each VREFx pin is approximately 50 k Ω . The
valid data is written, all DAC outputs remain in this
divide-by-two function provided by the resistor string
state. This is particularly useful in applications where
is compensated by a gain-of-two amplifier
it is important to know the state of the DAC outputs
configuration. The voltage is tapped off by closing
while the device is powering up. In order not to turn
one of the switches connecting the string to the
on ESD protection devices, V
DD
should be applied
amplifier. Because it is a string of resistors, it is
before any other pin is brought high.
specified monotonic. The DAC7558 architecture uses
eight separate resistor strings to minimize
During power up, all digital input pins should be set at
channel-to-channel crosstalk.
logic-low voltages. Shortly after power up, if RSTSEL
pin is low, then all DAC outputs are at their
zero-scale voltages. If RSTSEL pin is brought high,
then all DAC outputs are at their mid-scale voltages.
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