Rainbow Electronics MAX522 - Manual (Page 7)

MAX522
Dual, 8-Bit, Voltage-Output
Serial DAC in 8-Pin SO Package
_______________________________________________________________________________________ 7
_______________Detailed Description
Analog Section
The MAX522 contains two 8-bit, voltage-output digital-
to-analog converters (DACs). The DACs are “inverted”
R-2R ladder networks using complementary switches
that convert 8-bit digital inputs into equivalent analog
output voltages in proportion to the applied reference
voltage.
The MAX522 has one reference input which is shared
by DAC A and DAC B. The device includes output
buffer amplifiers for both DACs and input logic for sim-
ple microprocessor (µP) and CMOS interfaces. The
power-supply range is from +5.5V down to +2.7V.
Reference Input and DAC Output Range
The voltage at REF sets the full-scale output of the
DACs. The input impedance of the REF input is code
dependent. The lowest value, approximately 8k
Ω ,
occurs when the input code is 01010101 (55hex). The
maximum value of infinity occurs when the input code
is zero.
In shutdown mode, the selected DAC output is set to
zero while the value stored in the DAC register remains
unchanged. This removes the load from the reference
input to save power. Bringing the MAX522 out of shut-
down mode restores the DAC output voltage. Because
the input resistance at REF is code dependent, the
DAC’s reference sources should have an output
impedance of no more than 5
Ω . The input capacitance
at the REF pin is also code dependent and typically
does not exceed 25pF.
The reference voltage on REF can range anywhere from
GND to V
DD
. See the Output Buffer Amplifier section for
more information.
Output Buffer Amplifiers
DAC A and DAC B voltage outputs are internally
buffered. The buffer amplifiers have a rail-to-rail
(GND to V
DD
) output voltage range.
The DAC outputs are internally divided by two and the
buffer is set to a gain of two, eliminating the need for a
buffer input voltage range to the positive supply rail.
DAC A’s output amplifier can source and sink up to
5mA of current (0.5mA for DAC B’s buffer). See the
Total Unadjusted Error vs. Digital Code graph in the
Typical Operating Characteristics. The amplifier is
unity-gain stable with a capacitive load of 0.1µF
(0.01µF for DAC B’s buffer) or greater. The slew rate is
limited by the load capacitor and is typically 0.1V/µs
with a 0.1µF load (0.01µF for DAC B’s buffer).
Shutdown Mode
When programmed to shutdown mode, the outputs of
DAC A and DAC B go into a high-impedance state.
Virtually no current flows into or out of the buffer ampli-
fiers in that state. In shutdown mode, the REF inputs
are high impedance (2M
Ω typical) to conserve current
drain from the system reference; therefore, the system
reference does not have to be powered down.
Coming out of shutdown, the DAC outputs return to the
values kept in the registers. The recovery time is equiv-
alent to the DAC settling time.
______________________________________________________________Pin Description
NAME FUNCTION
1
–
C
—
S
– Chip Select (active low). Enables data to be shifted into the 16-bit shift register. Programming commands
are executed at the rising edge of
–
C
—
S
–
.
2 SCLK Serial Clock Input. Data is clocked in on the rising edge of SCLK.
PIN
3 V DD Positive Power Supply (2.7V to 5.5V). Bypass with 0.22µF to GND.
4 GND Ground
8 DIN Serial Data Input of the 16-bit shift register. Data is clocked into the register on the rising edge of SCLK.
7 REF Reference Input for DAC A and DAC B
6 OUTB DAC B Output Voltage (Buffered). Connect 0.01µF capacitor or greater to GND.
5 OUTA DAC A Output Voltage (Buffered). Connect 0.1µF capacitor or greater to GND.