Rainbow Electronics MAX550A - Manual (Page 6)

MAX548A/MAX549A/MAX550A
+2.5V to +5.5V, Low-Power, Single/Dual,
8-Bit Voltage-Output DACs in µMAX Package
6 _______________________________________________________________________________________
_______________Detailed Description
Analog Section
The MAX548A/MAX549A/MAX550A are 8-bit, voltage-
output digital-to-analog converters (DACs). The
MAX548A/MAX549A are dual DACs, and the MAX550A
is a single DAC. Each DAC consists of an R-2R ladder
network that converts 8-bit digital inputs into equivalent
analog output voltages in proportion to the applied ref-
erence voltage (Figure 1).
The DACs feature double-buffered inputs and
unbuffered outputs. The MAX549A/MAX550A require
an external reference. The MAX548A’s reference inputs
are internally connected to V
DD . The power-supply
range is from +2.5V to +5.5V.
Reference Input
The voltage applied at REF (V DD for the MAX548A) sets
the full-scale output for all the DACs and may range
from +2.5V to V
DD . The REF input resistance is code
dependent, with the lowest value occurring with code
01010101 (55 hex). To minimize INL errors, the refer-
ence voltage source should have less than 3
Ω
output
impedance.
DAC Output
The MAX548A/MAX549A/MAX550A contain DACs with
unbuffered outputs; each output connects directly to an
R-2R ladder. Typical output impedance is 33.3k
Ω
. This
configuration minimizes power consumption and
reduces offset errors. For highest accuracy, apply high
resistive loads (1M
Ω
and up). Lower resistive loads can
be driven, but output loading increases full-scale error.
The magnitude of the expected error is the ratio of the
DAC output resistance to the DC load resistance at the
output.
Typically, an energy pulse is coupled into the DAC out-
put on CS’s rising edge. Since each DAC output is
unbuffered, connecting a small capacitor (200pF to
1000pF) from the output to ground creates a lowpass
filter that effectively suppresses the pulse for sensitive
applications (see
Typical Operating Characteristics ).
Shutdown Mode
When the MAX548A/MAX549A/MAX550A are in shut-
down mode, the R-2R ladder disconnects from the refer-
ence source. The MAX549A/MAX550A supply current
does not change, but the REF input current decreases to
less than 1µA. This allows the externally applied system
reference to remain active with minimal power
consumption. The MAX548A supply current also
decreases to less than 1µA in shutdown mode. When the
MAX548A/MAX549A/MAX550A exit shutdown mode,
recovery time is equivalent to the DAC’s settling time.
Serial Interface
The serial interface is SPI/QSPI and Microwire compati-
ble. An active-low chip select (CS) enables the input
shift register to receive data from the serial input (DIN).
Data is clocked into the shift register on the rising edge
of the serial-clock signal (SCLK). The clock frequency
can be as high as 10MHz.
Transmit data MSB first in one 16-bit word or two 8-bit
bytes. The write cycle can be segmented to allow two
8-bit-wide transfers when CS remains low. After all 16
bits are clocked into the input shift register, a rising
2R
R R R R
2R 2R 2R 2R 2R
REF
GND
NOTE: SWITCH POSITIONS SHOWN FOR DAC CODE FF HEX.
DAC_ REGISTER
OUT_
MSB LSB
GND
2R
R R R
2R 2R 2R 2R
Figure 1. DAC Simplified Circuit Diagram