Rainbow Electronics MAX5121 - Manual (Page 9)

MAX5120/MAX5121
+3V/+5V, 12-Bit, Serial Voltage-Output DACs
with Internal Reference
_______________________________________________________________________________________ 9
_______________Detailed Description
The MAX5120/MAX5121 12-bit, voltage-output DACs
are easily configured with a 3-wire serial interface. They
include a 16-bit data-in/data-out shift register and have
a double-buffered input consisting of an input register
and a DAC register. In addition, these devices employ
precision bandgap references and trimmed internal
resistors to produce a gain of 1.6384V/V, maximizing
the output voltage swing (Figure 1). The MAX5120/
MAX5121 output amplifier’s offset-adjust pin allows for
a DC shift in the DAC outputs. The full-scale output volt-
age is +4.095V for the MAX5120 and +2.0475V for the
MAX5121. These DACs are designed with an inverted
R-2R ladder network (Figure 2) that produces a weight-
ed output voltage proportional to the digital input code.
Internal Reference
Both the MAX5120 and MAX5121 use an on-board pre-
cision bandgap reference to generate an output volt-
age of +2.5V (MAX5120) or +1.25V (MAX5121). With a
low temperature coefficient of only 10ppm/°C (max),
the REF pin can source up to 100µA and may become
unstable with capacitive loads exceeding 100pF.
REFADJ can be used for minor adjustments (1%) to the
reference voltage. Use the circuits shown in Figure 3a
MAX5120
MAX5121
SR
CONTROL
16-BIT
SHIFT REGISTER
DECODE
CONTROL
INPUT
REGISTER
BANDGAP
REFERENCE
REFERENCE
BUFFER
DAC
REGISTER
DAC
2X
(1X)
DOUT
UPO
OUT
OS
R
0.6384R
GAIN = 1.6384X
4k
1.25V
AGND DGND V
DD DIN SCLK CS
2.5V, (1.25V)
LOGIC
OUTPUT
CLR
PDL
RSTVAL
PD
12
REFADJ REF ( ) FOR MAX5121 ONLY
Figure 1. Simplified Functional Diagram
OUT
OS
R
0.6384R
SHOWN FOR ALL 1s ON DAC
*INTERNAL 2.5V (MAX5120) AND 1.25V (MAX5121)
OR EXTERNAL REFERENCE.
D0 D9 D10 D11
2R 2R 2R 2R 2R
R R R
REF*
AGND
Figure 2. Simplified Inverted R-2R DAC Structure