MAX5500/MAX5501
Low-Power, Quad, 12-Bit
Voltage-Output DACs with Serial Interface
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Digitally Programmable Current Source
The circuit of Figure 12 places an npn transistor
(2N3904 or similar) within the op-amp feedback loop to
implement a digitally programmable, unidirectional cur-
rent source. This circuit drives 4mA to 20mA current
loops, which are commonly used in industrial-control
applications. The output current is calculated with the
following equation:
I
OUT
= (V
REF
/R) x (NB/4096)
where NB is the numeric value of the DAC’s binary input
code and R is the sense resistor shown in Figure 12.
Power-Supply Considerations
On power-up, all input and DAC registers are cleared
(set to zero code) and D
OUT
is in mode 0 (serial data is
shifted out of DOUT on the clock’s falling edge).
For rated MAX5500/MAX5501 performance, limit V
REFAB
/
V
REFCD
to 1.4V below V
DD
. Bypass V
DD
with a 4.7µF
capacitor in parallel with a 0.1µF capacitor to AGND.
Use short lead lengths and place the bypass capaci-
tors as close as possible to the supply inputs.
Grounding and Layout Considerations
Digital or AC transient signals between AGND and
DGND create noise at the analog outputs. Connect
AGND and DGND together at the DAC, and then con-
nect this point to the highest-quality ground available.
Good PCB ground layout minimizes crosstalk between
DAC outputs, reference inputs, and digital inputs.
Reduce crosstalk by keeping analog lines away from
digital lines. Do not use wire-wrapped boards.
Chip Information
PROCESS: BiCMOS
