Rainbow Electronics MAX5302 - Manual (Page 10)

MAX5302
Low-Power, 12-Bit Voltage-Output DAC
with Serial Interface
10 ______________________________________________________________________________________
DAC
V OUT
V+
+5V
V-
R1 = R2 = 10k Ω ±0.1%
REF
R1 R2
FB
OUT
MAX5302
V DD
GND
Figure 10. Bipolar Output Circuit
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 current source.
The output current is calculated with the following
equation:
I OUT = (V REF / R) (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, the input and DAC registers are cleared
(set to zero code).
For rated MAX5302 performance, V REF must be at least
1.4V below V DD . Bypass V DD with a 4.7µF capacitor in
parallel with a 0.1µF capacitor to GND. Use short lead
lengths and place the bypass capacitors as close to
the supply pins as possible.
Grounding and Layout Considerations
Digital or AC transient signals on GND can create noise
at the analog output. Connect GND to the highest-quality
ground available.
Good PC board ground layout minimizes crosstalk
between the DAC output, reference input, and digital
input. Reduce crosstalk by keeping analog lines away
from digital lines. Wire-wrapped boards are not recom-
mended.
Table 3. Bipolar Code Table
MAX5302
DAC
REF
OUT
10k
10k
GND
+5V
V
DD FB
Figure 9. Unipolar Rail-to-Rail Output Circuit
ANALOG OUTPUT
1 1 1 1 1 1 1 1 11 1 1 ( 0 )
1 0 0 0 0 0 0 0 00 0 1 ( 0 )
DAC CONTENTS
MSB LSB
1 0 0 0 0 0 0 0 00 0 0 ( 0 ) 0V
0 1 1 1 1 1 1 1 11 1 1 ( 0 )
0 0 0 0 0 0 0 0 00 0 0 ( 0 )
0 0 0 0 0 0 0 0 00 0 1 ( 0 )
+V
2047
2048
REF

 

+V
1
2048
REF

 

-V
1
2048
REF

 

-V
2047
2048
REF

 

-V
2048
2048
- V
REF REF

 

=
Note: ( ) are for sub-bit.