Rainbow Electronics MAX5295 - Manual (Page 28)

MAX5290–MAX5295
Buffered, Fast-Settling, Dual, 12-/10-/8-Bit,
Voltage-Output DACs
28 ______________________________________________________________________________________
Applications Information
Unipolar Output
Figure 7 shows the unity gain of the MAX5290 in a
unipolar output configuration. Table 24 lists the unipolar
output codes.
Bipolar Output
The MAX5290 outputs can be configured for bipolar
operation, as shown in Figure 8. The output voltage is
given by the following equation:
V
OUT_
= V
REF
x (CODE - 2048) / 2048
where CODE represents the numeric value of the
DAC’s binary input code (0 to 4095 decimal). Table 25
shows digital codes and the corresponding output volt-
age for the Figure 8 circuit.
Configurable Output Gain
The MAX5291/MAX5293/MAX5295 have force-sense out-
puts, which provide a connection directly to the inverting
terminal of the output op amp, yielding the most flexibility.
The advantage of the force-sense output is that specific
gains can be set externally for a given application. The
gain error for the MAX5291/MAX5293/MAX5295 is speci-
fied in a unity-gain configuration (op-amp output and
inverting terminals connected) and additional gain error
results from external resistor tolerances. The force-sense
DACs allow many useful circuits to be created with only a
few simple external components.
An example of a custom, fixed gain using the
MAX5291’s force-sense output is shown in Figure 9. In
this example, the external reference is set to 1.25V, and
the gain is set to +1.1V/V with external discrete resis-
tors to provide an approximate 0 to 1.375V DAC output
voltage range.
V OUT_ = [(0.5 x V REF x CODE) / 4096] x [1 + (R2 / R1)]
where CODE represents the numeric value of the
DAC’s binary input code (0 to 4095 decimal).
In this example, if R2 = 12k Ω and R1 = 10kΩ, set the
gain = 1.1V/V:
V OUT_ = [(0.5 x 1.25V x CODE) / 4096] x 2.2
Table 25. Bipolar Code Table (Gain = +1)
DAC CONTENTS
MSB LSB
ANALOG OUTPUT
1111 1111 1111 +V
REF
(2047 / 2048)
1000 0000 0001 +V REF (1 / 2048)
1000 0000 0000 0
0111 1111 1111 +V REF (1 / 2048)
0000 0000 0001 -V REF (2047 / 2048)
0000 0000 0000 -V REF (2048 / 2048) = -V REF
Figure 8. Bipolar Output Circuit
MAX5290
DAC_ REF
OUT_
10k Ω 10k Ω
V+
V-
V OUT_
MAX5291
DAC_ REF
OUT_
FB_
R2 = 12k Ω
0.1%
25ppm
R1 = 10k Ω
0.1%
25ppm
Figure 9. Configurable Output Gain