Rainbow Electronics MAX5515 - Manual (Page 18)

MAX5512–MAX5515
Voltage Biasing a
Current-Output Transducer
See the circuit in Figure 5 for an illustration of how to
configure the MAX5514/MAX5515 to bias a current-out-
put transducer. In
Figure 5, the output voltage of the
MAX5514/MAX5515 is a function of the voltage drop
across the transducer added to the voltage drop
across the feedback resistor R.
Unipolar Output
Figure 6 shows the MAX5514 in a unipolar output con-
figuration with unity gain. Table 4 lists the unipolar out-
put codes.
Bipolar Output
The MAX5514 output can be configured for bipolar
operation as shown in
Figure 7. The output voltage is
given by the following equation:
V OUT_ = V REFIN x [(N A - 128) / 128]
where N A represents the decimal value of the DAC’s
binary input code. Table 5 shows the digital codes (off-
set binary) and the corresponding output voltage for
the circuit in
Figure 7.
Configurable Output Gain
The MAX5514/MAX5515 have force-sense outputs,
which provide a connection directly to the inverting ter-
minal 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 MAX5514/MAX5515 is specified in a
unity-gain configuration (op-amp output and inverting
terminals connected), and additional gain error results
from external resistor tolerances. Another advantage of
the force-sense DAC is that it allows many useful circuits
to be created with only a few simple external components.
An example of a custom fixed gain using the MAX5514/
MAX5515 force-sense output is shown in
Figure 9. In
this example, R1 and R2 set the gain for V
OUTA
.
V OUTA = [(V REFIN x N A ) / 256] x [1 + (R2 / R1)]
where N A represents the numeric value of the DAC
input code.
Dual, Ultra-Low-Power,
8-Bit, Voltage-Output DACs
18 ______________________________________________________________________________________
Table 4. Unipolar Code Table (Gain = +1)
DAC CONTENTS
MSB LSB
ANALOG OUTPUT
1111 1111 0000 +V
REF
(255/256)
1000 0001 0000 +V REF (129/256)
1000 0000 0000 +V REF (128/256) = +V REF /2
0111 1111 0000 +V REF (127/256)
0000 0001 0000 +V REF (1/256)
0000 0000 0000 0V
Table 5. Bipolar Code Table (Gain = +1)
DAC CONTENTS
MSB LSB
ANALOG OUTPUT
1111 1111 0000 +V REF (127/128)
1000 0001 0000 +V REF (1/128)
1000 0000 0000 0V
0111 1111 0000 -V REF (1/128)
0000 0001 0000 -V REF (127/128)
0000 0000 0000 -V REF (128/128) = -V REF
N
A
IS THE DAC_ INPUT CODE
(0 TO 255 DECIMAL).
REFIN
MAX5514
OUT_
FB_
V OUT =
V REFIN × N A
256
DAC
Figure 6. Unipolar Output Circuit
REFIN
1/2 MAX5514
OUT_
V OUT
FB_
V+
10kΩ 10kΩ
V-
DAC
Figure 7. Bipolar Output Circuit