Datasheet

Data Sheet AD5231
Rev. D | Page 23 of 28
APPLICATIONS
BIPOLAR OPERATION FROM DUAL SUPPLIES
The AD5231 can be operated from dual supplies ±2.5 V, which
enables control of ground referenced ac signals or bipolar
operation. AC signals as high as V
DD
/V
SS
can be applied directly
across Terminal A to Terminal B with output taken from
Terminal W. See Figure 46 for a typical circuit connection.
±
2.5V p-p
AD5231
V
SS
GND
SDI
CLK
SS
SCLK
MOSI
GND
µC
±
1.25V p-p
V
DD
V
DD
+2.5V
–2.5V
CS
D = MIDSCALE
A
W
B
02739-045
Figure 46. Bipolar Operation from Dual Supplies
HIGH VOLTAGE OPERATION
The digital potentiometer can be placed directly in the feedback
or input path of an op amp for gain control, provided that the
voltage across Terminals A–B, Terminals W–A, or Terminals
W–B does not exceed |5 V|. When high voltage gain is needed,
users should set a fixed gain in an op amp operated at a higher
voltage and let the digital potentiometer control the adjustable
input. Figure 47 shows a simple implementation.
R 2R
5V
AD5231
A
W
B
15V
V+
V–
V
O
0V
TO15V
A1
–
+
C
C
2.2pF
02739-046
Figure 47. 15 V Voltage Span Control
BIPOLAR PROGRAMMABLE GAIN AMPLIFIER
There are several ways to achieve bipolar gain. Figure 48 shows
one versatile implementation. Digital potentiometer U1 sets the
adjustment range; the wiper voltage V
W2
can, therefore, be
programmed between V
i
and −KV
i
at a given U2 setting. For
linear adjustment, configure A2 as a noninverting amplifier and
the transfer function becomes






−+××






+= KK
D
R1
R2
V
V
2
I
O
)1(
1024
1
(4)
where:
K is the ratio of R
WB
/R
WA
that is set by U1.
D is the decimal equivalent of the input code.
V+
V–
OP2177
AD5231
V+
V–
OP2177
AD5231
Vi
A
W
B
–KVi
A
B
W
V
DD
V
O
V
SS
R1
R2
A
U2
A2
U1
C
C
2.2pF
V
DD
V
SS
02739-047
Figure 48. Bipolar Programmable Gain Amplifier
In the simpler (and much more usual) case where K = 1,
a pair of matched resistors can replace U1. Equation 4 can be
simplified to






−×






+= 1
1024
2
1
2
D
R1
R2
V
V
I
O
(5)
Table 20 shows the result of adjusting D with A2 configured as a
unity gain, a gain of 2, and a gain of 10. The result is a bipolar
amplifier with linearly programmable gain and 1024-step
resolution.
Table 20. Result of Bipolar Gain Amplifier
D R1 = ∞, R2 = 0 R1 = R2 R2 = 9 × R1
0 −1 −2 −10
256 −0.5 −1 −5
512 0 0 0
768 0.5 1 5
1023 0.992 1.984 9.92
10-BIT BIPOLAR DAC
If the circuit in Figure 48 is changed with the input taken from a
voltage reference and A2 configured as a buffer, a 10-bit bipolar
DAC can be realized. Compared to the conventional DAC, this
circuit offers comparable resolution but not the precision
because of the wiper resistance effects. Degradation of the
nonlinearity and temperature coefficient is prominent near
both ends of the adjustment range. On the other hand, this
circuit offers a unique nonvolatile memory feature that in some
cases outweighs any shortfall in precision.
The output of this circuit is
REF
O
V
D
V ×






−=
1
1024
2
2
(6)