AD5626
Rev. A | Page 15 of 20
For the ±2.5 V output range and the circuit values shown in the
table in Figure 31, the transfer equation becomes
V
O
= 1.22 mV × Digital Code − 2.5 V
Similarly, for the 5 V output range, the transfer equation
becomes
V
O
= 2.44 mV × Digital Code − 5 V
GENERATING A NEGATIVE SUPPLY VOLTAGE
Some applications may require bipolar output configuration
but only have a single power supply rail available. This is very
common in data acquisition systems using microprocessor-
based systems. In these systems, only 12 V, 15 V, and/or 5 V
are available.
Figure 32 shows a method for generating a negative supply
voltage using one CD4049, a CMOS hexadecimal inverter, and
operating on 12 V or 15 V. The circuit is essentially a charge
pump where two of the six inverters are used as an oscillator.
For the values shown, the frequency of oscillation is approx-
imately 3.5 kHz and is fairly insensitive to supply voltage
because R1 > 2 × R2.
The remaining four inverters are wired in parallel for higher
output current. The square wave output is level translated by C2
to a negative-going signal rectified using a pair of 1N4001s, and
then filtered by C3. With the values shown, the charge pump
provides an output voltage of −5 V for currents loading in the
range 0.5 mA ≤ I
OUT
≤ 10 mA with a 15 V supply and 0.5 mA ≤
I
OUT
≤ 7 mA with a 12 V supply.
06757-032
7
9
11
14
6
5432
R1
510k
R3
470
D2
1N4001
R2
5.1k
C1
0.02µF
C3
47µF
D1
1N4001
1N5231
5.1V
ZENER
C2
47µF
–5V
INVERTERS = CD4049
10
12
15
+
+
Figure 32. Generating a –5 V Supply When Only 12 V or 15 V Is Available
A SINGLE-SUPPLY, PROGRAMMABLE
CURRENT SOURCE
The circuit in Figure 33 shows how the AD5626 can be used
with an OP295 single-supply, rail-to-rail, output op amp to
provide a digitally programmable current sink from V
SOURCE
that consumes less than 3.8 mA, maximum. The DAC output
voltage is applied across R1 by placing the 2N2222 transistor in
the feedback loop of the OP295. For the circuit values shown,
the full-scale output current is 1 mA, which is given by the
following equation:
R1
D
I
OUT
V095.4×
=
where DW = the binary digital input code of the AD5626.
06757-033
CS
CLR
SCLK
LDAC
SDIN
2
8
6
5
3
4
1
7
AD5626
+5V
V
S
2N2222
LOAD
P1
200
R1
4.02k
A1 = 1/2 OP295
FULL-SCALE
ADJUST
0.1µF
5
V
DD
A1
2
1
3
+
–
V
OUT
GND
Figure 33. A Single-Supply, Programmable Current Source
The usable output voltage range of the current sink is 5 V to
60 V. The low limit of the range is controlled by transistor
saturation, and the high limit is controlled by the collector-base
breakdown voltage of the 2N2222.
GALVANICALLY-ISOLATED INTERFACE
In many process control type applications, it is necessary to
provide an isolation barrier between the controller and the unit
being controlled to protect and isolate the controlling circuitry
from any hazardous common-mode voltages that may occur.
An iCoupler® can provide isolation in excess of 2.5 kV. The
serial loading structure of the AD5626 makes it ideal for
isolated interfaces as the number of interface lines is kept to
a minimum. Figure 34 illustrates a 4-channel isolated interface
using an ADuM1400. For further information, visit
http://www.analog.com/icouplers.
ENCODE DECODE
ENCODE DECODE
ENCODE DECODE
V
IA
V
IB
V
IC
V
ID
V
OA
V
OB
V
OC
V
OD
ENCODE DECODE
DuM1
00*
MICROCONTROLLER
SERIAL CLOCK OUT
SERIAL DATA OUT
SYNC OUT
CONTROL OUT
TO SCLK
TO SDIN
TO SYNC
TO LDAC
*ADDITIONAL PINS OMITTED FOR CLARITY.
06757-034
Figure 34. An iCoupler-Isolated DAC Interface
