DAC8512 Datasheet, PDF (11/20 Pages) Analog Devices – % V, Serial Input Complete 12-Bit DAC

DAC8512
Table III. Bipolar Code Table
Hexadecimal Number Decimal Number Analog Output
in DAC Register
in DAC Register Voltage (V)
FFF
4095
–4.9976
801
2049
–2.44E–3
800
2048
0
7FF
2047
+2.44E–3
000
0
+5
To maintain monotonicity and accuracy, R1, R2, and R4 should
be selected to match within 0.01% and must all be of the same
(preferably metal foil) type to assure temperature coefficient
matching. Mismatching between R1 and R2 causes offset and gain
errors while an R4 to R1 and R2 mismatch yields gain errors.
For applications that do not require high accuracy, the circuit
illustrated in Figure 29 can also be used to generate a bipolar
output voltage. In this circuit, only one op amp is used and no
potentiometers are used for offset and gain trim. The output
voltage is coded in offset binary and is given by:
V_O = 1 mV × Digital Code ×
 R4 
 R3 + R4
×
₁₊
R2
R1
–2.5 ×
R2
R1
+5V
0.1µF
CS
CLR
LD
SCLK
SDI
2
REF03
4
R1
6
+2.5V
R2
+5V
+5V
0.1µF
2
8
A1 1
V_O
3
4
1
2
V_DD
6 _{DAC8512 R3}
5
8
–5V
A1 = 1/2 OP295
3
R4
4
GND
7
V_OUT RANGE R1 R2 R3
R4
؎2.5V 10k 10k 10k 15.4k + 274
؎5V 10k 20k 10k 43.2k + 499
Figure 29. Bipolar Output Operation without Trim
For the ± 2.5 V output range and the circuit values shown in the
table, 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 com-
mon in data acquisition systems using microprocessor-based
systems. In these systems, +12 V, +15 V, and/or +5 V are only
available. Shown in Figure 30 is a method of generating a nega-
tive supply voltage using one CD4049, a CMOS hex inverter,
operating on +12 V or +15 V. The circuit is essentially a charge
pump where two of the six are used as an oscillator. For the val-
ues shown, the frequency of oscillation is approximately 3.5 kHz
and is fairly insensitive to supply voltage because R1 > 2 × R2.
The remaining four inverters are wired in parallel for higher out-
put 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
will provide an output voltage of –5 V for current loadings 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.
7
INVERTERS = CD4049
9
3
25
4
11
R1
R2
510kΩ
5.1kΩ
14
C1
0.02µF
6
10
C2
47µF
D2
1N4001
R3
470Ω
–5V
12
15
D1
C3
1N5231
1N4001 47µF 5.1V
ZENER
Figure 30. Generating a –5 V Supply When Only +12 V
or +15 V Is Available
A High-Compliance, Digitally Controlled Precision Current
Source
The circuit in Figure 31 shows the DAC8512 controlling a
high-compliance precision current source using an AMP05 in-
strumentation amplifier. The AMP05’s reference pin becomes
the input, and the “old” inputs now monitor the voltage across a
precision current sense resistor, R_CS. Voltage gain is set to unity,
so the transfer function is given by the following equation:
I_OUT
=
V_IN
R_CS
If R_CS equals 100 Ω, the output current is limited to +10 mA
with a 1 V input. Therefore, each DAC LSB corresponds to
2.4 µA. If a bipolar output current is required, then the circuit
in Figure 28 can be modified to drive the AMP05’s reference
pin with a ± 1 V input signal.
Potentiometer P1 trims the output current to zero with the in-
put at 0 V. Fine gain adjustment can be accomplished by adjust-
ing R1 or R2.
REV. A
–11–