AD7545A Analog Devices, AD7545A Datasheet - Page 4

• Manufacturer Part Number: AD7545A
• Description: CMOS 12-Bit Buffered Multiplying DAC
• Manufacturer: Analog Devices

AD7545A
CIRCUIT INFORMATION—D/A CONVERTER SECTION
Figure 1 shows a simplified circuit of the D/A converter section
of the AD7545A, and Figure 2 gives an approximate equivalent
circuit. Note that the ladder termination resistor is connected to
AGND. R is typically 15 kΩ.
The binary weighted currents are switched between the OUT1
bus line and AGND by N-channel switches, thus maintaining a
constant current in each ladder leg independent of the switch
state.
The capacitance at the OUT1 bus line, C
dependent and varies from 70 pF (all switches to AGND) to
150 pF (all switches to OUT1).
One of the current switches is shown in Figure 2. The input
resistance at V
the V
a reference voltage or a reference current, ac or dc, of positive or
negative polarity. (If a current source is used, a low temperature
coefficient external R
CIRCUIT INFORMATION—DIGITAL SECTION
Figure 3 shows the digital structure for one bit.
The digital signals CONTROL and CONTROL are generated
from CS and WR.
The input buffers are simple CMOS inverters designed such
that when the AD7545A is operated with V
convert TTL input levels (2.4 V and 0.8 V) into CMOS logic
levels. When V
REF
Figure 2. N-Channel Current Steering Switch
Figure 1. Simplified D/A Circuit of AD7545A
pin is constant, the reference terminal can be driven by
Figure 3. Digital Input Structure
REF
IN
is in the region of 2.0 volts to 3.5 volts, the
(Figure 1) is always equal to R. Since R
FB
is recommended to define scale factor.)
OUT1
DD
= 5 V, the buffers
, is code-
IN
at
–4–
input buffers operate in their linear region and draw current
from the power supply. To minimize power supply currents it is
recommended that the digital input voltages be as close to the
supply rails (V
The AD7545A may be operated with any supply voltage in the
range 5 ≤ V
levels are CMOS compatible only, i.e., 1.5 V and 13.5 V.
BASIC APPLICATIONS
Figures 4 and 5 show simple unipolar and bipolar circuits using
the AD7545A. Resistor R1 is used to trim for full scale. The L,
C, U grades have a guaranteed maximum gain error of ± 1 LSB
at +25°C, and in many applications it should be possible to
dispense with gain trim resistors altogether. Capacitor C1 pro-
vides phase compensation and helps prevent overshoot and
ringing when using high speed op amps. Note that all the cir-
cuits of Figures 4, 5 and 6 have constant input impedance at the
V
The circuit of Figure 4 can either be used as a fixed reference
D/A converter so that it provides an analog output voltage in the
range 0 to –V
or V
an attenuator (2-Quadrant Multiplier). V
in the range –20 ≤ V
handle such voltages) since V
Table II shows the code relationship for the circuit of Figure 4.
Binary Number in
DAC Register
1 1 1 1
1 0 0 0
0 0 0 0
0 0 0 0
Table II. Unipolar Binary Code Table for Circuit of Figure 4
REF
Table I. Recommended Trim Resistor Values vs. Grades
IN
terminal.
can be an ac signal in which case the circuit behaves as
1 1 1 1
0 0 0 0
0 0 0 0
0 0 0 0
DD
Figure 4. Unipolar Binary Operation
IN
Trim Resistor
R1
R2
DD
≤ 15 volts. With V
(note the inversion introduced by the op amp)
and DGND) as is practically possible.
IN
≤ +20 volts (provided the op amp can
1 1 1 1
0 0 0 0
0 0 0 1
0 0 0 0
REF
K/B/T
200 Ω
68 Ω
DD
is permitted to exceed V
= +15 V the input logic
Analog Output
–V
–V
–V
0 Volts
IN
IN
IN
IN









L/C/U
100 Ω
33 Ω
can be any voltage
4096
2048
4096
4096
4095
1




 = –1/2 V




REV. C
DD
.
IN