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.
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
DD
and DGND) as is practically possible.
The AD7545A may be operated with any supply voltage in the
range 5
≤
V
DD
≤
15 volts. With V
DD
= +15 V the input logic
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
REF
terminal.
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
IN
(note the inversion introduced by the op amp)
or V
IN
can be an ac signal in which case the circuit behaves as
an attenuator (2-Quadrant Multiplier). V
IN
can be any voltage
in the range –20
≤
V
IN
≤
+20 volts (provided the op amp can
handle such voltages) since V
REF
is permitted to exceed V
DD
.
Table II shows the code relationship for the circuit of Figure 4.
Figure 1. Simplified D/A Circuit of AD7545A
The capacitance at the OUT1 bus line, C
OUT1
, is code-
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
REF
(Figure 1) is always equal to R. Since R
IN
at
the V
REF
pin is constant, the reference terminal can be driven by
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
FB
is recommended to define scale factor.)
Figure 4. Unipolar Binary Operation
Table I. Recommended Trim Resistor Values vs. Grades
Trim Resistor
Figure 2. N-Channel Current Steering Switch
CIRCUIT INFORMATION—DIGITAL SECTION
K/B/T
200
Ω
68
Ω
L/C/U
100
Ω
33
Ω
R1
R2
Figure 3 shows the digital structure for one bit.
The digital signals CONTROL and
CONTROL
are generated
from
CS
and
WR.
Table II. Unipolar Binary Code Table for Circuit of Figure 4
Binary Number in
DAC Register
Analog Output
4095
–V
IN
4096
1111
1111
1111
1000
Figure 3. Digital Input Structure
0000
0000
–V
IN
2048
4096
= –1/2 V
IN
1
4096
The input buffers are simple CMOS inverters designed such
that when the AD7545A is operated with V
DD
= 5 V, the buffers
convert TTL input levels (2.4 V and 0.8 V) into CMOS logic
levels. When V
IN
is in the region of 2.0 volts to 3.5 volts, the
–4–
0000
0000
0000
0000
0001
0000
–V
IN
0 Volts
REV. C
AD [ ANALOG DEVICES ]
