AD7524
CIRCUIT DESCRIPTION
CIRCUIT INFORMATION
WRITE MODE
The AD7524, an 8-bit multiplying D/A converter, consists of a
highly stable thin film R-2R ladder and eight N-channel current
switches on a monolithic chip. Most applications require the
addition of only an output operational amplifier and a voltage
or current reference.
The simplified D/A circuit is shown in Figure 1. An inverted
R-2R ladder structure is used—that is, the binarily weighted
currents are switched between the OUT1 and OUT2 bus lines,
thus maintaining a constant current in each ladder leg indepen-
dent of the switch state.
When
CS
and
WR
are both LOW, the AD7524 is in the
WRITE mode, and the AD7524 analog output responds to data
activity at the DB0–DB7 data bus inputs. In this mode, the
AD7524 acts like a nonlatched input D/A converter.
HOLD MODE
When either
CS
or
WR
is HIGH, the AD7524 is in the HOLD
mode. The AD7524 analog output holds the value correspond-
ing to the last digital input present at DB0–DB7 prior to
WR
or
CS
assuming the HIGH state.
MODE SELECTION TABLE
CS
L
H
X
WR
L
X
H
Mode
Write
Hold
Hold
DAC Response
DAC responds to data bus
(DB0–DB7) inputs.
Data bus (DB0–DB7) is
Locked Out:
DAC holds last data present
when
WR
or
CS
assumed
HIGH state.
L = Low State, H = High State, X = Don't Care.
WRITE CYCLE TIMING DIAGRAM
Figure 1. Functional Diagram
EQUIVALENT CIRCUIT ANALYSIS
The equivalent circuit for all digital inputs LOW is shown in
Figures 2. In Figure 2 with all digital inputs LOW, the refer-
ence current is switched to OUT2. The current source I
LEAKAGE
is composed of surface and junction leakages to the substrate
while the
1
current source represents a constant 1-bit cur-
256
rent drain through the termination resistor on the R-2R ladder.
The “ON” capacitance of the output N-channel switches is
120 pF, as shown on the OUT2 terminal. The “OFF” switch
capacitance is 30 pF, as shown on the OUT1 terminal. Analysis
of the circuit for all digital inputs high is similar to Figure 2
however, the “ON” switches are now on terminal OUT1, hence
the 120 pF appears at that terminal.
Figure 2. AD7524 DAC Equivalent Circuit—All Digital
Inputs Low
INTERFACE LOGIC INFORMATION
MODE SELECTION
AD7524 mode selection is controlled by the
CS
and
WR
inputs.
Figure 3. Supply Current vs. Logic Level
Typical plots of supply current, I
DD
, versus logic input voltage,
V
IN
, for V
DD
= +5 V and V
DD
= +15 V are shown above.
–4–
REV. B
AD [ ANALOG DEVICES ]
