AD7541A Data Sheet
Rev. C | Page 8 of 13
THEORY OF OPERATION
The simplified digital-to-analog circuit is shown in Figure 4. An
inverted R-2R ladder structure was used, meaning the binarily
weighted currents are switched between the OUT 1 and OUT 2
bus lines, thus maintaining a constant current in each ladder leg
independent of the switch state.
DIGITAL INPUTS (DTL/TTL/CMOS COMPATIBLE)
LOGIC: A SWITCH IS CLOSED TO I
OUT 1
FOR
ITS DIGITAL INPUT IN A HIGH STATE.
20kΩ
S1
10kΩ
20kΩ
S2
10kΩ
20kΩ
S3
10kΩ
20kΩ
S12
20kΩ
V
REF
OUT 2
BIT 1 (MSB)
OUT 1
BIT 2 BIT 3 BIT 12 (LSB)
R
FEEDBACK
10kΩ
00718-005
Figure 4. Functional Diagram (Inputs High)
The input resistance at V
REF
(see Figure 4) is always equal to
R
LDR
, which is the R-2R ladder characteristic resistance and is
equal to value R. Because 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
FEEDBACK
is recommended to define the scale factor.
EQUIVALENT CIRCUIT ANALYSIS
The equivalent circuits for all digital inputs low and all digital
inputs high are shown in Figure 5 and Figure 6. In Figure 5 with
all digital inputs low, the reference current is switched to OUT 2.
The current source, I
LEAKAGE
, is composed of surface and junction
leakages to the substrate, while the I/
4096
current source represents a
constant 1-bit current drain through the termination resistor on
the R-2R ladder. The on capacitance of the output N-channel
switch is 200 pF, as shown on the OUT 2 terminal. The off
switch capacitance is 70 pF, as shown on the OUT 1 terminal.
Analysis of the circuit for all digital inputs high, as shown in
Figure 5, is similar to Figure 4; however, the on switches are
now on the OUT 1 terminal; therefore, 200 pF at that terminal.
R
FEEDBACK
R
70pF
OUT 1
I
LEAKAGE
200pF
OUT 2
I
LEAKAGE
I
/4096
R
≈
15kΩ
V
REF
I
REF
00718-006
Figure 5. DAC Equivalent Circuit, All Digital Inputs Low
R
FEEDBACK
R
OUT 1
70pF
I
LEAKAGE
OUT 2
200pF
I
LEAKAGE
I
/4096
R
≈
15kΩ
V
REF
I
REF
00718-007
Figure 6. DAC Equivalent Circuit All Digital Inputs High