AD7533 Analog Devices, AD7533 Datasheet - Page 7

• Manufacturer Part Number: AD7533
• Description: CMOS Low Cost, 10-Bit Multiplying DAC
• Manufacturer: Analog Devices

Specifications of AD7533

• Resolution (bits): 10bit
• Dac Update Rate: 1.7MSPS
• Dac Settling Time: 600ns
• Max Pos Supply (v): +16.5V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

CIRCUIT DESCRIPTION
GENERAL CIRCUIT INFORMATION
The AD7533 is a 10-bit multiplying DAC that consists of a
highly stable thin-film R-2R ladder and ten CMOS 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 7. An inverted
R- 2R ladder structure is used, that is, the binarily weighted
currents are switched between the I
thus maintaining a constant current in each ladder leg
independent of the switch state.
V
One of the CMOS current switches is shown in Figure 8. The
geometries of Device 1, Device 2, and Device 3 are optimized to
make the digital control inputs DTL/TTL/CMOS compatible
over the full military temperature range. The input stage drives
two inverters (Device 4, Device 5, Device 6, and Device 7),
which in turn drive the two output N channels. The on
resistances of the switches are binarily sealed so that the voltage
drop across each switch is the same. For example, Switch 1 in
Figure 8 is designed for an on resistance of 20 Ω, Switch 2 for
40 Ω, and so on. For a 10 V reference input, the current through
Switch 1 is 0.5 mA, the current through Switch 2 is 0.25 mA,
and so on, thus maintaining a constant 10 mV drop across each
switch. It is essential that each switch voltage drop be equal if
the binarily weighted current division property of the ladder is
to be maintained.
REF
BIT 1 (MSB)
20kΩ
S1
DIGITAL INPUTS (DTL/TTL/CMOS COMPATIBLE)
10kΩ
BIT 2
Figure 7. Functional Diagram
20kΩ
S2
10kΩ
BIT 3
20kΩ
OUT
S3
1 and I
10kΩ
BIT 10 (LSB)
OUT
20kΩ
2 bus lines,
SN
10kΩ
20kΩ
I
I
R
OUT
OUT
FB
Rev. C | Page 7 of 12
2
1
EQUIVALENT CIRCUIT ANALYSIS
The equivalent circuits for all digital inputs high and digital
inputs low are shown in Figure 9 and Figure 10. In Figure 9 with
all digital inputs low, the reference current is switched to I
The current source I
leakages to the substrate, while the I/1024 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 100 pF, as shown on the I
capacitance is 35 pF, as shown on the I
the circuit for all digital inputs high, as shown in Figure 10, is
similar to Figure 9; however, the on switches are now on
Terminal I
DTL/TTL/
CMOS
INPUT
V
V
REF
REF
V+
I
I
REF
REF
Figure 10. Equivalent Circuit—All Digital Inputs High
250Ω
OUT
Figure 9. Equivalent Circuit—All Digital Inputs Low
1. Therefore, there is the 100 pF at that terminal.
R
R
R
10kΩ
1
LEAKAGE
I/1024
I/1024
Figure 8. CMOS Switch
R
2
10kΩ
3
is composed of surface and junction
I
I
I
I
LEAKAGE
LEAKAGE
LEAKAGE
LEAKAGE
OUT
5
4
2 terminal. The off switch
OUT
1 terminal. Analysis of
35pF
100pF
100pF
35pF
7
6
R
R
I
OUT
8
2
AD7533
R
I
I
R
I
I
TO LADDER
I
OUT
OUT
OUT
OUT
OUT
9
FB
FB
1
2
1
1
2
OUT
2.