Datasheet

8
LTC1590
Unipolar 2-Quadrant Multiplying Mode
(V
OUT
= 0V to –V
REF
)
The LTC1590 can be used with a dual op amp to provide
a dual 2-quadrant multiplying DAC as shown in Figure 2.
The unipolar DAC transfer function is shown in Table 1.
The 33pF feedback capacitor is recommended to compen-
sate for the pole caused by the internal feedback resistor
and the OUT1 output capacitance. For high speed op amps
this feedback capacitor is required for stability, and a
smaller value, 8pF to 15pF, may be desired to get the
fastest transient response and shortest settling time. A
larger feedback capacitor can be used to reduce wideband
noise, glitch impulse and distortion for lower frequency
signals. A pole is introduced in the DAC transfer function
at approximately (C
FB
)(R
FB
). For example, a 100pF feed-
back capacitor will typically give a pole at:
145
1
2100 11
kHz
pF k
=
()()
πΩ
APPLICATIONS INFORMATION
WUU
U
Figure 3. Bipolar Operation (4-Quadrant Multiplication)
–15V
5V
0.1µF
V
CC
V
REF
LTC1590
DAC A OR DAC B
R
FB
DGND AGND
V
REF
–10V TO 10V
OUT1
OUT2
V
OUT
–V
REF
TO V
REF
1590 F03
15V
–
+
1/2
LT1112
R3
20k
R1
10k
R2
20k
15V
–15V
–
+
1/2
LT1112
33pF
Table 1. Unipolar Binary Code Table
DIGITAL INPUT
BINARY NUMBER ANALOG OUTPUT
IN DAC REGISTER V
OUT
MSB LSB
1111 1111 1111 –V
REF
(4095/4096)
1000 0000 0000 –V
REF
(2048/4096) = –V
REF
/2
0000 0000 0001 –V
REF
(1/4096)
0000 0000 0000 0V
Bipolar 4-Quadrant Multiplying Mode
(V
OUT
= –V
REF
to +V
REF
)
The circuit of Figure 3 can be used to provide a dual
4-quadrant multiplying DAC. This circuit starts with the
unipolar application circuit and adds three resistors and an
op amp. These extra devices provide a gain of –2 from the
unipolar output to the bipolar output, plus an offset of
(–1)(V
REF
) to produce the transfer function shown in Table
2. A pack of matched 20k resistors, with two resistors in
parallel forming the 10k resistor, is recommended.
Table 2. Bipolar Offset Binary Code Table
DIGITAL INPUT
BINARY NUMBER ANALOG OUTPUT
IN DAC REGISTER V
OUT
MSB LSB
1111 1111 1111 +V
REF
(2047/2048)
1000 0000 0001 +V
REF
(1/2048)
1000 0000 0000 0V
0111 1111 1111 –V
REF
(1/2048)
0000 0000 0000 –V
REF
(2048/2048) = – V
REF
V
CC
V
REF
LTC1590
DAC A OR DAC B
R
FB
DGND AGND
5V
V
REF
–10V TO 10V
OUT2
OUT1
33pF
V
OUT
0V TO –V
REF
1590 F02
0.1µF
–
+
1/2
LT1112
15V
–15V
Figure 2. Unipolar Operation (2-Quadrant Multiplication)