8

LTC1590

Unipolar 2-Quadrant Multiplying Mode

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

back capacitor will typically give a pole at:

145

1

2100

11

kHz

pF

k

=

()( )

πΩ

Figure 3. Bipolar Operation (4-Quadrant Multiplication)

–15V

5V

0.1

µF

LTC1590

DAC A OR DAC B

DGND

AGND

–10V TO 10V

OUT1

OUT2

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

MSB

LSB

1111 1111 1111

1000 0000 0000

0000 0000 0001

0000 0000 0000

0V

Bipolar 4-Quadrant Multiplying Mode

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

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

MSB

LSB

1111 1111 1111

1000 0000 0001

1000 0000 0000

0V

0111 1111 1111

0000 0000 0000

LTC1590

DAC A OR DAC B

DGND

AGND

5V

–10V TO 10V

OUT2

OUT1

33pF

1590 F02

0.1

µF

1/2

LT1112

15V

–15V

Figure 2. Unipolar Operation (2-Quadrant Multiplication)