LTC1099

11

APPLICATIONS

Analog Multiplier

The schematic Figure 9 shows the LTC1099 configured

with a DAC to form a two quadrant analog multiplier. An

input waveform is applied to the LTC1099 where it is

digitized at a 300kHz rate. The digitized signal is fed to the

DAC in “flow-through” mode where another signal is input

to the DAC reference input. In this way, the two analog

signals are multiplied to produce a double sideband ampli-

tude modulated output. Figure 10 shows a 3kHz sine wave

multiplied by a 100Hz triangle.

–

+

1

2

3

4

5

6

7

1

2

3

4

5

6

7

8

9

10

1

2

3

4

5

6

7

8

9

10

11

12

14

13

12

11

10

9

8

20

19

18

17

16

15

14

13

12

11

24

23

22

21

20

19

18

17

16

15

14

13

N/C

N/C

N/C

CLK

3MHz

OSC

5V

5V

OUT

74LS90

LT1019-5

÷ 10 = 300kHz

= ANALOG GROUND

= DIGITAL GROUND

AGND

25k

OUT

TRIM

GND

IN

LTC1099

DB0

DB1

DB2

DB3

WR/RDY

MODE

RD

INT

GND

CS

WR1

GND

DI5

DI4

DI3

DI2

DI1

DI0

RFB

WR2

XFER

DI6

DI7

DI8

DI9

DI10

DI11

DB7

DB6

DB5

DB4

CS

BYTE 1/

BYTE 2

ANALOG

INPUT

ANALOG

INPUT

CS AND RD LOW

4

DB0-DB3

4

DB4-DB7

88

MICROLINEAR

MP1208 DAC

12V

10

µF

4.7

µF

0.01

µF

10

µF

50k

OFFSET NULL

15V

–15V

15V

10pF

1

5

LT1056

5V

Figure 9

Note that since this is only a two quadrant multiplier, a

carrier component (the input to the LTC1099) will appear

in the output spectrum. Figure 11 shows the frequency

spectrum of a 42.5kHz sine wave multiplied by a 5kHz sine

wave. The depth of modulation is about 30dB. Figure 12

shows a 42.375kHz sine wave multiplied by a 30.875kHz

sine wave. Note that at these higher frequencies, the depth

of modulation is still about 30dB. The carrier feed-through

is seen in Figure 12.

TRIANGLE INTO LTC1099

~100Hz

INTO DAC ~ 3kHz

1099 F10

Figure 10