10

LTC1657

TYPICAL APPLICATIO S

This circuit shows a digitally programmable current source

from an external voltage source using an external op amp,

an LT1218 and an NPN transistor (2N3440). Any digital

word from 0 to 65535 is loaded into the LTC1657 and its

output correspondingly swings from 0V to 4.096V. This

chosen to be 412

Ω, the output current will range from

0mA at zero scale to 10mA at full scale. The minimum

voltage source can be 5V.

0.1

µF

5V

1657 TA04

Q1

2N3440

412

Ω

1%

≈ 0mA TO 10mA

DATA (0:15)

CSLSB

CSMSB

WR

LDAC

CLR

GND

REFOUT

REFHI

REFLO

X1/X2

LTC1657

µP

7

6

4

26

21

20

22

23

25

3

5:19

2

3

1

28

27

2

–

+

LT1218

Digitally Programmable Current Source

A Wide Swing, Bipolar Output 16-Bit DAC

This circuit shows how to make a bipolar output 16-bit

DAC with a wide output swing using an LTC1657 and an

LT1077. R1 and R2 resistively divide down the LTC1657

output and an offset is summed in using the LTC1657

onboard 2.048V reference and R3 and R4. R5 ensures that

the onboard reference is always sourcing current and

scale. The LT1077 output will have a wide bipolar output

swing of – 4.096V to 4.096V as shown in the figure below.

With this output swing, 1LSB = 125

µV.

DATA (0:15)

CSLSB

CSMSB

WR

LDAC

CLR

0.1

µF

24

25

7

6

4

26

21

20

22

23

3

5:19

2

3

1

28

27

2

GND

REFHI REFOUT

REFLO

X1/X2

R1

100k

1%

5V

LTC1657

1657 TA05

–

+

LT1077

5V

–5V

R2

200k

1%

R3

100k

1%

R5

100k

1%

R4

200k

1%

65536

– 4.096V

4.096

– 4.096

32768

0

65535

TRANSFER CURVE

µP