Corporation

SIGNAL PROCESSING EXCELLENCE

163

Output

00

0

0

1

1/4 Full-Scale

1

0

1/2 Full-Scale

1

1

3/4 Full-Scale

Table 1. Contribution of the two MSB's

VREF

VDD

470

Ω

DIGITAL

INPUTS

RFEEDBACK

I O1

+

-

I O2

GND

ROS

A

V OUT

SP7514

HS3140

200

Ω

400

Ω

Figure 2. Unipolar Operation

accuracy required of any switch for a given overall

converter accuracy.

With the decoded converter described above, a 1%

change in any of the converter’s switches will affect

the output by no more than 0.25% of full-scale as

compared to 0.5% for a conventional converter. In

other words the conventional D/A converter can be

made less sensitive to the quality of its individual bits

by decoding.

In the SP7514/HS3140 the first four MSB’s are

decodedinto16levelswhichdrive15equallyweighted

current sources. The sensitivity of each switch on the

output is reduced by a factor of 8. Each of the 15

sources contributes 6.25% output change rather than

an MSB change of 50% for the common approach.

FollowingthedecodedsectionoftheDACastandard

binary weighted R-2R approach is used. This divides

each of the 16 levels (or 6.25% of F.S.) into 4096

discrete levels (the 12 LSB’s).

Output Capacitance

The SP7514/HS3140 have very low output capaci-

tance(C

andallswitchesOFF.Outputcapacitancevariesfrom

50pF to 100pF over all input codes. This low capaci-

tance is due in part to the decoding technique used.

Smaller switches are used with resulting less capaci-

tance. Three important system characteristics are

affected by C

settlingtime,andbandwidth.TheDACoutputequiva-

lent circuit can be represented as shown in Figure 1.

Digitalfeedthroughisthechangeinanalogoutputdue

to the toggling conditions on the converter input data

lines when the analog input V

SP7514/HS3140verylowC

low digital feedthrough. Inputs to the DAC can be

buffered.Thisinputlatchwithmicroprocessorcontrol

is shown in Figure 4.

SettlingtimeisdirectlyaffectedbyC

combines with R

response, reducing bandwidth and causing excessive

phase shift - which could result in ringing and/or

oscillation.Afeedbackcapacitor,C

restore stability. Even with C

mismatch due to R

codedependentmismatchisminimizedwhenC

R

compensate for worst case

∆R

ducedbandwidthandincreasedsettlingtime.Withthe

SP7514/HS3140, small values for C

DIGITAL

INPUTS

RFEEDBACK

I O1

+

-

IO2

GND

ROS1

A

VOUT

1

+

-

A2

ROS2

V OUT1

A1, A2, OP-07

4K

Ω

4K

Ω

ROS2

R

200

Ω

VREF

VDD

470

Ω

400

Ω

SP7514

HS3140

Figure 3. Bipolar Operation

TRANSFER FUNCTION (N=14)

BINARY INPUT UNIPOLAR OUTPUT BIPOLAR OUTPUT

111...111

–V

–N

)–V

–(N – 1)

)

100...001

–V

–N

)–V

–(N – 1)

)

100...000

–V

0

011...111

–V

–N

)V

–(N – 1)

)

000…001

–V

(N – 1)

)V

–(N – 1)

)

000...000

0

V

REF

Table 2. Transfer Function