Corporation

SIGNAL PROCESSING EXCELLENCE

131

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

SP7516

HS3160

200

Ω

400

Ω

Figure 2. Unipolar Operation

The most common technique for building a D/A

converter of n bits is to use n switches to turn n current

or voltage sources on or off. The n switches and n

sourcesaredesignedsothateachswitchorbitcontrib-

utestwiceasmuchtotheD/Aconverter’soutputasthe

preceding bit. This technique is commonly known as

binary weighting and allows an n-bit converter to

combination of bits.

In such a binary-weighted converter, the switch

with the smallest contribution (the LSB) accounts

Similarly, the switch with the largest contribution

full-scale output. Thus it is easy to see that a given

percent change in the MSB will have a greater

effect on the converter’s output than would a

similar percent change in the LSB. For example, a

1% change in the LSB of a 10 bit converter would

only affect the output by 0.001% of full-scale. A

1% change in the MSB of the same converter

would affect the output by 0.5% of FSR.

In order to overcome the problem which results from

the large weighting of the MSB, the two MSB’s can

be decoded to three equally weighted sources. Table

1 shows that all combinations of the two MSB’s of a

converter result in four output levels. So by replacing

the two MSB’s with three bits equally weighted at 1/

4 full-scale and decoding the two MSB digital inputs

into three lines which drive the equally weighted bits,

the same functional performance can be obtained.

ThusbyreplacingthetwoMSBswitchesofaconven-

tional converter with three switches properly de-

coded, the contribution of any switch is reduced from

1/2to1/4.Thisreductioninsensitivityalsoreducesthe

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 SP7516/HS3160 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.

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

SP7516

HS3160

Figure 3. Bipolar Operation

TRANSFER FUNCTION (N=16)

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