Corporation

SIGNAL PROCESSING EXCELLENCE

186

Figure 4. Bipolar Guarding

-

+

15

16

14

Guard

SP9316

-

+

Figure 3. Unipolar Guarding

-

+

15

16

14

SP9316

Guard

rated from the device drift, and that all measure-

ments are taken at identical temperatures. The

long–term drift of the SP9316 voltage output

system transfer function, after initial op amp

trim, will largely be determined by the choice of

the external components. For minimum offset

error the op amps should be trimmed after one

hour of continuous operation. The SP9316 con-

tribution to the offset drift after that time will be

±0.1ppm/°C per 1000 hours. The long–term

gain drift error contribution of the SP9316 is

±1

ppm/

°C per 1000 hours. Also to be considered

are the temperature coefficients of the external

resistors, the op amp drift specifications and the

stability of the reference. The SIPEX

HS2700LD, with a stability drift of 3ppm/

°C, is

recommended as a suitable voltage reference.

The unique segmented architecture of the

SP9316 resistor network, plus the low–TCR

nichrome materials and processes used in its

manufacture, results in an exceptionally low

linearity drift with time. Typical differential

linearity drift with temperature is 0.1ppm/

°C.

APPLICATION

Digitally–Controlled Low Pass Filter

The SP9316 can be used to construct active

filters which display very low noise and distor-

tion characteristics. Low pass filters can be

designed to provide digital control over center

frequency, gain and Q-factor. The SP9316 is an

ideal high resolution element for this applica-

tion. Figure 5 shows a low–pass filter designed

to be independent of the resistance of the SP9316

network by using it as a programmable gain

element. The filter characteristic is given by:

1

1+ j

()

=

()

where D is the binary code applied to the DAC

and C is the value of the capacitor.

LSB LATCH

CONTROL

REFERENCE

INPUT

MSB LATCH

CONTROL

FEEDBACK

5KΩ

15

14

16

19

13

20

PRECISION 16-BIT

RESISTOR NETWORK

& SWICHES

BIT: 9

10 11 12 13 14 15 16 ANA GND

+15V

5

418

17

3

2

1

24 232221

BIT: 8

7

6

5

4

3

2

1(MSB)

678

910 11 12

INPUT REGISTER

INPUT REGISTER

SP9316

–

+

–

+

+5V

–

+

C

1

1+ j

()

=

()

Figure 5. Digitally–Controlled Low–Pass Filter