AD7937

–6–

REV. 0

SEPARATE AGND PINS

The DACs in the AD7937 have separate AGND lines taken to

pins AGNDA and AGNDB on the package. This increases the

applications versatility of the part. Figure 6 is an example of this.

DAC A is connected in standard fashion as a programmable

attenuator. AGNDA is at ground potential. DAC B is operating

with AGND B biased to 5 V by the AD584. This gives an out-

put range of 5 V to 10 V.

AGNDA

A1

AGNDB

DGND

DATA

INPUT

AD7937*

DB7

DB0

DAC A

20V p–p

A2

DAC B

AD584

5V

SIGNAL

GROUND

TO 10V

Figure 6. DACs Used in Different Modes

PROGRAMMABLE OSCILLATOR

Figure 7 shows a conventional state variable oscillator in which

the AD7937 controls the programmable integrators. The frequency

of oscillation is given by:

f

=

1

2

π

R6

R5

×

1

C1

The same digital code is loaded into both DACs. If C1 = C2

and R5 = R6, the expression reduces to

f

=

1

2

π

×

1

C

1

× R

EQ2

=

× R

LAD

N

f

=

1

2

π

×

1

C

× R

LAD2

=

1

2

π

×

D

C

1

× R

LAD2

D

=

N









=

1

2

π

×

D

C

× R

LAD m

where m is the DAC ladder resistance mismatch ratio, typically

1.005.

With the values shown in Figure 7, the output frequency varies

from 0 Hz to 1.38 kHz. The amplitude of the output signal at

the A3 output is 10 V peak-to-peak and is constant over the

entire frequency span.

AGNDA

1/2 AD7937

R6

10k

R4

200k

A1

1/2

AD712

10k

A2

DAC A

C1

10,000pF

AGNDB

1/2 AD7937

1/2

AD712

A3

DAC B

C2

10,000pF

R5

10k

AD711

FREQUENCY

SELECT CODE

5.1V

NOTE

DAC CONTROL INPUTS OMITTED FOR CLARITY

Figure 7. Programmable State Variable Oscillator