AD7537

REV.

–6–

SEPARATE AGND PINS

The DACs in the AD7537 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 program-

mable attenuator. AGNDA is at ground potential. DAC B is op-

erating with AGND B biased to +5 V by the AD584. This gives

an output range of +5 V to +10 V.

Figure 6. AD7537 DACs Used in Different Modes

PROGRAMMABLE OSCILLATOR

Figure 7 shows a conventional state variable oscillator in which

the AD7537 controls the programmable integrators. The fre-

quency of oscillation is given by:

f

=

1

2

π

R

6

R

5

×

1

C

1

DACs. 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

Since R

EQ

=

× R

LAD

N

f

=

1

2

π

×

1

C

=

1

2

π

×

D

C

1

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.

Figure 7. Programmable State Variable Oscillator

A