AD7112

REV. 0

–11–

OUTPUT

INPUT

VARIABLE

GAIN ELEMENT

1/2 AD7112

D

UP/DOWN COUNTER

U

END STOP AND

CONTROL LOGIC

DETECTOR

COMPARATOR

Figure 21. Automatic Gain Control System

Programmable State Variable Filter

The AD7112 with its multiplying capability and fast settling

time is ideal for many types of signal conditioning applications.

The circuit of Figure 22 shows its use in a state variable filter

design. This type of filter has three outputs: low pass, bandpass

and high pass. The particular version shown in Figure 22 uses

stead of several fixed resistors, the circuit uses the DAC equiva-

lent resistances as circuit elements. Thus, R1 in Figure 22 is

controlled by the 8-bit word loaded to DAC A1 of the AD7112.

This is also the case with R2, R3 and R4.

DAC Equivalent Resistance,

10

× EXP (–0.375 × N /20)

where:

N is the DAC code in Decimal (0

≤N≤240).

DACs A1 and B1 control the gain and Q of the filter character-

istic while DACs A2 and B2 control the cutoff frequency.

Circuit equations:

C1 = C2, R3 = R4, R7 = R8.

π R3C1).

Quality factor, Q = (R6/R8)

× (R2/R

to 15 kHz and Q = 0.3 to 4.5.

DATA 2

OUT A

AD7112

DAC A1

(R1)

C3

10pF

NOTES

1. A1, A2, A3, A4 : 1/4 x AD713

2. C3 IS A COMPENSATION CAPACITOR TO ELIMINATE Q AND GAIN VARIATIONS

CAUSED BY AMPLIFIER GAIN BANDWIDTH LIMITATIONS

HIGH

PASS

OUTPUT

OUT B

DAC B1

(R2)

BANDPASS

OUTPUT

LOW-PASS

OUTPUT

R7

30k

Ω

R8

30k

Ω

R6

10k

Ω

C1

1000pF

C2

1000pF

R5

DB0–DB7

CS

WR

DAC A/

DAC B

DATA 1

DAC A2

(R3)

DAC B2

(R4)

A1

OUT A

AD7112

OUT B

DB0–DB7

CS

WR

DAC A/

DAC B

A2

A3

A4

Figure 22. Programmable State Variable Filter