OP290

Rev. C | Page 11 of 12

VARIABLE SLEW RATE FILTER

The circuit shown in Figure 21 can be used to remove pulse

noise from an input signal without limiting the response rate to

a genuine signal. The nonlinear filter has use in applications

where the input signal of interest is known to have physical

limitations. An example of this is a transducer output where a

change of temperature or pressure cannot exceed a certain rate

due to physical limitations of the environment. The filter consists

of a comparator that drives an integrator. The comparator

compares the input voltage to the output voltage and forces

the integrator output to equal the input voltage. A1 acts as a

comparator with its output high or low. Diode D1 and Diode

D2 clamp the voltage across R3, forcing a constant current to

flow in or out of C2. R3, C2, and A2 form an integrator with the

output of A2 slewing at a maximum rate of

C2

R3

C2

R3

V

rate

slew

Maximum

D

×

≈

×

=

V

6

.

0

For an input voltage slewing at a rate under this maximum slew

rate, the output simply follows the input with A1 operating in

its linear region.

1/2 A2

OP290GP

1

1/2 A1

OP290GP

3

2

7

5

6

R1

250k

Ω

C1

0.1µF

+15V

–15V

VOUT

R2

100k

Ω

R3

1M

Ω

D2

D1

DIODES ARE 1N4148

4

R4

25k

Ω

C2

4700pF

8

Figure 21. Variable Slew Rate Filter

LOW OVERHEAD VOLTAGE REFERENCE

Figure 22 shows a voltage reference that requires only 0.1 V of

overhead voltage. As shown, the reference provides a stable

4.5 V output with a 4.6 V to 36 V supply. Output voltage drift is

only 12 ppm/°C. Line regulation of the reference is under

5 μV/V with load regulation better than 10 μV/mA with up to

50 mA of output current.

The REF43 provides a stable 2.5 V that is multiplied by the

OP290. The PNP output transistor enables the output voltage to

approach the supply voltage.

Resistor R1 and Resistor R2 determine the output voltage.











 +

=

R1

R2

VOUT

1

V

5

.

2

The 200 Ω variable resistor is used to trim the output voltage.

For the lowest temperature drift, parallel resistors can be used

in place of the variable resistor and taken out of the circuit as

required to adjust the output voltage.

6

2

4

REF43FZ

VOUT

VIN

VOUT

GND

C1

10µF

C2

0.1µF

V+

R2

2k

Ω

1%

R1A

2.37

Ω

1%

R1B

200

Ω

20-TURN

BOURNS 3006P-1-201

2N2907A

1

1/2

OP290GP

3

2

8

4

Figure 22. Low Overhead Voltage Reference