AD5260/AD5262

Rev. A | Page 21 of 24

–5V

OP1177

+

–

U2

+5V

102

Ω

100

Ω

A

B W

AD5260

C1

1µF

GND

REF191

SLEEP

OUTPUT

+5V

U1

2

3

4

6

Figure 64. Programmable 4-to-20 mA Current Source

The circuit is simple, but be aware that dual-supply op amps are

ideal because the ground potential of REF191 can swing from

setting. Although the circuit works under single supply, the

programmable resolution of the system is reduced.

PROGRAMMABLE BIDIRECTIONAL CURRENT

SOURCE

For applications that require bidirectional current control or

higher voltage compliance, a Howland current pump can be a

solution (see Figure 65). If the resistors are matched, the load

current is

()

W

L

V

B

R

R1

B

R

A

R

I

×

+

=

2

2

2

(8)

AD8016

+15V

–15V

+5V

–5V

OP2177

AD5260

A1

W

A

B

C2

10pF

R1'

150k

Ω

R1

150k

Ω

R2'

15k

Ω

A2

C1

10pF

R2A

14.95k

Ω

500

Ω

50

Ω

+15V

–15V

Figure 65. Programmable Bidirectional Current Source

PROGRAMMABLE LOW-PASS FILTER

Digital Potentiometer AD5262 can be used to construct a

second-order, Sallen-Key low-pass filter (see Figure 66). The

design equations are

2

2

2

O

O

O

i

O

S

Q

S

V

V

ω

ω

ω

+

+

=

(9)

R1R2C1C2

O

1

=

ω

(10)

R2C2

R1C1

Q

1

1

+

=

(11)

Users can first select any convenient value for the capacitors. To

achieve maximally flat bandwidth where Q = 0.707, let C1 be

twice the size of C2 and let R1 = R2. As a result, users can adjust

R1 and R2 to the same settings to achieve the desirable bandwidth.

A

B

AD8601

+2.5V

–2.5V

W

R

R2

R1

A

B

W

R

C1

C2

ADJUSTED TO

SAME SETTINGS

Figure 66. Sallen Key Low-Pass Filter

PROGRAMMABLE OSCILLATOR

In a classic Wien-bridge oscillator (see Figure 67), the Wien

network (R, R’, C, C’) provides positive feedback, whereas R1

the overall phase shift is zero, and the positive feedback causes

the circuit to oscillate. With R = R’, C = C’, and R2 = R2A//(R2B +

RC

O

1

=

ω

or

RC

f

O

π

2

1

=

(12)

AB

R

D

R

256

256 −

=

(13)

At resonance, setting

2

=

R1

R2

(14)

balances the bridge. In practice, R2/R1 should be set slightly

larger than 2 to ensure the oscillation can start. However, the

alternate turn-on of the diodes, D1 and D2, ensures R2/R1 to

be smaller than 2 momentarily and therefore stabilizes the

oscillation.

When the frequency is set, the oscillation amplitude can be

tuned by R2B because

D

D

O

V

B

R

I

V

+

=

2

3

2

(15)

converges. R2B can be in series with a discrete resistor to

increase the amplitude, but the total resistance cannot be too

large to saturate the output.

In both circuits in Figure 66 and Figure 67, the frequency tuning

requires that both RDACs be adjusted to the same settings.

Because the two channels are adjusted one at a time, an intermedi-