AD5231 datasheet(25/28 Pages) AD

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Data Sheet

AD5231

Rev. D | Page 25 of 28

PROGRAMMABLE BIDIRECTIONAL CURRENT

SOURCE

For applications that require bidirectional current control or

higher voltage compliance, a Howland current pump can be a

solution. If the resistors are matched, the load current is

(

)

R2B

R2A

(8)

–15V

OP2177

V+

V–

+15V

–

10pF

15kΩ

150kΩ

500Ω

R2B

50Ω

150kΩ

R2A

14.95kΩ

OP2177

V+

V–

+15V

–

–15V

AD5231

B W

+2.5V

–2.5V

Figure 53. Programmable Bidirectional Current Source

R2B, in theory, can be made as small as necessary to achieve the

current needed within the A2 output current-driving capability.

In this circuit, OP2177 delivers ±5 mA in both directions, and

the voltage compliance approaches 15 V. It can be shown that

the output impedance is

)

(

)

(

R2B

R2A

R1'

R1R2'

R2A

R2B

R1'

−

(9)

ZO can be infinite if resistors R1 and R2 match precisely with R1

and R2A + R2B,respectively. On the other hand, ZO can be

negative if the resistors are not matched. As a result, C1, in the

range of 1 pF to 10 pF, is needed to prevent oscillation from the

negative impedance.

RESISTANCE SCALING

The AD5231 offers 10 kΩ, 50 kΩ, and 100 kΩ nominal

resistance. For users who need lower resistance but want to

maintain the number of adjustment steps, they can parallel

multiple devices. For example, Figure 54 shows a simple scheme

of paralleling two AD5231s. To adjust half the resistance

linearly per step, users need to program both devices coherently

with the same settings and tie the terminals as shown.

Figure 54. Reduce Resistance by Half with Linear Adjustment Characteristics

In voltage diver mode, by paralleling a discrete resistor as

shown in Figure 55, a proportionately lower voltage appears at

Terminals A–B. This translates into a finer degree of precision,

because the step size at Terminal W is smaller. The voltage can

be found as follows:

)

1024

(

)

(

(10)

Figure 55. Lowering the Nominal Resistance

Figure 54 and Figure 55 show that the digital potentiometers

change steps linearly. On the other hand, pseudo log taper

adjustment is usually preferred in applications such as audio

control. Figure 56 shows another type of resistance scaling. In

this configuration, the smaller the R2 with respect to R1, the

more the pseudo log taper characteristic of the circuit behaves.

Figure 56. Resistor Scaling with Pseudo Log Adjustment Characteristics

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