AD5231 datasheet(20/24 Pages) AD

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REV. 0

AD5231

–20–

Programmable 4 mA to 20 mA Current Source

A programmable 4 mA to 20 mA current source can be imple-

mented with the circuit shown in Figure 27.

V+

V–

OP1177

VIN

SLEEP

REF191

GND

VOUT

1 F

AD5231

102

100

+5V

–2.048V TO VL

–5V

0 TO (2.048 + VL)

+5V

Figure 27. Programmable 4 mA to 20 mA Current Source

REF191 is a unique low supply headroom precision reference

that can deliver the 20 mA needed at 2.048 V. The load current

is simply the voltage across terminals B-to-W of the digital

potentiometer divided by RS:

REF









(7)

The circuit is simple, but be aware that there are two issues. First,

dual supply op amps are ideal because the ground potential of

REF191 can swing from –2.048 V at zero scale to VL at full scale

of the potentiometer setting. Although the circuit works under

single supply, the programmable resolution of the system will be

reduced. Second, the voltage compliance at VL is limited to 2.5 V or

equivalently a 125

Ω load. Should higher voltage compliance be

needed, users may consider digital potentiometers AD5260, AD5280,

and AD7376. Figure 28 below shows an alternate circuit for high

voltage compliance.

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:

R B

()

(8)

–15V

OP2177

V+

V–

+15V

–

10pF

15k

150k

R2B

500

R2A

14.95k

150k

10pF

OP2177

V+

V–

+15V

–

–15V

AD5231

B W

+2.5V

–2.5V

Figure 28. Programmable Bidirectional Current Source

R2B in theory can be made as small as needed to achieve the

current needed within 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:









–

(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 nega-

tive if the resistors are not matched. As a result, C1 and C2, in

the range of 1 pF to 10 pF, are needed to prevent the oscillation.

Resistance Scaling

AD5231 offers 10 k

Ω, 50 kΩ, and 100 kΩ nominal resistance. For

users who need lower resistance but want to maintain the numbers

of step adjustment, they can parallel multiple devices. For example,

Figure 29 shows a simple scheme of paralleling two AD5231. In

order to adjust half of the resistance linearly per step, users need to

program both devices coherently with the same settings and tie

the terminals as shown.

Figure 29. Reduce Resistance by Half with Linear

Adjustment Characteristics

In voltage divider mode, a much lower resistance can be achieved

by paralleling a discrete resistor as shown in Figure 30. The equiva-

lent resistance become:

WBeq

() +

1024

(10)

WAeq

= 



 () +

1024

–/ /

(11)

R2 << R1

Figure 30. Lowering the Nominal Resistance

Figures 29 and 30 show that the digital potentiometers change

steps linearly. On the other hand, log taper adjustment is usually

preferred in applications like audio control. Figure 31 shows

another way of resistance scaling. In this configuration, the

smaller the R2 with respect to R1, the more the pseudo log

taper characteristic behaves.

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