EVAL-AD5235EBZ Analog Devices Inc, EVAL-AD5235EBZ Datasheet - Page 26

• Manufacturer Part Number: EVAL-AD5235EBZ
• Description: BOARD EVALUATION FOR AD5235
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD5235EBZ

• Main Purpose: Digital Potentiometer
• Utilized Ic / Part: AD5235
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Secondary Attributes: -
• Embedded: -
• Primary Attributes: -

AD5235
PROGRAMMABLE CURRENT SOURCE
A programmable current source can be implemented with the
circuit shown in Figure 54.
The
handling precision reference that can deliver 20 mA at 2.048 V.
The load current is simply the voltage across Terminal W to
Terminal B of the digital potentiometer divided by R
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 V
scale of the potentiometer setting. Although the circuit works
under single supply, the programmable resolution of the system
is reduced by half. Second, the voltage compliance at V
limited to 2.5 V, or equivalently, a 125 Ω load. When higher
voltage compliance is needed, consider digital potentiometers,
such as, AD5260, AD5280, and AD7376. Figure 55 shows an
alternate circuit for high voltage compliance.
To achieve higher current, such as when driving a high power
LED, replace U1 with an LDO, reduce R
series with the A terminal of the digital potentiometer. This
limits the current of the potentiometer and increases the current
adjustment resolution.
REF191
I
L
=
3
–2.048V TO V
R
V
+5V
SLEEP
GND
2
V
4
S
S
REF
is a unique low supply headroom and high current
×
1024
Figure 54. Programmable Current Source
×
U1
AD5235
REF191
D
OUTPUT
L
6
C1
0V TO (2.048V + V
V+
V–
OP1177
1µF
+5V
–5V
U2
B
A
–
+
W
S
, and add a resistor in
100Ω
L
)
R
L
R
102Ω
S
V
L
S
I
.
L
L
L
at full
is
Rev. D | Page 26 of 32
(7)
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 55). If the resistors are matched, the load
current is
AD5235
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,
the voltage compliance approaches 15 V. Without the additions
of C1 and C2, the output impedance (looking into V
Z
R1 and R2A + R2B, respectively, which is desirable. On the other
hand, if the resistors do not match, Z
oscillation. As a result, C1, in the range of a few picofarad, is
needed to prevent oscillation from the negative impedance.
O
can be infinite, if Resistors R1
Z
I
O
L
=
+2.5V
–2.5V
=
A
B
Figure 55. Programmable Bidirectional Current Source
R2A
R1
W
R1'
R2'
R2B
OP2177
R1
+
A1
+
–
R2B
OP2177
R2B
−
–15V
+15V
V+
V–
R1'
×
(
R1
delivers ±5 mA in either direction, and
(
V
R2A
W
+
R2A
+
150kΩ
R2B
R1
)
150kΩ
'
and R2
R1
)
O
can be negative and cause
'
–
+
OP2177
match precisely with
–15V
+15V
V+
V–
15k
14.95kΩ
R2
R2A
Ω
A2
10pF
C1
L
) can be
R2B
50Ω
R
500Ω
L
V
L
I
L
(8)
(9)