參數(shù)資料
型號(hào)： EVAL-AD5233SDZ
廠商： Analog Devices Inc
文件頁(yè)數(shù)： 20/32頁(yè)
文件大?。?/td> 0K
描述： BOARD EVAL FOR AD5233
標(biāo)準(zhǔn)包裝： 1
系列： *
AD5233
Rev. B | Page 27 of 32
PROGRAMMABLE CURRENT SOURCE
A programmable current source can be implemented with the
circuit shown in Figure 58.
V+
V–
OP1177
U2
VS
SLEEP
REF191
GND
OUTPUT
3
2
4
6
U1
C1
1F
AD5233
W
A
B
RS
102
RL
100
VL
IL
+5V
–2.048V TO VL
–5V
0 TO (2.048 + VL)
+5V
+
–
0
27
94
-059
Figure 58. Programmable 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 Terminal B to Terminal W of the
digital potentiometer divided by RS.
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 is reduced. Second, the voltage compliance at VL
is limited to 2.5 V or equivalently a 125 Ω load. If higher voltage
compliance is needed, users can consider digital potentiometers
AD5260, AD5280, and AD7376. Figure 58 shows an alternative
circuit for high voltage compliance.
To achieve higher current, such as when driving a high power
LED, the user can replace the U1 with an LDO, reduce RS, and
add a resistor in series with the AD5233’s A terminal. This
limits the potentiometer’s current and enhances the current
adjustment resolution.
PROGRAMMABLE BIDIRECTIONAL CURRENT
SOURCE
For applications that require bidirectional current control or
higher voltage compliance, a Howland current pump can be
used (see Figure 59).
–15V
OP2177
V+
V–
+15V
+
–
C1
10pF
R2
15k
R1
150k
R2B
50
RL
500
VL
R2A
14.95k
R1
150k
IL
OP2177
V+
V–
+15V
+
–
–15V
A1
AD5233
A
BW
+2.5V
–2.5V
A2
02
79
4-
06
0
Figure 59. Programmable Bidirectional Current Source
If the resistors are matched, the load current is
(
)
W
L
V
R2B
R1
R2B
R2A
I
×
+
=
(17)
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. Without C1, it can be
shown that the output impedance is
)
(
'
)
(
'
R2B
R2A
R1
R2
R1
R2A
R1
R2B
R1
Z
O
+
×
+
×
=
(18)
ZO can be infinite if the R1 and R2 resistors 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
AD5233 offers 10 kΩ, 50 kΩ, and 100 kΩ nominal resistance.
Users who need lower resistance but want to maintain the
number of adjustment steps can parallel multiple devices. For
example, Figure 60 shows a simple scheme of paralleling two
AD5233 channels. To adjust half the resistance linearly per step,
users need to program both devices concurrently with the same
settings.
A1
B1
W1
W2
A2
B2
LD
VDD
027
94
-06
1
Figure 60. Reduce Resistance by Half with Linear Adjustment Characteristics
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