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Programmable Current Source Circuit
I
L
+
(
R2)R3
)
ń R1
R3
V
REF
D
4096
(3)
Z
O
+
R1ȀR3
(
R1)R2
)
R1
(
R2Ȁ)R3Ȁ
)
* R1Ȁ
(
R2)R3
)
(4)
R2¢
15kW
I 1
OUT
R
FB
GND
V
DD
V
DD
V
REF
VOUT
I
L
LOAD
C
1
10pF
U3
OPA277
R1¢
150kW
R3¢
50W
R3
50W
R1
150kW
R2
15kW
U2
OPA277
U1
DAC7811
I 2
OUT
Cross-Reference
DAC7811
SBAS337C – APRIL 2005 – REVISED JULY 2007
A DAC7811 can be integrated into the circuit in Figure 32 to implement an improved Howland current pump for
precise voltage to current conversions. Bidirectional current flow and high voltage compliance are two features of
the circuit. With a matched resistor network, the load current of the circuit is shown by Equation 3 :
The value of R3 in Equation 3 can be reduced to increase the output current drive of U3. U3 can drive ± 20mA in
both directions with voltage compliance limited up to 15V by the U3 voltage supply. Elimination of the circuit
compensation capacitor C
1
in the circuit is not suggested as a result of the change in the output impedance Z
O
,
according to Equation 4 :
As shown in Equation 4 , with matched resistors, Z
O
is infinite and the circuit is optimum for use as a current
source. However, if unmatched resistors are used, Z
O
is positive or negative with negative output impedance
being a potential cause of oscillation. Therefore, by incorporating C
1
into the circuit, possible oscillation problems
are eliminated. The value of C
1
can be determined for critical applications; for most applications, however, a
value of several pF is suggested.
Figure 32. Programmable Bidirectional Current Source Circuit
The DAC7811 has an industry-standard pinout. Table 5 provides the cross-reference information.
Table 5. Cross-Reference
SPECIFIED
TEMPERATURE PACKAGE PACKAGE CROSS-
PRODUCT INL (LSB) DNL (LSB) RANGE DESCRIPTION OPTION REFERENCE PART
DAC7811 ± 1 ± 1 –40 ° C to +125 ° C 10-Lead MicroSOIC MSOP-10 AD5443YRM
17
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