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Programmable Current Source Circuit
ĂI
L
+
(
R2)R3
)
ńR1
R3
V
REF
D
(3)
ĂZ
O
+
R1ȀR3
(
R1)R2
)
R1
(
R2Ȁ)R3Ȁ
)
* R1Ȁ
(
R2)R3
)
(4)
R2¢
15kW
I
L
LOAD
C
1
10pF
U3
OPA277
R1¢
150kW
R3¢
50 W
R3
50 W
R1
150kW
R2
15kW
OPA277
DAC7821
GND
I 1
OUT
R
FB
V
REF
V
DD
V
DD
V
OUT
I 2
OUT
U1
U2
DAC7821
SBAS365B – OCTOBER 2005 – REVISED JULY 2007
A DAC7821 can be integrated into the circuit in Figure 30 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 the previous equation 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 30. Programmable Bidirectional Current Source Circuit
15
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