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PROGRAMMABLE CURRENT SOURCE CIRCUIT
I
L
A/B=
(R +R )/R
3 12
R
3
´ ´
REF
V
D
16384
(3)
Z
O
+
R
1
ȀR
3
(
R
1
)R
2
)
R
1
(
R
2
Ȁ)R
3
Ȁ
)
* R
1
Ȁ
(
R
2
)R
3
)
(4)
R ´
150kW
1
R
150kW
1
U3
OPA2277
DAC8805
R
15kW
2
R
50W
3
R
50
´
W
3
I
L
LOAD
V
REF
R
15k
´
W
2
C
10pF
1
C
10pF
2
U2
I A/B
OUT
V
DD
R
OFS
R
FB
U1
V
DD
GND
V
REF
OPA2277
V
OUT
CROSS-REFERENCE
DAC8805
SBAS391A – DECEMBER 2006 – REVISED MAY 2007
APPLICATION INFORMATION (continued)
The DAC8805 can be integrated into the circuit in Figure 44 to implement an improved Howland current pump
for precise V/I 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 R
3
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 , Z
O
with matched resistors 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 44. Programmable Bidirectional Current Source Circuit
The DAC8805 has an industry-standard pinout. Table 4 provides the cross-reference information.
Table 4. Cross-Reference
SPECIFIED CROSS-
INL DNL TEMPERATURE PACKAGE PACKAGE REFERENCE
PRODUCT BIT (LSB) (LSB) RANGE DESCRIPTION OPTION PART
DAC8805Q 14 1 1 –40 ° C to +125 ° C TSSOP-38 DBT AD5557
18
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