Datasheet

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R2R1
DAC
ParallelBus
Input
Register
Control
Logic
DAC
Register
WR
D13
D0
¼
RST
LDAC
REF
R
COM
R1
R
FB
R
OFS
R
FB
R
OFS
V
DD
I
OUT
AGND
DGND
DAC8806
OPA2277
V
REF
U1
OPA2277
V
OUT
C1
U2
Programmable Current Source Circuit
I
L
+
(
R2 ) R3
)
ńR1
R3
V
REF
D
(3)
Z
O
+
R1ȀR3
(
R1 ) R2
)
R1
(
R2Ȁ ) R3Ȁ
)
* R1Ȁ
(
R2 ) R3
)
(4)
DAC8806
SBAS385B – APRIL 2006 – REVISED FEBRUARY 2008
Figure 42. Bipolar Output Circuit
A DAC8806 can be integrated into the circuit in Figure 43 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 R3 in the previous equation can be reduced to increase the output current drive of U3. U3 can drive
± 20 mA in both directions with voltage compliance limited up to 15 V by the U3 voltage supply. Elimination of the
circuit compensation capacitor (C1) 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 C1 into the circuit, possible oscillation problems
are eliminated. The value of C1 can be determined for critical applications; for most applications, however, a
value of several pF is suggested.
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