DAC8806 Datasheet, PDF (15/20 Pages) Texas Instruments – 14-Bit, Parallel Input Multiplying Digital-to-Analog Converter

DAC8806
www.ti.com
SBAS385A – APRIL 2006 – REVISED JUNE 2006
APPLICATION INFORMATION (continued)
V_REF
U1
OPA2277
D0
D13
WR
RST
LDAC
DAC8806
V_DD
R1
R_COM
R1
Parallel Bus
Input
Register
Control
Logic
REF
R2
R_OFS
R_OFS
R_FB
R_FB
DAC
Register
DAC
C1
I_OUT
AGND
U2
OPA2277
DGND
V_OUT
Figure 42. Bipolar Output Circuit
Programmable Current Source 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:
(R2 ) R3)ńR1
I_L +
R3
V_REF D
(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 (C1) in the circuit is not suggested as a result of the change in the output
impedance (Z_O), according to Equation 4:
Z_O
+
R1(R2Ȁ
R1ȀR3(R1
) R3Ȁ) *
) R2)
R1Ȁ(R2
)
R3)
(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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