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| DAC8822_07 Datasheet, PDF (18/24 Pages) Burr-Brown (TI) – 16-Bit, Dual, Parallel Input, Multiplying Digital-to-Analog Converter | |||
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DAC8822
SBAS390A â DECEMBER 2006 â REVISED MARCH 2007
www.ti.com
APPLICATION INFORMATION (continued)
PROGRAMMABLE CURRENT SOURCE CIRCUIT
The DAC8822 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:
ILAÅB
+
(R2)R3)
R3
Å
R1
V REF
D
65536
(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 (ZO), according to Equation 4:
ZO
+
R1ÈR3(R1)R2)
R1(R2È)R3È) * R1È(R2)R3)
(4)
As shown in Equation 4, ZO with matched resistors is infinite and the circuit is optimum for use as a current
source. However, if unmatched resistors are used, ZO 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.
R2´
15kW
C1
10pF
VREF
VDD
U1
VDD ROFS RFB
VREF DAC8822 IOUTA/B
C2
10pF
U2
OPA2277
R1´
150kW
U3
OPA2277
R1
150kW
R2
15kW
R3´
50W
VOUT
R3
50W
IL
GND
LOAD
Figure 44. Programmable Bidirectional Current Source Circuit
CROSS-REFERENCE
The DAC8822 has an industry-standard pinout. Table 4 provides the cross-reference information.
PRODUCT
BIT
DAC8822QB
16
DAC8822QC
16
INL
(LSB)
2
1
Table 4. Cross-Reference
DNL
(LSB)
1
1
SPECIFIED
TEMPERATURE
RANGE
â40°C to +125°C
â40°C to +125°C
PACKAGE
DESCRIPTION
TSSOP-38
TSSOP-38
PACKAGE
OPTION
DBT
DBT
CROSS-
REFERENCE
PART
AD5547B
N/A
18
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