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DAC7821IPW Datasheet(PDF) 13 Page - Texas Instruments |
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DAC7821IPW Datasheet(HTML) 13 Page - Texas Instruments |
13 / 18 page www.ti.com +2.5 V (+10 V) U4 OPA277 10 kW 10 kW 5 kW - £ 2.5 V £ +2.5 V V OUT ( 10 V - £ £ +10 V) V OUT U2 GND I 1 OUT R FB V REF V DD V DD V OUT I 2 OUT C 1 OPA277 DAC7821 C 2 Programmable Current Source Circuit I L + (R2)R3) R1 R3 V REF D (3) Z O + R1 R3 (R1 R2) R1 (R2 R3 ) ) R1 (R2 R3) (4) R2¢ 15 kW I L LOAD C1 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 SBAS365A – OCTOBER 2005 – REVISED FEBRUARY 2006 APPLICATION INFORMATION (continued) External resistance mismatching is the significant error in Figure 29. Figure 29. Bipolar Output Circuit 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 ±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 ZO, according to Equation 4: As shown in Equation 4, with matched resistors, ZO 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. Figure 30. Programmable Bidirectional Current Source Circuit 13 |
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