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DAC7801 Datasheet(PDF) 9 Page - Texas Instruments |
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DAC7801 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 17 page DAC7800, 7801, 7802 9 SBAS005A www.ti.com DGND should be connected together at one point only, preferably at the power-supply ground point. Separate re- turns minimize current flow in low-level signal paths if properly connected. Output op amp analog common (+ input) should be connected as near to the AGND pins of the DAC780x as possible. WIRING PRECAUTIONS To minimize AC feedthrough when designing a PC board, care should be taken to minimize capacitive coupling be- tween the VREF lines and the IOUT lines. Similarly, capacitive coupling between DACs may compromise the channel-to- channel isolation. Coupling from any of the digital control or data lines might degrade the glitch and digital crosstalk performance. Solder the DAC780x directly into the PC board without a socket. Sockets add parasitic capacitance (which can degrade AC performance). AMPLIFIER OFFSET VOLTAGE The output amplifier used with the DAC780x should have low input offset voltage to preserve the transfer function linearity. The voltage output of the amplifier has an error component which is the offset voltage of the op amp multiplied by the “noise gain” of the circuit. This “noise gain” is equal to (RF/RO + 1) where RO is the output impedance of the DAC IOUT terminal and RF is the feedback network impedance. The nonlinearity occurs due to the output impedance varying with code. If the 0 code case is excluded (where RO = infinity), the RO will vary from R-3R providing a “noise gain” variation between 4/3 and 2. In addition, the variation of RO is nonlinear with code, and the largest steps in RO occur at major code transitions where the worst differential nonlinearity is also likely to be experienced. The nonlinearity seen at the amplifier output is 2VOS – 4VOS/3 = 2VOS/3. Thus, to maintain good nonlinearity the op amp offset should be much less than 1/2 LSB. UNIPOLAR CONFIGURATION Figure 3 shows DAC780x in a typical unipolar (two-quadrant) multiplying configuration. The analog output values versus digital input code are listed in Table II. The operational amplifiers used in this circuit can be single amplifiers such as the OPA602, or a dual amplifier such as the OPA2107. C1 and C2 provide phase compensation to minimize settling time and overshoot when using a high speed operational amplifier. If an application requires the DAC to have zero gain error, the circuit shown in Figure 4 may be used. Resistors R2 and R4 induce a positive gain error greater than worst-case initial negative gain error. Trim resistors R1 and R3 provide a variable negative gain error and have sufficient trim range to correct for the worst-case initial positive gain error plus the error produced by R2 and R4. BIPOLAR CONFIGURATION See Figure 5 for the DAC780x in a typical bipolar (four- quadrant) multiplying configuration. See Table III for the listing of the analog output values versus digital input code. DATA INPUT ANALOG OUTPUT MSB ↓↓ LSB 1111 1111 1111 –VREF (4095/4096) 1000 0000 0000 –VREF (2048/4096) = –1/2VREF 0000 0000 0001 –VREF (1/4096) 0000 0000 0000 0 Volts TABLE II. Unipolar Output Code. The operational amplifiers used in this circuit can be single amplifiers such as the OPA602, a dual amplifier such as the OPA2107, or a quad amplifier like the OPA404. C1 and C2 provide phase compensation to minimize settling time and overshoot when using a high speed operational amplifier. The bipolar offset resistors R5–R7 and R8–R10 should be ratio- matched to 0.01% to ensure the specified gain error perfor- mance. DAC A I OUT A DAC B AGND A I OUT B R FB B R FB A C1 10pF C2 10pF DAC780X V OUT A V OUT B – + – + A1 A2 DGND V REF B V REF A V DD +5V C D A1, A2 OPA602 or 1/2 OPA2107. DAC7802 has a single analog common, AGND. + 1µF AGND B R 100 3 Ω REF B R 2 Ω 47 R 4 Ω 47 DAC A I OUT A DAC B AGND A I OUT B R FB B R FB A C1 10pF C2 10pF DAC780X V OUT A V OUT B – + – + A1 A2 DGND V DD +5V C D A1, A2 OPA602 or 1/2 OPA2107. DAC7802 has a single analog common, AGND. + 1µF AGND B V IN A R 100 1 Ω REF A V V IN B V FIGURE 4. Unipolar Configuration with Gain Trim. FIGURE 3. Unipolar Configuration. |
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