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HS7541AJS Datasheet(PDF) 5 Page - Sipex Corporation |
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HS7541AJS Datasheet(HTML) 5 Page - Sipex Corporation |
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5 / 7 page  HS7541A 12-Bit CMOS Multiplying DAC © Copyright 2000 Sipex Corporation 5 VREF 10K Ω 20K Ω D11 (MSB) D1 D2 D0 (LSB) IOUT2 IOUT1 RFB 10K Ω 10K Ω 20K Ω 20K Ω 20K Ω 20K Ω S1 S2 S11 S0 Switches shown for digital inputs "high" R = 10K Ω ILEAKAGE 30pF RFEEDBACK IOUT1 ILEAKAGE 85pF IOUT2 1/4096 IREF VREF R = 10K Ω The twelve output current-steering switches are in series with the R-2R ladder, and therefore, can introduce bit errors. It is essential then, that the switch “on” resistance be binarily scaled so that the voltage drop across each switch remains constant. If, for example, switch S 0 of Figure 1 was designed with an “on” resistance of 10 ohms, switch S 1 for 20 ohms, etc., then with a 10V reference input, the current through S 0 is 0.5mA, S 1 is 0.25mA, etc.; a constant 5mV drop will then be maintained across each switch. To further insure accuracy across the full tem- perature range, permanently “on” MOS switches are included in series with the feedback resistor and the R-2R ladder’s terminating resistor. These series switches are equivalently scaled to two times switch S 11 (MSB) and to switch S0 (LSB) respectively to maintain constant relative volt- age drops with varying temperature. During any testing of the resistor ladder or R FB (such as incoming inspection), V DD must be present to turn “on” these series switches. Figure 3. Equivalent Circuit – All Inputs High Figure 1. Simplified DAC Circuit 2001V ESD Protection In the design of the HS7541A’s data inputs, 2001V ESD resistance has been incorporated through careful layout and the inclusion of input protection circuitry. Equivalent Circuit Analysis Figures 2 and 3 show the equivalent circuits for all digital inputs LOW and HIGH respectively. The reference current is switched to I OUT2 when all inputs are LOW, and to I OUT1 when all inputs are HIGH. The I LEAKAGE current source is the combination of surface and junction leakages to the substrate; the 1/4096 current source represents the constant 1-bit current drain through the ladder terminating resis- tor. The output capacitance is dependent upon the digital input code, and therefore varies between the low and high values. Output Impedance The output resistance, as in the case of the output capacitance, varies with the digital input code. The resistance, looking back into the I OUT1 ter- R = 10K Ω ILEAKAGE 30pF RFEEDBACK IOUT1 ILEAKAGE 85pF IOUT2 1/4096 IREF VREF R = 10K Ω Figure 2. Equivalent Circuit – All Inputs Low VREF (–10V) D11 (MSB) D0 (LSB) INPUT DATA 2K Ω GAIN TRIM 1K Ω GAIN TRIM 15pF +15V –15V VOUT – + RFB IO1 IO2 GND VREF VDD +15V 4 15 3 2 1 18 16 17 HS7541A Figure 4. Unipolar Operation |
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