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DAC8512 Datasheet(PDF) 11 Page - Analog Devices |
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DAC8512 Datasheet(HTML) 11 Page - Analog Devices |
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11 / 20 page  DAC8512 –11– REV. A Table III. Bipolar Code Table Hexadecimal Number Decimal Number Analog Output in DAC Register in DAC Register Voltage (V) F FF 4095 –4.9976 801 2049 –2.44E–3 800 2048 0 7FF 2047 +2.44E–3 000 0 +5 To maintain monotonicity and accuracy, R1, R2, and R4 should be selected to match within 0.01% and must all be of the same (preferably metal foil) type to assure temperature coefficient matching. Mismatching between R1 and R2 causes offset and gain errors while an R4 to R1 and R2 mismatch yields gain errors. For applications that do not require high accuracy, the circuit illustrated in Figure 29 can also be used to generate a bipolar output voltage. In this circuit, only one op amp is used and no potentiometers are used for offset and gain trim. The output voltage is coded in offset binary and is given by: VO = 1 mV × Digital Code × R4 R3 + R4 × 1 + R2 R1 –2.5 × R2 R1 43.2k + 499 R1 10k 10k 2.5V 5V VOUT RANGE R2 10k 20k R3 10k 10k R4 15.4k + 274 4 6 1 8 4 2 3 3 5 2 4 6 7 8 1 2 CS CLR LD SCLK SDI VDD GND DAC8512 +5V 0.1µF +2.5V R1 R2 REF03 +5V –5V A1 = 1/2 OP295 R3 R4 VO +5V 0.1µF A1 Figure 29. Bipolar Output Operation without Trim For the ±2.5 V output range and the circuit values shown in the table, the transfer equation becomes: VO = 1.22 mV × Digital Code – 2.5 V Similarly, for the ±5 V output range, the transfer equation becomes: VO = 2.44 mV × Digital Code – 5 V Generating a Negative Supply Voltage Some applications may require bipolar output configuration but only have a single power supply rail available. This is very com- mon in data acquisition systems using microprocessor-based systems. In these systems, +12 V, +15 V, and/or +5 V are only available. Shown in Figure 30 is a method of generating a nega- tive supply voltage using one CD4049, a CMOS hex inverter, operating on +12 V or +15 V. The circuit is essentially a charge pump where two of the six are used as an oscillator. For the val- ues shown, the frequency of oscillation is approximately 3.5 kHz and is fairly insensitive to supply voltage because R1 > 2 × R2. The remaining four inverters are wired in parallel for higher out- put current. The square wave output is level translated by C2 to a negative-going signal, rectified using a pair of 1N4001s, and then filtered by C3. With the values shown, the charge pump will provide an output voltage of –5 V for current loadings in the range 0.5 mA ≤ I OUT ≤ 10 mA with a +15 V supply and 0.5 mA ≤ I OUT ≤ 7 mA with a +12 V supply. 910 6 11 12 14 15 7 32 5 4 R2 5.1k Ω R1 510k Ω C1 0.02 µF C2 47 µF D1 1N4001 C3 47 µF 1N5231 5.1V ZENER D2 1N4001 R3 470 Ω –5V INVERTERS = CD4049 Figure 30. Generating a –5 V Supply When Only +12 V or +15 V Is Available A High-Compliance, Digitally Controlled Precision Current Source The circuit in Figure 31 shows the DAC8512 controlling a high-compliance precision current source using an AMP05 in- strumentation amplifier. The AMP05’s reference pin becomes the input, and the “old” inputs now monitor the voltage across a precision current sense resistor, RCS. Voltage gain is set to unity, so the transfer function is given by the following equation: IOUT = VIN RCS If RCS equals 100 Ω, the output current is limited to +10 mA with a 1 V input. Therefore, each DAC LSB corresponds to 2.4 µA. If a bipolar output current is required, then the circuit in Figure 28 can be modified to drive the AMP05’s reference pin with a ±1 V input signal. Potentiometer P1 trims the output current to zero with the in- put at 0 V. Fine gain adjustment can be accomplished by adjust- ing R1 or R2. |
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