Precision 16-Bit, Bipolar Output Voltage Source with +12 V to ±5 V Supply
The circuit offers an efficient solution to a problem often encountered in systems with a single +12 V supply rail. Proper printed circuit board (PCB) layout and grounding techniques ensure that the ADP2300 switching regulator does not degrade the overall performance of the circuit.
Figure 1. Bipolar Output DAC Circuit with ±5 V Power Supplies
The output voltage of the AD5668 is 0 V to 2.5 V at TP1, and this signal drives the noninverting input of the AD8638 op amp. The signal gain of the op amp is 1 + R2/R1, which is 2 for R1 = R2. A negative 2.5 V offset is injected into the op amp output by driving R1 with the 2.5 V reference. The result is a bipolar output voltage at TP2 that swings from −2.5 V to +2.5 V.
The circuit operates on a single supply voltage of nominally 12 V, which can vary between 10 V and 15 V. The regulated −5 V supply rail is developed from an ADP2300 switching regulator connected in the inverting buck-boost configuration. The circuit can be designed using the ADIsimPower program available at www.analog.com/ADIsimPower. The L1 coupled inductor is used to develop an unregulated 5 V supply for the circuit using a Zeta configuration. This circuit yields high efficiency for small output currents.
The integral nonlinearity (INL) and differential nonlinearity (DNL) measured at TP2 (bipolar output) are shown in Figure 2 and Figure 3, respectively.
Figure 2. INL Performance of Bipolar Output (TP2)
Figure 3. DNL Performance of Bipolar Output (TP2)
The INL and DNL measured at TP1 (unipolar DAC output) are shown in Figure 4 and Figure 5, respectively.
Figure 4. INL Performance of Unipolar DAC Output (TP1)
Figure 5. DNL Performance of Unipolar DAC Output (TP1)
The AD8639 is a dual version of the AD8638 and can be used, if desired. The circuit in Figure 1 uses the single AD8638 to minimize crosstalk between the eight channels.
Other 2.5 V references can be used, such as the ADR4525, which has an accuracy of ±0.02% and a temperature coefficient of 2 ppm/°C maximum (B grade).
The following equipment is needed:
- The System Demonstration Platform (EVAL-SDP-CB1Z)
- The CN-0183 circuit evaluation board (EVAL-CN0183-SDZ)
- The CN-0183 evaluation software
- The Tektronix TDS2024, 4-channel oscilloscope
- The HP E3630A 0 V to 6 V/2.55 A; ±20 V/0.5 A triple output dc power supply
- A PC (Windows 32-bit or 64-bit)
Load the evaluation software by placing the CN-0183 evaluation software CD in the CD drive of the PC. Using My Computer, locate the drive that contains the evaluation software CD and open the Readme file. Follow the instructions contained in the Readme file for installing and using the evaluation software. The evaluation software main window is shown in Figure 6.
Figure 6. Evaluation Software Main Window
Functional Diagram of Test Setup
A functional diagram of the test setup is shown in Figure 7. This setup allows the DAC output (TP1) and the bipolar output (TP2) to be observed with an oscilloscope.
Linearity measurements require a precision, digital voltmeter (DVM) that can be read by the PC via a USB port.
Figure 7. Functional Diagram of Test Setup
Connect the 120-pin connector on the EVAL-CN0183-SDZ to the CON A connector or the CON B connector on the EVAL-SDP-CB1Z. Use nylon hardware to firmly secure the two boards, using the holes provided at the ends of the 120-pin connectors. After successfully setting the dc output supply to +5 V, −5 V, and +12 V, turn the power supply off.
With power to the supply off, connect the −5 V power supply to the −5V pin on J5-3, connect the +5 V power supply to the AVDD pin on J5-1, connect GND to the AGND pins on J5-2 and J4-2, and connect the +12 V power supply to the +12V pin on J4-1. Alternatively, place Link 2 and Link 3 in Position B to power the circuitry using the ADP2300 to supply +5 V and −5 V. Note that AVDD and the −5 V are not needed in this case.
Turn on the power supply and then connect the USB cable from the SDP board to the USB port on the PC. Do not connect the USB cable to the mini-USB connector on the SDP before turning on the dc power supply for the EVAL-CN0183-SDZ.
Table 1.Jumper Settings for EVAL-CN0183-SDZ (Default Settings in Bold)
|LK1||Short AD5668 reference pin to REF192 output
|It shorts AD5668 reference pin to REF192 output allowing the use of an external DAC reference.
Only the internal reference of the AD5668 can be used.
|LK2||AVDD supply source
|The circuit is powered by an external 5 V supply applied to the AVDD pin on J5-1.
The digital power is supplied by the 5V voltage supplied by the ADP2300 regulator.
|LK3||-5V voltage source
|The analog circuit is supplied by an external power supply apply to the -5Vpin on J5-3.
The digital power is supplied by the -5V voltage obtained by inverting the output of the ADP2300 regulator.
After setting up the test equipment, connect the probes of the oscilloscope to the TP1 and TP2 test points. The TP3, TP4, and TP5 test points are connected to the reference, the regulated +5 V, and the regulated −5 V, respectively. Check these test points for the correct voltages (use TP6 for the ground).
The software provided on the CD allows users to set the value of VOUTA by loading a code into the DAC and by choosing the source of the reference. If users keep the default setting, they will have to supply the +5 V and −5 V voltages, and the +12 V is not required. The default setting uses the external REF192 reference, giving you a full-scale DAC output range of 2.5 V (TP1), and −2.5 V to +2.5 V on the bipolar output (TP2). Loading 0x0000 sets the DAC output to 0 V and the bipolar output to −2.5 V. Loading 0x8000 sets the DAC output to 1.25 V and the bipolar output to 0 V. Loading 0xFFFF sets the DAC output to 2.5 V and the bipolar output to 2.5 V.
|AD8638||16 V Auto-Zero, Rail-to-Rail Output Operational Amplifier||
|AD5668||Octal, 16-Bit, SPI Voltage Output denseDAC With 5 PPM/°C On-Chip Reference||
|ADP2300||1.2 A, 20 V, 700 kHz Nonsynchronous Step-down Switching Regulator||
|REF192||2.5V Precision Micropower, Low Dropout, Low Voltage Reference|