Precision, 16-Bit Bipolar Output Voltage Source from +12V to ±5V Supplies

Circuit Function and Advantages

The circuit shown in Figure 1 provides precision, 16-bit, ±2.5 V low drift bipolar voltage outputs from a single +10 V to +15 V supply. The unipolar voltage outputs of the AD5668 8-channel denseDAC are amplified and level shifted by the AD8638 auto-zero op amp. The maximum drift contribution of the AD8638 is only 0.06 ppm/°C. The external reference REF192 ensures a maximum drift of 5 ppm/°C (E grade) and provides a low impedance pseudo ground voltage for the AD8638 level gain and shift circuitry.

This circuit provides an efficient solution to a problem that often occurs in systems with a single +12 V 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 using ±5V supplies.

Circuit Description

The AD5668 is a 16-bit, 8-channel, voltage output denseDAC controlled via an SPI interface. It contains an on-chip reference with a maximum drift of 10 ppm/°C. At power-up, the on-chip reference is off, allowing an external reference to be used. The internal reference is enabled by a software write. An external REF192 is used in the circuit shown in Figure 1 because a low output impedance is required to drive the 2.5 V pseudo-ground reference of the AD8638 op amp.

The output voltage of the AD5668 is 0V to 2.5V 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, so it is equal to 2 when R1=R2. By driving R1 with a 2.5V reference voltage, a negative offset of 2.5V is injected into the output of the op amp. Therefore, the bipolar output voltage swing at TP2 is −2.5V to +2.5V.

The circuit operates from a single supply, nominally 12 V, which can vary between 10 V and 15 V. The regulated −5 V rail is generated by connecting the ADP2300 switching regulator in an 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 generate the unregulated 5 V supply for the circuit in a Zeta configuration. The circuit is able to produce high efficiency for small output currents.

Figure 2 and Figure 3 show the integral nonlinearity (INL) and differential nonlinearity (DNL), respectively, measured at TP2 (bipolar output).

Figure 2. INL performance of bipolar output (TP2)

Figure 3. DNL performance of bipolar output (TP2)

Figure 4 and Figure 5 show the INL and DNL, ​​respectively, measured at TP1 (unipolar DAC output).

Figure 4. INL performance of a unipolar DAC output (TP1).

Figure 5. DNL performance of unipolar DAC output (TP1).

Common changes

The AD5628 and AD5648 are 12-bit and 14-bit versions of the AD5668, respectively. They both have an on-chip reference with an internal gain of 2. The AD5628-1/AD5648-1/AD5668-1 have an internal 1.25 V, 5 ppm/°C reference that allows a full-scale output range of 2.5 V; the AD5628-2/AD5648-2/AD5668-2 and AD5668-3 have an internal 2.5 V, 5 ppm/°C reference that allows a full-scale output range of 5 V. At power-up, the on-chip reference is turned off, allowing an external reference to be used. The internal reference is enabled by a software write. The devices contain a power-on reset circuit that ensures that the DAC output is at 0 V (AD5628-1/AD5648-1/AD5668-1, AD5628-2/AD5648-2/AD5668-2) or midscale (AD5668-3) after power-up and remains at that level until a valid write operation is performed.

The AD8639 is a dual-channel version of the AD8638 and can be used if desired. The circuit in Figure 1 uses a 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).

Circuit Evaluation and Testing

Equipment requirements (equivalent equipment may be substituted)

The following equipment is needed:

System Demonstration Platform (EVAL-SDP-CB1Z)

CN-0183 Circuit Evaluation Board (EVAL-CN0183-SDZ)

The CN-0183 Evaluation Software

Tektronix TDS2024, 4-channel oscilloscope

HP E3630A 0V to 6V/2.55A, ±20 V/0.5 A Triple Output DC Power Supply

PC (Windows 32-bit or 64-bit)

Get Started

Load the evaluation software by placing the CN-0183 Evaluation Software CD into the CD drive of the PC. Open My Computer, locate the drive that contains the evaluation software CD, and open the Readme file. Follow the instructions in the Readme file to install and use the evaluation software. Figure 6 shows the evaluation software main window.

Figure 6. Evaluation software main window

Test Setup Functional Block Diagram

A functional block 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 a PC through the USB port.

Figure 7. Test setup functional block diagram

set up

Connect the 120-pin connector on the EVAL-CN0183-SDZ to the CON A or CON B connector on the EVALSDP-CB1Z. Use nylon hardware to secure the two boards firmly, using the holes provided at the ends of the 120-pin connectors. After successfully setting the dc output power supply to +5 V, -5 V, and +12 V outputs, turn off the power supply.

With power removed, connect the −5 V supply to the −5 V pin on J5-3, the +5 V supply to the AVDD pin on J5-1, GND to the AGND pins on J5-2 and J4-2, and the +12 V supply to the +12 V pin on J4-1. Alternatively, place Link 2 and Link 3 in Position B to use the ADP2300 to provide +5 V to −5 V to the circuit. Note that AVDD and −5 V are not required in this case.

Apply power and connect the USB cable that comes with the SDP board to a USB port on the PC. Do not connect the USB cable to the micro-USB connector on the SDP board before applying dc power to the EVAL-CN0183-SDZ.

After setting up the test equipment, connect the oscilloscope probe to the TP1 and TP2 test points. Connect the TP3, TP4, and TP5 test points to the reference voltage, regulated +5 V, and regulated −5 V, respectively. Check that these test points are at the correct voltage (use TP6 for ground).

The software provided on the CD enables the user to set the VOUTA value by loading a code into the DAC and selecting the reference source. If the user keeps the default settings, +5 V and −5 V will need to be provided, and +12 V is not required. The default settings use the external REF192 reference, which provides a full-scale DAC output of 2.5 V (TP1) and −2.5 V to +2.5 V for the bipolar output (TP2). Loading 0x0000 sets the DAC output to 0 V and the bipolar output to −2.5 V, respectively. Loading 0x8000 sets the DAC output to 1.25 V and the bipolar output to 0 V, respectively. Loading 0xFFFF sets both the DAC output and the bipolar output to 2.5 V.