Design of high-precision DC voltage output circuit based on AD760

With the development of technology, the accuracy requirements of the test system in the industrial production process monitoring system are getting higher and higher. For the test system that requires high-precision DC voltage output, the selection of D/A chip and its peripheral circuits deserves careful consideration. This paper presents a high-precision DC voltage output circuit based on AD760 design, which has been successfully applied in industrial control test systems.

1 System Design
Figure 1 shows the block diagram of the designed high-precision DC voltage output circuit, which consists of bus terminal, digital isolation, D/A conversion, and voltage drive circuit. D/A uses 18-bit chip AD760, and the voltage drive circuit is realized by the follower circuit of op amp LF411.

2 Hardware Circuit Design
2.1 Digital Isolation
In order to prevent the digital signal at the bus end from affecting the analog output voltage, the digital signals on both sides need to be isolated. This board uses ADI's ADUM1400 and ADUM1201 high-speed magnetic isolation for isolation.
2.2 D/A Conversion
D/A conversion uses ADI's AD760 chip. Figure 2 shows the D/A conversion circuit diagram. AD760 is a 16/18-bit selectable, self-calibrated DAC with built-in on-chip reference voltage source, double buffer latch and output amplifier. It is manufactured using ADI's BiMOSII process and can implement high-precision bipolar linear circuits and low-power CMOS logic functions on the same chip. The self-calibration function is started by simply sending a pulse to make the CAL pin low. The CALOK pin will show when the calibration is successfully completed. During calibration, the output multiplexer (MUXOUT) can be used to send the output to the bottom of the output range. Data can be loaded into the AD760 in standard binary, serial data or double 8-bit byte format. In serial mode, 16-bit or 18-bit data can be used; the serial mode input format is pin-selectable and can be either MSB first or LSB first. This is accomplished through three digital input pins (pins 12, 13, and 14) with dual functionality. In byte mode, the user can also define whether the high byte or low byte data is loaded first. The double-buffered latch structure not only eliminates data skew errors, but also enables simultaneous update of each DAC in a multi-DAC system. When CLR is enabled, the asynchronous CLR function can be used to clear the output to negative full scale or mid-scale, depending on the state of pin 17.

[page]2.2 Voltage Drive
The output drive capability of AD760 is only 5mA, and the load capacity is limited. Therefore, an op amp follower circuit must be added to reduce the output resistance, as shown in Figure 4.

3 PCB Design
The power supply of AD760 should be bypassed with 10μF and 0.1μF capacitors. These capacitors should be as close to the device as possible. The 10μF capacitor is a tantalum electrolytic capacitor and the 0.1μF capacitor is a ceramic capacitor. The reference chip AD587 should be as close to AD760 as possible. The connection between its output and the REF pin of AD760 should be thickened, and a bypass capacitor should be added to this connection to reduce the impact of the output noise of AD760 on the output accuracy of D/A.

4 Test results
This circuit outputs a bipolar voltage of ±10V. By setting 5 fixed code values: 3FFFF, 2FFFF, 1FFFF, 0FFFF, 0, and testing with Agilent34401 high-precision 6.5-digit multimeter, after 8 hours of testing after power-on, its resolution can reach 17 bits and its accuracy can reach 16.7 bits.

5 Conclusion
Based on the design of high-precision DC voltage output circuit of AD760, it can provide high-precision voltage with a resolution of 17 bits and an accuracy of 16.7 bits, which is suitable for the field of industrial control system requiring high-precision measurement.