Circuit Note CN-0123 Devices Connected/Referenced Circuit Designs Using Analog Devices Products Apply these product pairings quickly and with confidence. For more information and/or support call 1-800-AnalogD (1-800-262-5643) or visit www.analog.com/circuit. AD5360 16-Channel, 16-Bit Voltage-Output DAC AD790 High Speed Precision Comparator AD8597 Low Noise Amplifier ADR435 5 V Ultralow Noise Voltage Reference Automated Calibration Technique That Reduces the AD5360 16-Channel, 16-Bit DAC Offset Voltage to Less Than 1 mV CIRCUIT FUNCTION AND BENEFITS CIRCUIT DESCRIPTION The circuit described in this document and shown in Figure 1 provides a method of calibrating that removes an unknown offset error. When using high precision, high resolution DACs in industrial process control and instrumentation applications, low offset is often a critical specification. The circuit uses builtin features of the AD5360 in conjunction with an external comparator and an operational amplifier to determine if the DAC output voltages are above or below a ground reference signal. With the amount of offset known, the user can adjust the codes sent to the DAC to null out the offset. The AD5360 is a 16 channel, 16-bit digital to analog converter. The nominal output range is ±10 V when used with a 5 V reference. The AD5360 contains two offset DACs. Each offset DAC is connected to a group of eight DACs and is used to adjust the mid-scale point of the output span. For example, the offset DAC can be programmed to change the output span from ±10 V to −8 V to +12 V, or other values as required by the application. INPUT AND SPECIAL FUNCTION REGISTERS SERIAL INTERFACE DVCC VSS VDD AD5360 +5V –15V +15V DAC 0 REGISTER DAC 0 VOUT0 DAC 1 REGISTER DAC 1 VOUT1 DAC 14 REGISTER DAC 14 VOUT14 DAC 15 REGISTER DAC 15 VOUT15 GPIO MUX MON_OUT VREF0 VREF1 GPIO AGND DGND +5V +5V +15V 1kΩ +15V LATCH ADR435 +5V VLOGIC +15V 510Ω +VS AD790 AD8597 100nF –15V –15V 08521-001 –VS Figure 1. Autocalibration Circuit for AD5360 DAC That Reduces the Offset Voltage to Less Than 1 mV (Simplified Schematic: Decoupling and All Connections Not Shown). Rev.0 “Circuits from the Lab” from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any“Circuit from the Lab”. (Continued on last page) One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2009 Analog Devices, Inc. All rights reserved. CN-0123 Circuit Note The AD5360 is factory trimmed to have a very low offset. The trimming is done with the offset DAC at its default value, and the offset error due to the offset DAC is effectively removed. When the value of the offset DAC is changed from its default value, however, its offset error affects the offset error of the main DACs. The circuit described here allows for the offset error of the main DACs to be measured and calibrated out under those conditions. The circuit relies on a general-purpose I/O pin and an on-chip monitor multiplexer. The GPIO (general-purpose I/O) pin is set as an input, and by reading the GPIO internal register, the logic status of the GPIO pin is determined. The analog multiplexer is programmable to connect any of the 16 DAC outputs to a single pin, the MON_OUT pin. The multiplexer switches have a low but finite on resistance, RON so that any current drawn from MON_OUT creates a voltage drop across RON and, therefore, an output error. To prevent this, MON_OUT is buffered by an AD8597 low-noise amplifier. The low pass filter following the amplifier reduces the amount of noise seen by the AD790 high speed precision comparator and prevents false triggering. The AD790 can be operated on ±15 V supplies, making it compatible with the AD5360. The AD790 also requires an additional +5 V VLOGIC supply when operating on ±15 V supplies. In addition, the AD790 has a 15 V maximum differential input voltage; therefore, it can tolerate the output voltages from the AD5360 without attenuation. In Figure 1, the comparator output is low if the channel offset is positive, indicating that the output voltage must be reduced to remove the offset. The comparator output is high if the channel offset is negative, indicating that the output voltage must be increased to remove the offset. To calibrate a DAC, the DAC channel is loaded with the digital value, which should ideally provide a voltage equal to SIGGND (that is, 0 V). In this example the DAC channel is assumed to have a negative offset. Reading the GPIO register shows that the comparator output is low, indicating that the input must be incremented until the output toggles. As progressively higher codes are written to the DAC input register, the GPIO register is read until the comparator trips to the high state. The AD790 has a maximum hysteresis band of 0.65 mV; therefore, reducing the DAC code again allows a more accurate determination of the DAC offset. When comparator output trips back to the low state, SIGGND is somewhere between those two codes. Due to the errors of the components used in the circuit, there is typically a span of three or four codes between comparator trip points. There is no way to determine exactly which code gives the lowest offset output using this method, but by picking a code that is the average of the two trip point codes, the DAC channel offset is typically less than 1 mV from SIGGND. Excellent layout, grounding, and decoupling techniques must be used to achieve the desired performance from the circuits discussed in this note (see MT-031 Tutorial and MT-101 Tutorial). As a minimum, a 4-layer PCB should be used with one ground plane layer, one power plane layer, and two signal layers. COMMON VARIATIONS The AD5362 is an 8-channel version of the AD5360. The AD5361 and AD5363 are 14-bit versions of the AD5360 and AD5362, respectively. The AD8599 is a dual version of the AD8597. The circuit described here can be used with any of the AD536x devices mentioned above. The reference can also be changed to give different output ranges if required. LEARN MORE MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND," Analog Devices. MT-083 Tutorial, Comparators, Analog Devices. MT-101 Tutorial, Decoupling Techniques, Analog Devices. Data Sheets and Evaluation Boards AD5360 AD5361 AD5362 AD5363 AD5360, AD5361, AD5362, AD5363 Evaluation Board AD8597 AD790 ADR435 REVISION HISTORY 9/09—Revision 0: Initial Version (Continued from first page) "Circuits from the Lab" are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the "Circuits from the Lab" in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the "Circuits from the Lab". Information furnished by Analog Devices is believed to be accurate and reliable. However, "Circuits from the Lab" are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any "Circuits from the Lab" at any time without notice, but is under no obligation to do so. Trademarks and registered trademarks are the property of their respective owners. ©2009 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN08521-0-9/09(0) Rev. 0 | Page 2 of 2