Circuit Note CN-0104 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. AD5522 Quad Parametric Measurement Unit with Integrated 16-Bit Level Setting DACs AD7685 16-Bit, 250 kSPS PulSAR® ADC ADR435 5 V Ultralow Noise XFET® Voltage Reference ADG1209 Low Capacitance, 4 Channel Differential Multiplexer ADG412 Quad SPST Switch Parametric Measurement Unit and Supporting Components for ATE Applications Using the AD5522 PMU and the AD7685 16-Bit ADC CIRCUIT FUNCTION AND BENEFITS This circuit is a quad parametric measurement unit (PMU) with supporting components to service a minimum of four deviceunder-test (DUT) channels. Typically, PMU channels are shared among a number of DUT channels. Although the AD5522 is very integrated and delivers four full PMU solutions, an external reference and an ADC are required as a minimum to complete this portion of the ATE signal chain. Typically, this reference and the ADC can be shared among multiple PMU packages. For further flexibility, additional external switches can be used to extend the capabilities of the PMU by extending the range of DUT capacitances that the AD5522 can drive. CIRCUIT DESCRIPTION The AD5522 quad PMU provides the forcing and measuring functions for the DUT, but digitizing is required external to the PMU. This can be achieved as follows: • An ADC can be dedicated to each individual PMU channel, providing the fastest throughput and result. • An ADC can be shared across multiple channels. In Figure 1, one AD7685 ADC is shared across the four PMU channels. In some applications, an ADC might be shared across many more channels, sometimes 8 or 16 PMU channels. The ADC can be shared across channels using the internal Disable feature of each MEASOUTx pin. This requires a write command to the PMU register to enable/disable the appropriate switches. If this method is chosen, note that no more than one MEASOUTx channel should be selected at any one time. Alternatively, an external 4:1 multiplexer can be used to control the measurement channel selection. In this way, all four MEASOUTx paths can be enabled, and the multiplexer makes the selection. Similarly, an 8:1 or 16:1 multiplexer would allow more measurement paths to share the one ADC. The choice of this multiplexer will depend on the ADC used and its input voltage range. For bipolar input ADCs, one member of the ADG1404/ADG1204 family would be ideal; for single-supply usage, the ADG706 and ADG708 would be more suitable. The output impedance of the MEASOUTx path is typically 60 Ω in addition to the switch impedance. Therefore, an ADC buffer, such as the ADA4898-1, should be considered to drive the ADC (buffer not shown). The AD7685 16-bit, 250 kSPS ADC was chosen for this application because of its ability to handle the 0 V to 4.5 V output range of the MEASOUTx path. In addition, the availability of other ADCs with faster speeds in the same footprint (for example, the 16-bit, 500 kSPS AD7686) also makes it very attractive for upgrade paths. The AD5522 requires a 5 V reference if a 20 V output range is required. The ADR435 5 V XFET reference was chosen because of its low tempco (10 ppm/°C, A Grade; 3 ppm/°C, B Grade), low noise (8 µV p-p, 0.1 Hz to 10 Hz), and ability to drive multiple PMU channels (30 mA source, 20 mA sink). Some applications require the PMU to drive a wide range of DUT capacitances, especially applications where the PMU is connected to a power supply pin or where the PMU is used as a device power supply and will see the decoupling/bypass capacitance of the DUT. In such cases, an external switch connected to the CCOMP pin, rather than a fixed capacitor, allows for additional CCOMP capacitors to be switched in and out, thereby allowing for optimization of settling time and stability with various capacitive loads. The switch chosen for this circuit was the ADG412 quad SPST, which has an onresistance of less than 50 Ω. The quad SPST switch was chosen 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-0104 Circuit Note AVDD +5V VOUT VIN VIO GND VREF TRIM 0.1µF 22nF ADR435 0.1µF VDD 10µF +5V +5V SDI SCK SDO IN– GND CNV IN+ AVSS AVDD DVCC 10µF 10µF 2.7nF ADG412 10µF VL = +5V 10µF 0.1µF 0.1µF AVSS AVDD 0.1µF ADG1209 (1/2) S1, S2, S3, S4 DVCC VREF MEASOUT CCOMP[0:2] CCOMP[3] A0, A1 EXTFOH3 EXTFOH0 CFF0 CFF3 AVSS FOH3 FOH0 MEASVH3 MEASVH0 EXTMEASIH3 EXTMEASIH0 RSENSE RSENSE EXTMEASIL0 12.5Ω MIN EXTMEASIL3 AD5522 DUT AVDD ≥ +10V AVSS ≤ –5V |AVDD – AVSS| ≥ 20V |AVDD – AVSS| ≤ 33V DVCC = 2.3V TO 5.25V DUT EXTFOH2 EXTFOH1 CFF2 CFF1 FOH2 FOH1 MEASVH2 MEASVH1 EXTMEASIH2 EXTMEASIH1 RSENSE RSENSE 12.5Ω MIN 12.5Ω MIN EXTMEASIL1 DUTGND AGND DGND EXTMEASIL2 SERIAL INTERFACE DUT DUT 08387-001 12.5Ω MIN SERIAL INTERFACE AVDD 10µF AVSS AVDD AD7685 Figure 1. Parametric Measurement Unit and Supporting Components (Simplified Schematic) instead of a multiplexer because most multiplexers allow only one of a number of channels to be on at any one time. Using the quad switch, each of the drains can be connected together and the sources connected to the each of the compensation capacitors, thereby providing 24 − 1 possible combinations of CCOMP. Similarly, the ADG1209 differential multiplexer is used in this circuit to accommodate a wider range of feedforward capacitances connected to the CFFx pins, enabling the AD5522 to drive a wider range of DUT capacitance. The series resistance of the multiplexer used should be such that 1/(2π × RON × CDUT) > 100 kHz. In this example, the ADG1209 services two AD5522 channels. voltage rating of the switch and capacitors should take this into account. CFF capacitors can have a ≤10% tolerance; this extra variation directly affects settling times, especially in the measure current mode for low currents. Selection of CCOMP capacitors should be ≤5% tolerance. Table 1 gives suggested nominal compensation capacitors CCOMP and CFF for various values of load capacitance. Table 1. Suggested Compensation Capacitor Selection CCOMP CFF CLOAD ≤1 nF 100 pF 220 pF ≤10 nF 100 pF 1 nF ≤100 nF CLOAD/100 CLOAD/10 The switch and capacitors will see the same voltage excursion as the voltage range at the FOH pin of the AD5522. Therefore, the Rev. 0 | Page 2 of 4 Circuit Note CN-0104 The circuit must be constructed on a multilayer PC board with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see Tutorial MT-031, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND" and Tutorial MT-101, Decoupling Techniques). Note that Figure 1 is a simplified schematic and does not show all the necessary decoupling. 12 10 CCOMP = 1nF CCOMP = 220pF 6 0V TO 11.25V FORCE VOLTAGE CLOAD = 220pF ADG412 USED TO PROVIDE SELECTABLE CCOMP 4 0 –60 –40 –20 0 20 40 60 80 100 120 140 08387-002 2 TIME (µs) Figure 2. Output Voltage Step Response for 11.25 V Force Voltage Step with 220 pFLoad for Various Values of CCOMP Using ADG412 SPST Switch Careful consideration of the power supply and ground return layout helps to ensure the rated performance. Design the printed circuit board (PCB) on which the AD5522 is mounted so that the analog and digital sections are separated and confined to certain areas of the board. If the AD5522 is in a system where multiple devices require an AGND-to-DGND connection, the connection should be made at one point only. Establish the star ground point as close as possible to the device. 0.010 0.015 0.008 MEASURE VOLTAGE INTEGRAL LINEARTY ERROR (%FSR) MEASURE VOLTAGE INTEGRAL LINEARTY ERROR (%FSR) 0.010 0.005 0 –0.005 –0.010 –0.015 –0.020 0.006 0.004 0.002 0 –0.002 –0.004 –0.006 08387-003 –0.008 –0.025 0 10k 20k 30k 40k 50k 60k –0.010 –10 –5 5 10 FORCE VOLTAGE (V) AD5522 FIN DAC CODE (FI MODE) Figure 5. Integral Linearity Performance Using the AD7685 to Measure FVMV Error (FV Range = ±10 V, Measout Gain = 0.2) Figure 3. Integral Linearity Performance Using the AD7685 to Measure FIMV Error (FI Range = ±2 mA, Measout Gain = 0.2) 0.015 0.015 0.010 0.010 MEASURE CURRENT INTEGRAL LINEARTY ERROR (%FSR) MEASURE CURRENT INTEGRAL LINEARTY ERROR (%FSR) 0 08387-005 OUTPUT (V) 8 0.005 0 –0.005 –0.010 0.005 0 –0.005 –0.010 –0.015 –8 –6 –4 –2 0 2 4 6 8 10 –0.025 0 FORCE VOLTAGE (V) 10k 20k 30k 40k 50k 60k FORCE CURRENT (DAC CODE) Figure 4. Integral Linearity Pperformance Using the AD7685 to Measure FVMI Error (FV Range = ±10 V in 2mA Range into a 5.6 kΩ Load) Figure 6. Integral Linearity Performance Using the AD7685 to Measure FIMI Error (FI Range = ±2 mA, Measout Gain = 0.2) Rev. 0 | Page 3 of 4 08387-006 –0.015 –10 08387-004 –0.020 CN-0104 Circuit Note For supplies with multiple pins (AVSS and AVDD), it is recommended that these pins be tied together and that each supply be decoupled only once. The AD5522 should have ample supply decoupling of 10 μF in parallel with 0.1 μF on each supply located as close to the package as possible, ideally right up against the device. The 10 μF capacitors are the tantalum bead type. The 0.1 μF capacitors should have low effective series resistance (ESR) and low effective series inductance (ESL)—typical of the common ceramic types that provide a low impedance path to ground at high frequencies—to handle transient currents due to internal logic switching. Avoid running digital lines under the device because they can couple noise onto the device. However, allow the analog ground plane to run under the AD5522 to avoid noise coupling (this applies only to the package with paddle up). The power supply lines of the AD5522 should use as large a trace as possible to provide low impedance paths and reduce the effects of glitches on the power supply line. Fast switching digital signals should be shielded with digital ground to avoid radiating noise to other parts of the board, and they should never be run near the reference inputs. It is essential to minimize noise on all VREF lines. Avoid crossover of digital and analog signals. Traces on opposite sides of the board should run at right angles to each other to reduce the effects of feedthrough through the board. As is the case for all thin packages, care must be taken to avoid flexing the package and to avoid a point load on the surface of this package during the assembly process. DUT requirements by using the on-chip OFFSET DAC. (See the AD5522 data sheet for more detail). There is also the added benefit of being able to use lower supply rails. This helps by reducing the power dissipated in the AD5522, especially when operating at the full 80 mA current range per channel. Variations in terms of the partitioning of PMU measurement channels per ADC channel could mean that one ADC channel is shared among more PMU channels (sometimes in 8:1 or 16:1 ratios). Alternatively, the on-chip MEASOUT disable feature or an analog multiplexer can be used for this function. Multiplexers add more series resistance to the measurement path; therefore, buffering may be required prior to the ADC input. Other variations include the use of ADCs, which handle bipolar signal ranges, or ADCs with faster sampling rates. LEARN MORE Automatic Test Equipment (ATE). MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of AGND and DGND. Analog Devices. MT-101 Tutorial, Decoupling Techniques. Analog Devices. Voltage Reference Wizard Design Tool. Data Sheets and Evaluation Boards AD5522 Data Sheet. AD5522 Evaluation Board. AD7685 Data Sheet. Also, note that the exposed paddle of the AD5522 is connected to the negative supply, AVSS. AD7685 Evaluation Board. COMMON VARIATIONS ADG412 Data Sheet. PMU circuits do not always need to use the full 20 V output range of the AD5522. Many applications require only a portion of that voltage. For example, the use of the ADR421 2.5 V voltage reference will allow the user to achieve a nominal output voltage range of ±5.6 V, which can be further scaled to suit the ADR435 Data Sheet. ADG1209 Data Sheet. REVISION HISTORY 7/09—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. CN08387-0-7/09(0) Rev. 0 | Page 4 of 4