Circuit Note CN-0217 Devices Connected/Referenced Circuits from the Lab™ reference circuits are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0217. AD5933 1 MSPS, 12-Bit Impedance Converter, Network Analyzer AD5934 250 kSPS, 12-Bit Impedance Converter, Network Analyzer AD8606 Precision, Low Noise, Dual CMOS Op Amp High Accuracy Impedance Measurements Using 12-Bit Impedance Converters EVALUATION AND DESIGN SUPPORT CIRCUIT FUNCTION AND BENEFITS Circuit Evaluation Boards AD5933 Evaluation Board (EVAL-AD5933EBZ) Design and Integration Files Schematics, Layout Files, Bill of Materials The AD5933 and AD5934 are high precision impedance converter system solutions that combine an on-chip programmable frequency generator with a 12-bit, 1 MSPS (AD5933) or 250 kSPS (AD5934) analog-to-digital converter (ADC). The tunable frequency generator allows an external complex impedance to be excited with a known frequency. The circuit shown in Figure 1 yields accurate impedance measurements extending from the low ohm range to several hundred kΩ, and it also optimizes the overall accuracy of the AD5933/AD5934. 1.98V p-p 1.98V p-p VDD MCLK AVDD VDD 1.48V VDD/2 DVDD VDD VDD DDS CORE (27 BITS) OSCILLATOR DAC VOUT ROUT SCL SDA I2C INTERFACE − 50kΩ A1 + 47nF 50kΩ TRANSMIT SIDE OUTPUT AMPLIFIER TEMPERATURE SENSOR ZUNKNOWN A1, A2 ARE ½ AD8606 REAL REGISTER AD5933/AD5934 IMAGINARY REGISTER RFB RFB 1024-POINT DFT 20kΩ VIN GAIN I-V VDD LPF VDD/2 AGND − A2 + DGND 50kΩ 50kΩ 09915-001 ADC (12 BITS) 20kΩ Figure 1. Optimized Signal Chain for Impedance Measurement Accuracy (Simplified Schematic, All Connections and Decoupling Not Shown) Rev. A Circuits from the Lab™ circuits 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 Circuits from the Lab circuits. (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 ©2011–2013 Analog Devices, Inc. All rights reserved. CN-0217 Circuit Note CIRCUIT DESCRIPTION Matching the DC Bias of Transmit Stage to Receive Stage The AD5933 and AD5934 have four programmable output voltage ranges; each range has an output impedance associated with it. For example, the output impedance for a 1.98 V p-p output voltage is typically 200 Ω (see Table 1). The four programmable output voltage ranges in the AD5933/ AD5934 have four associated bias voltages (see Table 2). For example, the 1.98 V p-p excitation voltage has a bias of 1.48 V. However, the current-to-voltage (I-V) receive stage of the AD5933/ AD5934 is set to a fixed bias of VDD/2 as shown in Figure 1. Therefore, for a 3.3 V supply, the transmit bias voltage is 1.48 V, and the receive bias voltage is 3.3 V/2 = 1.65 V. This potential difference polarizes the impedance under test and can cause inaccuracies in the impedance measurement. Table 1. Output Series Resistance (ROUT) vs. Excitation Range for VDD = 3.3 V Supply Voltage Range Range 1 Range 2 Range 3 Range 4 Output Excitation Amplitude (V p-p) 1.98 0.97 0.383 0.198 Output Resistance (ROUT) 200 Ω typical 2.4 kΩ typical 1.0 kΩ typical 600 Ω typical One solution is to add a simple high-pass filter with a corner frequency in the low Hz range. Removing the dc bias from the transmit stage and rebiasing the ac signal to VDD/2 keeps the dc level constant throughout the signal chain. The output impedance affects the impedance measurement accuracy, particularly in the low kΩ range, and must be taken into account when calculating the gain factor. Refer to the AD5933 or AD5934 data sheets for more details on the gain factor calculation. Table 2. Output Levels and Respective DC Bias for VDD = 3.3 V Supply Voltage A simple buffer in the signal chain prevents the output impedance from affecting the unknown impedance measurement. Select a low output impedance amplifier with sufficient bandwidth to accommodate the AD5933/AD5934 excitation frequency. An example of the low output impedance achievable is shown in Figure 2 for the AD8605/AD8606/AD8608 family of CMOS op amps. The output impedance for this amplifier for an AV of 1 is less than 1 Ω up to 100 kHz, which is the maximum operating range of the AD5933/AD5934. 100 VS = 2.7V 90 70 AV = 100 60 50 AV = 10 AV = 1 20 10 0 1k 10k 100k 1M FREQUENCY (Hz) Output DC Bias Level (V) 1.48 0.76 0.31 0.173 Selecting an Optimized I-V Buffer for the Receive Stage The I-V amplifier stage of the AD5933/AD5934 can also add minor inaccuracies to the signal chain. The I-V conversion stage is sensitive to the amplifier's bias current, offset voltage, and common-mode rejection ratio (CMRR). By selecting the proper external discrete amplifier to perform the I-V conversion, the user can choose an amplifier with lower bias current and offset voltage specifications along with excellent CMRR, making the I-V conversion more accurate. The internal amplifier can then be configured as a simple inverting gain stage. Optimized Signal Chain for High Accuracy Impedance Measurements 40 30 Output Excitation Amplitude (V p-p) 1.98 0.97 0.383 0.198 Selection of the RFB resistor still depends on the gain through the system as described in the AD5933/AD5934 data sheets. 09915-002 OUTPUT IMPEDANCE (Ω) 80 Range 1 2 3 4 10M Figure 2. Output Impedance of AD8605/AD8606/AD8608 100M Figure 1 shows a proposed configuration for measuring low impedance sensors. The ac signal is high-pass filtered and rebiased before buffering with a very low output impedance amplifier. The I-V conversion is completed externally before the signal returns to the AD5933/AD5934 receive stage. Key specifications that determine the required buffer are very low output impedance, the single-supply capability, low bias current, low offset voltage, and excellent CMRR performance. Some suggested parts are the ADA4528-1, AD8628, AD8629, AD8605, and AD8606. Depending on board layout, use a single-channel or dual-channel amplifier. Use precision 0.1% resistors for both the biasing resistors (50 kΩ) and gain resistors (20 kΩ and RFB) to reduce inaccuracies. Rev. A | Page 2 of 6 Circuit Note CN-0217 CIRCUIT EVALUATION AND TEST 35 25 20 15 1µF 0 29.95 30.00 30.05 30.10 30.15 FREQUENCY (kHz) 30.20 30.25 09915-003 5 Table 3. Low Impedance Range Setup for VDD = 3.3 V Supply Voltage Value 1.98 V (Range 1) 15 16 MHz 20.1 Ω 20.0 Ω 30 kHz to 30.2 kHz R1 = 10.3 Ω, R2 = 30.0 Ω, C3 = 1 µF (ZC = 5.3 Ω at 30 kHz) 10.3Ω 10 Example 1: Low Impedance Range Figure 3. Measured Low Impedance Magnitude Results 20 10.3Ω, 30Ω 0 –20 PHASE (Degrees) –40 –60 The results of the low impedance measurements are shown in Figure 3, Figure 4, and Figure 5. Figure 5 is for the 10.3 Ω measurement and is shown on an expanded vertical scale. In addition, note that to achieve a wider range of measurements a 200 mV p-p range was used. If the unknown Z is a small range, a larger output voltage range can be used to optimize the ADC dynamic range. –100 29.95 30.00 30.05 30.10 30.15 FREQUENCY (kHz) 30.20 30.25 09915-004 1µF Figure 4. Measured Low Impedance Phase Results 10.22 10.20 10.18 MAGNITUDE (Ω) The accuracy achieved is very much dependent on how large the unknown impedance range is relative to the calibration resistor, RCAL. Therefore, in this example, the unknown impedance of 10.3 Ω measured 10.13 Ω, an approximate 2% error. Choosing an RCAL closer to the unknown impedance achieves a more accurate measurement; that is, the smaller the unknown impedance range is centered on RCAL is the more accurate the measurement. Consequently, for large unknown impedance ranges, it is possible to switch in various RCAL resistors to break up the unknown impedance range using external switches. The RON error of the switch is removed by calibration during the RCAL gain factor calculation. Using a switch to select various RFB values can optimize the dynamic range of the signal seen by the ADC. –80 10.16 10.14 10.12 10.10 10.08 10.06 10.04 29.50 30.00 30.05 30.10 30.15 FREQUENCY (kHz) 30.20 30.25 Figure 5. Measured 10.3 Ω Magnitude Results (Expanded Scale) Rev. A | Page 3 of 6 09915-005 Parameter Voltage Peak-to-Peak (V p-p) Number of Settling Time Cycles MCLK RCAL RFB Excitation Frequency Range Unknown Impedances 30Ω 30 MAGNITUDE (Ω) The schematic in Figure 1 was developed to improve impedance measurement accuracy, and some example measurements were taken. The AD8606 dual-channel amplifier buffers the signal on the transmit path and converts the receive signal from current to voltage. For the three examples shown, the gain factor is calculated for each frequency increment to remove frequency dependent errors. A complete design package including schematics, bill of materials, layout, and Gerber files is available for this solution at www.analog.com/CN0217-DesignSupport. The software used is the same software that is available with evaluation boards and is accessible from the AD5933 and AD5934 product pages. CN-0217 Circuit Note Example 2: kΩ Impedance Range –89.3 Using an RCAL of 99.85 kΩ, a wide range of unknown impedances were measured according to the setup conditions listed in Table 4. Figure 6 to Figure 10 document accuracy results. To improve the overall accuracy, select an RCAL value closer to the unknown impedance. For example, in Figure 9, an RCAL closer to the ZC value of 217.5 kΩ is required. If the unknown impedance range is large, use more than one RCAL resistor. –89.4 –89.8 –89.9 –90.0 –90.1 Value 0.198 V (Range 4) 15 16 MHz 99.85 kΩ 100 kΩ 30 kHz to 50 kHz R0 = 99.85 kΩ, R1 = 29.88 kΩ, R2 = 14.95 kΩ, R3 = 8.21 kΩ, R4 = 217.25 kΩ, C5 = 150 pF, (ZC = 26.5 kΩ at 40 kHz), C6 = 47 pF (ZC = 84.6 kΩ at 40 kHz) –90.3 30 35 40 FREQUENCY (kHz) 45 50 09915-007 –90.2 Figure 7. Phase Result for ZC = 47 pF, RCAL = 99.85 kΩ 8280 R3 8260 120 110 8240 8220 8200 IDEAL 100 8160 30 IDEAL 35 90 40 FREQUENCY (kHz) 45 50 09915-008 8180 Figure 8. ZC = 8.21 kΩ, RCAL = 99.85 kΩ 218.5 80 MEASURED 70 35 40 FREQUENCY (kHz) 45 Figure 6. Magnitude Result for ZC = 47 pF, RCAL = 99.85 kΩ 50 IDEAL 217.5 217.0 216.5 216.0 R4 215.5 21.50 214.5 214.0 213.5 30 35 40 FREQUENCY (kHz) 45 Figure 9. ZC = 217.25 kΩ, RCAL = 99.85 kΩ Rev. A | Page 4 of 6 50 09915-009 60 30 IMPEDANCE MAGNITUDE (kΩ) 218.0 09915-006 IMPEDANCE MAGNITUDE (kΩ) –89.7 IMPEDANCE MAGNITUDE (Ω) Parameter Voltage Peak-to-Peak (V p-p) Number of Settling Time Cycles MCLK RCAL RFB Excitation Frequency Range Unknown Impedances –89.6 PHASE (Degrees) Table 4. kΩ Impedance Range Setup for VDD = 3.3 V Supply Voltage –89.5 Circuit Note CN-0217 120 –60 R0 –65 –70 PHASE (Degrees) 80 C6 60 40 –80 IDEAL MEASURED –85 R1 C5 20 –90 R2 –95 32 34 36 38 40 42 44 46 48 50 FREQUENCY (kHz) Figure 10. Magnitude Results for Example 2: R1, R2, R3, C5, C6 24 4 44 64 84 104 FREQUENCY (kHz) Figure 12. Phase Results for ZC = 10 kΩ||10 nF, RCAL = 1 kΩ Setup and Test Example 3: Parallel R-C (R||C) Measurement An R||C type measurement was also made using the configuration, using an RCAL of 1 kΩ, an R of 10 kΩ, and a C of 10 nF, measured across a frequency range of 4 kHz to 100 kHz. The magnitude and phase results vs. ideal are plotted in Figure 11 and Figure 12. Table 5. R||C Impedance Range Setup for VDD = 3.3 V Supply Voltage Parameter Voltage Peak-to-Peak (V p-p) Number of Settling Time Cycles MCLK RCAL RFB Excitation Frequency Range Unknown Impedance R||C 09915-010 R3 0 30 Value 0.383 V (Range 3) 15 16 MHz 1 kΩ 1 kΩ 4 kHz to 100 kHz R = 10 kΩ, C = 10 nF The evaluation board software is the software used on the EVAL-AD5933EBZ. Refer to the technical note available on the CD provided with the evaluation board for details on the board setup. Note that there are alterations to the schematic. Link connections on the EVAL-AD5933EBZ board are listed in Table 4. In addition, note that the location for RFB is located at R3 on the evaluation board, and the location for ZUNKNOWN is C4. Table 6. Link Connections for EVAL-AD5933EBZ Link Number LK1 LK2 LK3 LK4 LK5 LK6 4000 Default Position Open Open Insert Open Insert A 3500 Complete setup and operation for the hardware and software for the evaluation board can be found in User Guide UG-364. 3000 COMMON VARIATIONS 2500 Other op amps can be used in the circuit, such as the ADA4528-1, AD8628, AD8629, AD8605, and AD8608. 2000 Switching Options for System Applications IDEAL MEASURED 1500 1000 500 0 4 24 44 64 84 FREQUENCY (kHz) Figure 11. Magnitude Results for ZC = 10 kΩ||10 nF, RCAL = 1 kΩ 104 09915-011 IMPEDANCE MAGNITUDE (Ω) –75 09915–012 IMPEDANCE MAGNITUDE (kΩ) 100 For this particular circuit, the ZUNKNOWN and RCAL were interchanged manually. However, in production, use a low on-resistance switch. The choice of the switch depends on how large the unknown impedance range is and how accurate the measurement result needs to be. The examples in this circuit note use just one calibration resistor, and so a low on-resistance switch, such as the ADG849, can be used as shown in Figure 13. Multichannel switch solutions, such as the quad ADG812, can also be used. The errors caused by the switch resistance on the ZUNKNOWN are removed during calibration, but by choosing a very low RON switch, the effects can be further minimized. Rev. A | Page 5 of 6 CN-0217 Circuit Note LEARN MORE CN-0217 Design Support Package: http://www.analog.com/CN0217-DesignSupport A1 MT-085 Tutorial, "Fundamentals of Direct Digital Synthesis (DDS)," Analog Devices. D ADG849 Buchanan, David, "Choosing DACs for Direct Digital Synthesis," AN-237 Application Note, Analog Devices. IN Riordan, Liam, "AD5933 Evaluation Board Example Measurement," AN-1053 Application Note, Analog Devices. S1 UG-364 User Guide for AD5933 Evaluation Board S2 ADIsimDDS Design and Evaluation Tool RFB ZUNKNOWN RCAL AD5933/AD5934 Demonstration and Design Tool Data Sheets and Evaluation Boards A2 AD5933 Data Sheet VDD AD5933 Evaluation Board 50kΩ 09915–013 50kΩ AD5934 Data Sheet Figure 13. Switching Between RCAL and Unknown Z Using the ADG849 Ultralow RON SPDT Switch (Simplified Schematic, All Connections and Decoupling Not Shown) AD5934 Evaluation Board AD8606 Data Sheet ADG849 Data Sheet ADG812 Data Sheet REVISION HISTORY 3/13—Rev. 0 to Rev. A Updated Table Numbers; Renumbered Sequentially....................3 Changes to Evaluation and Design Support Section ....................1 Changes to Setup and Test Section and Table 6 ............................5 Changes to Learn More Section ......................................................6 6/11—Revision 0: Initial Version (Continued from first page) Circuits from the Lab circuits 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 circuits 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 circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab circuits 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 circuits at any time without notice but is under no obligation to do so. ©2011–2013 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN09915-0-3/13(A) Rev. A | Page 6 of 6