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Building a Simple SPICE Model for the THS3001 Application Report April 1999 Mixed Signal Products SLOA018 IMPORTANT NOTICE Texas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinue any product or service without notice, and advise customers to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgement, including those pertaining to warranty, patent infringement, and limitation of liability. TI warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with TI’s standard warranty. Testing and other quality control techniques are utilized to the extent TI deems necessary to support this warranty. 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Copyright 1999, Texas Instruments Incorporated Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 Measuring Zt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3 Measuring Zo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4 Measuring Re . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5 Constructing the Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 List of Figures 1 Basic CF Op Amp Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Test Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 THS3001 Open Loop Transimpedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Measuring Zo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 ZoCL vs Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Measuring Re . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Model for zo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Simple THS3001 SPICE Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Comparison of Data Sheet and SPICE Model with Gain = 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Comparison of Data Sheet and SPICE Model with Gain = 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Comparison of Data Sheet and SPICE Model with Gain = 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Building a Simple SPICE Model for the THS3001 1 2 3 4 5 5 6 6 7 7 8 iii iv SLOA018 Building a Simple SPICE Model for the THS3001 James Karki ABSTRACT The application report Voltage Feedback Versus Current Feedback Op Amps SLVA051 outlines the basic operation of a current feedback operational amplifier (op amp) in relation to a voltage feedback op amp. One of the basic principles of a current feedback op amp is that its transfer function is a transimpedance equal to the output voltage divided Vo . by the current from the negative input terminal, i.e., Zt le This application report describes how to construct a simple model that will simulate the frequency response of the THS3001 in SPICE. The laboratory setup to measure the basic parameters of the THS3001 is illustrated. + 1 Introduction This report highlights the basic functioning of the THS3001 op amp, and helps give insight to its proper use. Limitations of the device such as input common mode range, power supply range, CMRR, PSRR, output voltage swing, etc., are purposely left out . The model is simple and easy to understand, simulates fast, and gives good results. Figure 1 is a basic current feedback op amp model. To transform this into a SPICE circuit model, Zt, Zo, and Re must be determined. Vp + x1 x1 Ie Vn Re Ie Zo VO Zt – Figure 1. Basic CF Op Amp Model 2 Measuring Zt A network analyzer is designed to measure the transfer function of a circuit. Therefore an HP8753E network analyzer is used to measure the transfer function of the THS3001. It has a lower frequency limit of 30 kHz. Below 30 kHz a servo-loop is used. Figure 2 shows the laboratory test setups. The servo-loop is on the left and the network analyzer is on the right. 1 Measuring Zt HP 8753E Network Analyzer V1 50 Ω THS3001 + V1 _ 1 kΩ 50 Ω 1 kΩ + + _ THS3001 V2 1 kΩ 10 kΩ V3 100 pF V In 10 kΩ 10 Ω R2 100 kΩ Ie Out TLE2227 + V2 _ Ie 10 Ω (b) Network Analyzer, f ≥ 30 kHz (a) Servo-Loop, f < 30 kHz Figure 2. Test Setup Solving the servo-loop circuit shown in Figure 2 (a), the transimpedance of the THS3001 is Zt + – DDVV12 × R2 . The 100-pF capacitor limits the bandwidth to 159 kHz and the 1 kΩ provides a load. Using the network analyzer as shown in Figure 2 (b), the transfer functions V2 V1 V 3 and are measured by switching the input between V2 and V3. To compute V1 V2 , divide V2 by V3 to get V2 , then multiply by 10 Ω to get the Zt le V1 V1 V3 + transimpedance of the THS3001. By default the network analyzer displays the results in dB. Therefore, the mechanics of the transimpedance calculation are: Zt ( dB ) + VV21 (dB) * VV31 (dB) ) 20 dB. Figure 3 is a plot of the THS3001 transimpedance magnitude and phase versus frequency from data collected using the test setups and methods described above. The simulation result of the SPICE model, constructed later, is also plotted. 2 SLOA018 Measuring Zo MAGNITUDE 140 130 120 SPICE Data 110 Lab Data dB – Ω 100 90 80 70 60 50 40 30 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 1.E+07 1.E+08 1.E+09 f – Frequency PHASE 20 0 –20 SPICE Data –40 –60 Lab Data Deg –80 –100 –120 –140 –160 –180 –200 –220 –240 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 f – Frequency – Hz Figure 3. THS3001 Open Loop Transimpedance Analyzing this information: dc gain = 138.5 dB Ω = 8.5 MΩ dominant pole near 10 kHz, multiple upper frequency poles above 200 MHz 3 Measuring Zo With the circuit shown in Figure 4, the amplifier tries to maintain 0 V at its output terminal. Because of the finite output impedance zo, there is a finite voltage at V2. By varying the voltage V1 and recording V2, the output impedance, ZoCL , is calculated by the formula shown. Building a Simple SPICE Model for the THS3001 3 Measuring Zo THS3001 V2 + IeZt 50 Ω ZO 100 Ω V1 – 1 kΩ Zo CL + 100 × ȡȧ ȣȧ Ȣ Ȥ 1 V1 –1 V2 1 kΩ Figure 4. Measuring Zo In the closed loop situation shown, Zo CL transmission as follows: Zo + zo × CL ȡȧ ȣȧ Ȣ) Ȥ 0 zo, but it is related by the loop 1 1 Zt 1k At low frequencies ZoCL is very low because Zt is very high, and decreases as frequency increases. The graph shown in Figure 5 shows the measured ZoCL from 100 kHz to 1 GHz. The SPICE simulation of the output impedance model (developed below) configured the same as the test circuit in Figure 4 is also plotted. Analyzing the graph in Figure 5: ZoCL increases at approximately 20 dB/dec from 100 kHz to over 100 MHz; it peaks at 600 MHz, and falls at about 20 dB/dec to 1 GHz. Correlating ZoCL to Zt, and the above equation: zo appears resistive up to about 100 MHz where Zt = 1000. After that zo appears reactive in nature. 4 SLOA018 Measuring Re CLOSED LOOP OUTPUT IMPEDANCE vs FREQUENCY 100 ZoCL – Output Impedance – Ω 10 SPICE Data 1 Lab Data 0.1 0.01 0.001 1E+05 1E+06 1E+07 1E+08 1E+09 f – frequency – Hz Figure 5. ZoCL vs Frequency 4 Measuring Re By measuring the transfer function V2 in Figure 6, Re is approximated by the V1 formula shown. At higher frequencies, parasitic inductance and capacitance modify the impedance, but for the most part, these effects are included in the measurement of Zt. So, for the purposes of this report, take Re = 25 Ω resistive. V1 + _ THS3001 VO Re V2 + 10 × ǒ Ǔ+ V1 –1 V2 25 W 10 Ω Figure 6. Measuring Re 5 Constructing the Model To build a SPICE model for the THS3001, use Figure 1 as a template, and use the data collected to compute component values. E and F devices are used to construct the SPICE model. An E device is a voltage-controlled voltage source and an F device is a current-controlled current source. Unity gain is used for these devices. The input is modeled by an E device driving an F device. The current in the F device is set by Re between the negative input terminal and ground. The output of the F device emulates the error current le. Building a Simple SPICE Model for the THS3001 5 Constructing the Model The output of the F device drives Rt = 8.5 MΩ and Cc = 1.25 pF. The values are chosen to set the dc gain and dominant pole of Zt. Trial and error reveals that an RC single pole at 1 GHz, and an LRC double pole at 800 MHz (Q = 1) shapes the upper frequency response close to the measured values. E1 – E3 provide buffering. R1 and C1 form the RC single pole, and L2, R2, and C2 form the LRC double pole. The model shown in Figure 7 is proposed for zo. Rc C RI L Figure 7. Model for zo Ǹ Select component values as follows: 1. Rl = ZoCL divided by Zt/1000 at 100 kHz → Rl ≈ 7.7 Ω 2. sL = Rl at 100 MHz → L ≈ 11 nH 3. 1/sC = sL at 600 MHz → C ≈ 6.3 pF 4. Rc + ǒ Ǔ ǒǓ RI * CL ) sC RI * Zo ) (sL ) CL 2 Zo ǒ RI CL Ǔ 2 2 , with s 2 + 2p * 600 MHz, ³ Rc [ 18 W Figure 8 shows the SPICE model created. The simulated open loop transimpedance and closed loop output impedance are included in Figure 3 and Figure 5 to see the correlation between simulating this model and the lab data collected on the THS3001. Input Buffer Ein + IN Dominant Pole Multiple Upper Frequency Poles F1 + + – IN Output Impedance R1 1 kΩ E1 + Re 25 Ω Rt 8.5 MΩ Cc 1.25 pF + E2 + C1 0.159 pF L2 + R2 50 Ω 10 nH C2 4 pF Ro 18 Ω E3 + + RI 7.7 Ω To test the model further, SPICE simulations of circuits from the THS3001 data sheet, literature number SLOS217, is performed and compared to the data sheet graphs. The circuits and the results are shown in Figure 9 through Figure 11. Note that 1-pF capacitors are added to the output and negative input to simulate parasitic board capacitance. The data from the data sheet is prefaced by DS, followed by the circuit configuration. The data from the SPICE simulation is prefaced by SPICE, followed by the circuit configuration. SLOA018 L2 11 nH Figure 8. Simple THS3001 SPICE Model 6 C 6.3 pF OUT Constructing the Model GAIN vs FREQUENCY 3 (2) 2 (4) Gain = 1 1 pF† 1 THS3001 Gain – dB + _ VI (1) VO 1 pF† Rf (6) 0 (3) (1) DS RF = 750 Ω (2) SPICE RF = 750 Ω (3) DS RF = 1 kΩ (4) SPICE RF = 1 kΩ (5) DS RF = 1.5 kΩ (6) SPICE RF = 1.5 kΩ –1 † For SPICE simulation –2 –3 1.00E+06 (5) 1.00E+07 1.00E+08 1.00E+09 f – Frequency – Hz Figure 9. Comparison of Data Sheet and SPICE Model with Gain = 1 GAIN vs FREQUENCY 9 (1) 8 Gain = 2 1 pF† Rg † For SPICE simulation + _ 7 (3) THS3001 VO 1 pF† Rf 6 Gain – dB VI (2) (4) (6) 5 (5) 4 3 2 1 (1) DS RF = 560 Ω (2) SPICE RF = 560 Ω (3) DS RF = 680 Ω (4) SPICE RF = 680 Ω (5) DS RF = 1 kΩ (6) SPICE RF = 1 kΩ 0 1.00E+06 1.00E+07 1.00E+08 f – Frequency – Hz 1.00E+09 Figure 10. Comparison of Data Sheet and SPICE Model with Gain = 2 Building a Simple SPICE Model for the THS3001 7 Summary GAIN vs FREQUENCY 18 17 Gain = 5 1 pF† Rg Rf (2) 15 THS3001 VO 1 pF† 14 Gain – dB + _ VI (1) 16 13 (4) (5) 12 (6) 11 10 † For SPICE simulation 9 8 7 6 (1) DS RF = 390 Ω (2) SPICE RF = 390 Ω (3) DS RF = 560 Ω (4) SPICE RF = 560 Ω (5) DS RF = 1 kΩ (6) SPICE RF = 1 kΩ 5 1.00E+06 1.00E+07 (3) 1.00E+08 1.00E+09 f – Frequency – Hz Figure 11. Comparison of Data Sheet and SPICE Model with Gain = 5 6 Summary The simple model developed here is useful for simulating and understanding the basic operation of the THS3001. Hopefully this model will prove practical to you. Use it in good health. 8 SLOA018