Anatomy of a current-feedback op ampBy Ron Mancini, 12/5/2005 A previous column dealt with the VFOA (voltage-feedback op amp, Reference 1). This column explains the CFOA (current-feedback op amp) and includes a performance analysis. Slew rate and frequency performance are the CFOA's strong points, but its precision and CMRR (common-mode-rejection ratio) are subpar to those of a VFOA's. By Ron Mancini, 10/27/2005 One of the most common requests I get from engineers is for a comparison of voltage-feedback op amps and current-feedback op amps. It is impossible to determine which one better suits a given application without explaining how each op amp functions. Here, I tackle the voltage-feedback op amp (Figure 1). By Ron Mancini, 8/18/2005 I wanted to use Spice to predict the PSRR (power-supply-rejection-ratio) performance of an op amp. My plan was to evaluate the effects of adding decoupling capacitors or local power-supply filtering to improve the circuit's PSRR, but first I needed to establish a baseline-PSRR performance. I inserted an ac signal into the ground return to check the OPA132's PSRR performance, and ... Beyond the Spice model's dc and ac performance By Ron Mancini, 6/23/2005 Evaluating the dc and ac performance of a Spice model before using it is mandatory for success. But the model evaluation should not stop here; designers should also know how the model performs when operating the active device in the nonlinear area and how a critical ac specification behaves. Verify your ac Spice model By Ron Mancini, 5/26/2005 A few years ago, the ac parameters of Spice models were almost worthless; they had little correlation with reality. Time changes many things, and now, many ac Spice models are excellent representations of data sheets and, consequently, ICs. The designer still has to verify each new model before using it to ensure that it represents the IC in the design; the following description ... Understanding Spice models By Ron Mancini, 4/14/2005 To ensure a complete and accurate Spice analysis, you must verify your selected Spice model. Verification is a three-part exercise: Verify dc performance to ensure dc accuracy, ac performance to verify ac accuracy, and boundary conditions to investigate nonlinear behavior. Use the circuit in Figure 1 to verify the dc gain and input bias current of an op amp. Validate Spice models before use By Ron Mancini, 3/31/2005 Semiconductor vendors are notorious for handing out Spice models that don't work or that fail to represent the circuit. Filter your voltage reference for low-noise performance By Ron Mancini, 11/25/2004 The short-term variation in output voltage from a voltage reference is noise. Reference-voltage noise occurs in two frequency ranges: 0.1 to 10 Hz for short-term, peak-to-peak drift, and 10 Hz to 1 kHz for wideband noise. Expressing noise in parts per million is popular because the noise voltage is usually proportional to the reference voltage, thereby keeping the parts-per-million value relati... Low-voltage reference uses the ∆VBE circuit By Ron Mancini, Texas Instruments, 10/28/2004 Some of my recent columns discuss the functions and advantages of zener-diode references (references 1, 2, and 3). Although zener diodes are stable and precise voltage references, they require high bias voltage (usually 8V minimum). The bias-voltage requirement precludes most zener-diode references from circuits with supply voltages of 5V or less. The ultimate zener-diode reference By Ron Mancini, 9/30/2004 My recent columns discussed the physics and applications of discrete-zener-diode references, and the conclusion they reached is that discrete zener diodes make poor references because of tolerance buildup (references 1 and 2). Early-IC designers achieved zener characteristics using reverse-biased, npn-transistor base-emitter junctions. Designing a zener-diode regulator By Ron Mancini, 8/5/2004 IC references are popular with circuit designers because they are accurate and exhibit low drift. Some of my future columns will cover the three types of IC references: buried zener, bandgap, and XFET. You develop the reference-design procedure with a zener diode; the zener's simplicity illustrates the design procedure, and its problems make you appreciate IC references. Anatomy of a precision-voltage reference By Ron Mancini, 6/24/2004 Voltage references are critical components in any system because they determine system accuracy. They determine the accuracy of ADCs, DACs, and feedback systems. For example, in a correctly designed linear-feedback system, multiplying the voltage reference by the system-transfer function gives the output voltage (current). The nuances of op-amp integrators By Ron Mancini, 3/18/2004 An integrator that you configure with an op amp is a simple circuit consisting of a resistor, a capacitor, and an op amp, so how can anything go wrong? In Figure 1, when ZF is a capacitor, the closed-loop ideal-gain equation is G=–1/RGCs, where s is the Laplace complex operator. Thus, the circuit performs a pure integration. The perils of input capacitance By Ron Mancini, 12/11/2003 It is impossible to build an op amp without including some input capacitance, and the op amp's pc board adds even more (Figure 1). All of the capacitors except for the feedback capacitor, CF, are stray capacitances, and they influence the circuit's stability. When you set the capacitors to zero, an artificial condition, Equation 1 gives the loop gain (Reference 1). Choosing the correct amplifier By Ron Mancini, 10/30/2003 Operational amplifiers are fundamental because they can perform any amplifier function. The op-amp name stems from the days when analog computers reigned supreme and feedback amplifiers performed mathematical functions. Feedback makes an amplifier's performance a function of the passive components, resistors, and capacitors, rather than of the active devices. Terminating a differential-input signal By Ron Mancini, 9/25/2003 A previous column explains that terminating transmission lines at the driving and receiving ends minimizes signal reflections (Reference 1). You must calculate termination-resistor values for single-ended and fully differential circuits. The calculations for single-ended circuits are simple; the noninverting-circuit configuration separates the termination and gain-setting resistors. Fully differential amplifiers and transmission lines By Ron Mancini, 8/7/2003 Fully differential amplifiers aim at amplifying high-frequency signals while rejecting the noise that always exists in mixed-signal systems. You wire these amps on pc boards and in cabling schemes such that any injected noise is common-mode rather than single-ended. The amplifier-input stage rejects common-mode noise very well, so having two signal wires in close proximity carrying the signal t... Taming fully differential circuits By Ron Mancini, 6/26/2003 Manufacturers make fully differential amplifiers for designs requiring differential drive voltages. Example applications are high-speed ADC inputs, high-speed analog-signal transmission, high-frequency noise rejection, and low-distortion applications. Most fully-differential-amplifier applications are high-frequency; fully-differential-amplifier gain bandwidths are in the multiple-gigahertz reg... Developing equations for fully differential amplifiers By Ron Mancini, 5/29/2003 Fully differential amplifiers and op amps are similar, but they are not identical. You must consider the input voltages and both output voltages when developing fully-differential-amplifier transfer equations. (This column later addresses the development of the com- mon-mode-voltage equation.) You use the fully-differential-amplifier circuit in Figure 1 to develop the transfer equations. Take the analog high road By Ron Mancini, 5/15/2003 Op amps and instrumentation amps have differential front ends that make them excellent subtracters. Subtraction is important for noise reduction; you subtract any signal appearing on one amplifier input from the other input, thus eliminating common-mode input noise (ac and dc). The op amp has great versatility, but its versatility depends on external resistors, and their tolerances limit the op... Two op amps can ruin the stew By Ron Mancini, 3/20/2003 Op-amp circuits have a feedback loop, but sometimes it is advantageous to improve closed-loop performance by including another amplifier within the op amp's feedback loop. For example, a limited selection of high-drive-capability/precision-input op amps exists, and one option is to put a high-drive-capability amplifier in the op amp's feedback loop. Are single- and dual-supply op amps interchangeable? By Ron Mancini, 2/20/2003 As their names suggest, single-supply op amps aim to operate with one power supply, and their dual counterparts aim to operate with two power supplies. The application-circuit equations for both types of op amps are identical, so engineers often assume that the two types of op amps are interchangeable. Don't let noise ruin instrumentation-amplifier performance By Ron Mancini, 10/31/2002 You can't afford noise in your circuit designs, and certain applications, such as audio, demand low- noise performance. You can minimize external noise by considering noise during the board-layout stage. For example, you must make power and ground impedances small enough to minimize the effect of current spikes. Taming injected common-mode noise By Ron Mancini, 9/26/2002 External noise sources inject signals into analog-input circuits at every opportunity. Good grounding practice, prudent use of shields, decoupling capacitors, separate analog and digital grounds, Faraday-shield traces, and controlling noise sources minimize noise problems, but these tricks don't eliminate noise. Common instrumentation-amplifier traps By Ron Mancini, 8/8/2002 Designing the instrumentation amplifier, with its few external components, appears to be a simple task, but to obtain a reliable design, you must avoid certain traps. You must correctly match the instrumentation amp and the transducer to obtain the optimum performance with reliable operation. High-output-impedance sensors, such as piezoelectric and pH electrodes, require high-input-impedance, l... Small signals demand excellent CMRR performance By Ron Mancini, 7/25/2002 Transducer-output signals can range from a few picovolts to a few volts, but these signals mostly range from microvolts to millivolts. These differential signals often have a dc-voltage content due to transducer bias, ground-voltage differences, or amplifier bias. If this scenario isn't bad enough, radiated noise from the cables, electromechanical devices, switching power supplies, or CRT-defle... Don't fall in love with one type of instrumentation amp By Ron Mancini, 5/30/2002 The instrumentation amps I coveredin my last column are difference amps made with internal resistors (Reference 1). Difference amps have high CMR errors when you use them with high-resistance sensors. Their input resistance is low—usually in the tens of kilohms—so any mismatch in sensor resistance causes a CMR error. Mutant op amp becomes instrumentation amp By Ron Mancini, 4/11/2002 Op amps are the fundamental building blocks of analog circuits; you can build all analog functions from op amps. Add digital output circuitry to an op amp to get a comparator that can also function as a 1-bit ADC. Add switched resistors or current sources to the op-amp front end to get a DAC. Add some diodes in the feedback loop, and the op amp functions as a log or antilog circuit; thus,... Examining switch-debounce circuits By Ron Mancini, 2/21/2002 When a switch closes, the contacts do not close instantly and finally. The contacts close, then bounce open, and this cycle repeats for a period, depending on the switch-manufacturing technology. The length of the bounce cycle is the "bounce time" (TB). You must eliminate switch bounce because each switch closure causes the output to change its voltage level, and the multiple switching se... Understanding linear regulators By Ron Mancini, 10/25/2001 Linear regulators convert unregulated dc voltage to regulated dc voltage. They are good vehicles to use to begin a voltage-regulator study because with them, you can apply the feedback knowledge that I have mentioned in previous columns (references 1 and 2). Linear regulators couple excellent regulation characteristics with excellent noise performance and simplicity of use, but their lo... |
Analog guru Ron Mancini wrote the Analog Angle column for EDN from 1999 until late 2005, during which time he served as staff scientist with Texas Instruments. |
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