Analog Electronics In A Day Analog Electronic Design Resistor Equivalent Circuit Ideal Resistor R LL R/2 R/2 LL W 117 Inexperienced engineers assume that a resistor is just a resistor, and it really is a very complicated circuit. Li simulates the inductance of each lead. Cp is the capacitance across the resistor, thus it appears to be in parallel with the resistor Cp is about 0.5 pF for a 250 mW resistor Cg is formed by the resistor body and the ground plane, and it, like the rest of these stray effects, is really a distributed effect. Because it is small it appears as a capacitor connected to ground from the center of the resistor. Depending on the physical size of the resistor, Cp ranges from 0.01 pF on up to 0.5 pF The stray effects are reduced as the size of the resistor is reduced Surface mount resistors have the best high frequency performance primarily because of their small size
1-17 Analog Electronics In A Day Analog Electronic Design 1-17 Ideal Resistor R CG LL LL CP R/2 R/2 Inexperienced engineers assume that a resistor is just a resistor, and it really is a very complicated circuit. LL simulates the inductance of each lead. CP is the capacitance across the resistor, thus it appears to be in parallel with the resistor. CP is about 0.5 pF for a 250 mW resistor. CG is formed by the resistor body and the ground plane, and it, like the rest of these stray effects, is really a distributed effect. Because it is small it appears as a capacitor connected to ground from the center of the resistor. Depending on the physical size of the resistor, CP ranges from 0.01 pF on up to 0.5 pF. The stray effects are reduced as the size of the resistor is reduced. Surface mount resistors have the best high frequency performance primarily because of their small size
Analog Electronics In A Day Analog Electronic Design Potentiometers · Potentiometers are Variable resistors Used to adjust the voltage or current in the circuit · Potentiometers have All the problems associated with fixed resistors Severe drift problems caused by temperature and vibration Potentiometers or pots are used to adjust the voltage or current at some point in a circuit. When tolerances stack up or when the specifications fo a component can't be predicted accurately, pots are used to adjust out the tolerances thus obtaining the correct circuit parameter. The overuse of potentiometers is often a sign of poor design, but some equipment such as projection displays require many adjustments Pots have all of the bad problems associated with fixed resistors, and they exacerbate some of them and introduce new one. Pots are notorious for drifting under temperature or vibration stress. The connection from the resistive element to the lead is critical in fixed resistor design, and it is good that it isn,t considered a problem except with fractional ohm resistors. The wiper on a pot must slide across the resistive element, thus a good firm connection is impossible leading to a fractional ohm Systems that use large numbers of pots are converting to digital-to analog (DAC)converters. DACs come eight and twelve to a package, and their cost is equivalent to that of a pot. The adjustments are made through the keyboard or through a production test fixture. Smart systems self calibrate on start up thus eliminating the need for pots
1-18 Analog Electronics In A Day Analog Electronic Design 1-18 • Potentiometers are: – Variable resistors – Used to adjust the voltage or current in the circuit • Potentiometers have: – All the problems associated with fixed resistors – Severe drift problems caused by temperature and vibration Potentiometers or pots are used to adjust the voltage or current at some point in a circuit. When tolerances stack up or when the specifications for a component can’t be predicted accurately, pots are used to adjust out the tolerances thus obtaining the correct circuit parameter. The overuse of potentiometers is often a sign of poor design, but some equipment such as projection displays require many adjustments. Pots have all of the bad problems associated with fixed resistors, and they exacerbate some of them and introduce new one. Pots are notorious for drifting under temperature or vibration stress. The connection from the resistive element to the lead is critical in fixed resistor design, and it is so good that it isn’t considered a problem except with fractional ohm resistors. The wiper on a pot must slide across the resistive element, thus a good firm connection is impossible leading to a fractional ohm connection. Systems that use large numbers of pots are converting to digital-to analog (DAC) converters. DACs come eight and twelve to a package, and their cost is equivalent to that of a pot. The adjustments are made through the keyboard or through a production test fixture. Smart systems self calibrate on start up thus eliminating the need for pots
Analog Electronics In A Day Analog Electronic Design Potentiometer Applications -OOUT ● OUT INO -OOUT Used to set a reference voltage Used as a variable resistor 119 Pots are used in two major applications: voltage dividers for setting a reference and as variable resistors. The voltage divider application requires that the load be much higher in impedance than the pot to prevent loading of the pot by the load. This is a very popular application for pots, and the reference input voltage must be very stable because the circuit follows the reference. The reference voltage source should by well decoupled with a good grade capacitor to localize noise and keep it from spreading to other circuits Variable resistor applications can be very subtle, but the first thing to remember is that the pot has a limited current carrying capability. Do not connect the variable resistor configuration between the power supply and ground even if it connects through a semiconductor junction. When the variable resistor configuration is connected to ground in some manner series resistor must be inserted in the circuit to limit the current flow to a safe value
1-19 Analog Electronics In A Day Analog Electronic Design 1-19 Used to set a reference voltage Used as a variable resistor VOUT VREF IN OUT IN OUT Pots are used in two major applications: voltage dividers for setting a reference and as variable resistors. The voltage divider application requires that the load be much higher in impedance than the pot to prevent loading of the pot by the load. This is a very popular application for pots, and the reference input voltage must be very stable because the circuit follows the reference. The reference voltage source should by well decoupled with a good grade capacitor to localize noise and keep it from spreading to other circuits. Variable resistor applications can be very subtle, but the first thing to remember is that the pot has a limited current carrying capability. Do not connect the variable resistor configuration between the power supply and ground even if it connects through a semiconductor junction. When the variable resistor configuration is connected to ground in some manner, a series resistor must be inserted in the circuit to limit the current flow to a safe value
Analog Electronics In A Day Analog Electronic Design Capacitors The equation for the impedance of a capacitor is Xc= 1 /sc where s=jo Electrolytic capacitors are not suitable for high frequency applications Tantalum capacitors are suited for medium frequency applications Ceramic and mica capacitors are best for high frequency applications 1-20 The capacitor impedance is a function of frequency; at low frequencies the capacitor blocks signals, and at high frequencies the capacitor passes signals. Depending on the circuit connection, the capacitor can pass the signal to the next stage, or it can shunt it to ground All capacitors have a self-resonant frequency where they become ineffective as capacitors. Essentially, the capacitor goes to lunch at the self-resonant frequency. Aluminum electrolytic capacitors have a very low self-resonant frequency, so they are not effective in high frequency applications above a few hundred kHz. Tantalum capacitors have a mid range self-resonant frequency, thus they can be used up to several mhz Beyond several MHz ceramic and mica capacitors are the best choice because the have self-resonant frequencies ranging into the hundreds of MHZ. Beware; there are a lot of inexpensive ceramic capacitors on the market with poor high frequency performance Very low frequency and timing applications require another set of stable capacitors. The dielectric of these types of capacitor are made from polypropylene, polystyrene, and polyester. These capacitors have low leakage current, low dielectric absorption, and they come in large values
1-20 Analog Electronics In A Day Analog Electronic Design 1-20 • The equation for the impedance of a capacitor is XC=1/sC where s=jω • Electrolytic capacitors are not suitable for high frequency applications • Tantalum capacitors are suited for medium frequency applications • Ceramic and mica capacitors are best for high frequency applications The capacitor impedance is a function of frequency; at low frequencies the capacitor blocks signals, and at high frequencies the capacitor passes signals. Depending on the circuit connection, the capacitor can pass the signal to the next stage, or it can shunt it to ground. All capacitors have a self-resonant frequency where they become ineffective as capacitors. Essentially, the capacitor goes to lunch at the self-resonant frequency. Aluminum electrolytic capacitors have a very low self-resonant frequency, so they are not effective in high frequency applications above a few hundred kHz. Tantalum capacitors have a mid range self-resonant frequency, thus they can be used up to several MHz. Beyond several MHz ceramic and mica capacitors are the best choice because the have self-resonant frequencies ranging into the hundreds of MHz. Beware; there are a lot of inexpensive ceramic capacitors on the market with poor high frequency performance. Very low frequency and timing applications require another set of stable capacitors. The dielectric of these types of capacitor are made from polypropylene, polystyrene, and polyester. These capacitors have low leakage current, low dielectric absorption, and they come in large values
Analog Electronics In A Day Analog Electronic Design Capacitor Mod L models the lead and internal inductance of the capacitor. Except fo dielectrics such as ceramic and mica. the internal inductance is dominant at high frequencies. In high frequency capacitors the lead inductance can be approximated as 1/12 NH per foot. The combination of internal and lead inductance causes the capacitor to become self-resonant, and at frequencies above resonance the capacitor will appear to be an inductor High frequency applications demand capacitors with high self resonant frequencies and short leads which is why surface mount capacitors are used so often in high frequency circuits design The actual value of the capacitor is C. ESR stands for equivalent series resistance, and ESR is the effective resistance of the capacitor at the operating frequency. It is an important parameter when high currents are involved. Power supply filter design requires low ESr because voltage is dropped across the ESR, and the current flowing through the capacitor causes power dissipation resulting self heating. ESR is not an important parameter in the design of high frequency or signal processing circuits thus is only specified for aluminum electrolytic and tantalum capacitors The parallel resistance of a capacitor is modeled by Rp. This resistance is a function of the operating voltage and capacitor temperature; hence, it drifts quite a bit. The electrolytic capacitors exhibit the lowest parallel resistance, and aluminum electrolytic capacitors are often modeled with a parallel current source in place of Rp. Other types of capacitors have a relatively high Rp ranging in the hundreds of meg ohms
1-21 Analog Electronics In A Day Analog Electronic Design 1-21 L RP C ESR L models the lead and internal inductance of the capacitor. Except for dielectrics such as ceramic and mica, the internal inductance is dominant at high frequencies. In high frequency capacitors the lead inductance can be approximated as 1/12 NH per foot. The combination of internal and lead inductance causes the capacitor to become self-resonant, and at frequencies above resonance the capacitor will appear to be an inductor. High frequency applications demand capacitors with high self resonant frequencies and short leads which is why surface mount capacitors are used so often in high frequency circuits design. The actual value of the capacitor is C. ESR stands for equivalent series resistance, and ESR is the effective resistance of the capacitor at the operating frequency. It is an important parameter when high currents are involved. Power supply filter design requires low ESR because voltage is dropped across the ESR, and the current flowing through the capacitor causes power dissipation resulting self heating. ESR is not an important parameter in the design of high frequency or signal processing circuits, thus is only specified for aluminum electrolytic and tantalum capacitors. The parallel resistance of a capacitor is modeled by RP. This resistance is a function of the operating voltage and capacitor temperature; hence, it drifts quite a bit. The electrolytic capacitors exhibit the lowest parallel resistance, and aluminum electrolytic capacitors are often modeled with a parallel current source in place of RP. Other types of capacitors have a relatively high RP ranging in the hundreds of meg ohms