The tachom- eter uses a charge pump technique and offers frequency doubling for low ripple, full input protection in two versions LM, LM and its output swings to ground for a zero frequency input. This feature allows either a ground or supply referred load of up to 50 mA. The two basic configurations offered include an 8-pin device with a ground referenced tachometer input and an internal connection between the tachometer output and the op amp non-inverting input. This version is well suited for single speed or frequency switching or fully buffered frequency to voltage conversion applications. The more versatile configurations provide differential ta- chometer input and uncommitted op amp inputs. With this version the tachometer input may be floated and the op amp becomes suitable for active filter conditioning of the tachom- eter output.
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The tachometer uses a charge pump technique and offers frequency doubling for low ripple, full input protection in two versions LM, LM and its output swings to ground for a zero frequency input. This feature allows either a ground or supply referred load of up to 50 mA. The two basic configurations offered include an 8-pin device with a ground referenced tachometer input and an internal connection between the tachometer output and the op amp non-inverting input.
This version is well suited for single speed or frequency switching or fully buffered frequency to voltage conversion applications. The more versatile configurations provide differential tachometer input and uncommitted op amp inputs. With this version the tachometer input may be floated and the op amp becomes suitable for active filter conditioning of the tachometer output. Both of these configurations are available with an active shunt regulator connected across the power leads.
The regulator clamps the supply such that stable frequency to voltage and frequency to current operations are possible with any supply voltage and a suitable resistor. See test circuit. Note 4: Be sure when choosing the time constant R1 x C1 that R1 is such that the maximum anticipated output voltage at pin 3 can be reached with I3 x R1.
The maximum value for R1 is limited by the output resistance of pin 3 which is greater than 10 M typically. The first stage of operation is a differential amplifier driving a positive feedback flip-flop circuit. The input threshold voltage is the amount of differential input voltage at which the output of this stage changes state.
Two options LM, LM have one input internally grounded so that an input signal must swing above and below ground and exceed the input thresholds to produce an output. This is offered specifically for magnetic variable reluctance pickups which typically provide a single-ended ac output. The differential input options LM, LM give the user the option of setting his own input switching level and still have the hysteresis around that level for excellent noise rejection in any application.
Of course in order to allow the inputs to attain common-mode voltages above ground, input protection is removed and neither input should be taken outside the limits of the supply voltage being used.
It is very important that an input not go below ground without some resistance in its lead to limit the current that will then flow in the epi-substrate diode. Following the input stage is the charge pump where the input frequency is converted to a dc voltage. To do this requires one timing capacitor, one output resistor, and an integrating or filter capacitor. The timing capacitor also provides internal compensation for the charge pump and should be kept larger than pF for very accurate operation.
Smaller values can cause an error current on R1, especially at low temperatures. Several considerations must be met when choosing R1. If R1 is too large, it can become a significant fraction of the output impedance at pin 3 which degrades linearity. Also output ripple voltage must be considered and the size of C2 is affected by R1.
An expression that describes the ripple content on pin 3 for a single R1C2 combination is: It appears R1 can be chosen independent of ripple, however response time, or the time it takes VOUT to stabilize at a new voltage increases as the size of C2 increases, so a compromise between ripple, response time, and linearity must be chosen carefully. The size of C2 is dependent only on the amount of ripple voltage allowable and the required response time. The most important consideration in choosing a dropping resistor from the unregulated supply to the device is that the tachometer and op amp circuitry alone require about 3 mA at the voltage level provided by the zener.
At low supply voltages there must be some current flowing in the resistor above the 3 mA circuit current to operate the regulator. As an example, if the raw supply varies from 9V to 16V, a resistance of will minimize the zener voltage variation to mV. If the resistance goes under or over the zener variation quickly rises above mV for the same input variation. Flash rate increases with input frequency increase beyond trip point. As used herein: 1.
Life support devices or systems are devices or systems which, a are intended for surgical implant into the body, or b support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user.
A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness.
LTSpice models for LM2907, LM2917 attached
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