Texas Instruments LFN Series Sample & Hold Amplifiers are available at Mouser Electronics. Mouser offers inventory, pricing, & datasheets for Texas. lf Sample & Hold Amplifiers are available at Mouser Electronics. Mouser offers inventory, pricing, & datasheets for lf Sample & Hold Amplifiers. Understand the working of LF IC (sample-and-hold circuit). • Describe the concept of sampling a time varying signal. • Obtain the sampled and hold otuput.

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The LF is, however, an excellent circuit suitable lff398 most peaceful requirements. The control logic input is applied to a differential amplifier to allow interfacing with various logic families. For a hold capacitor of 0.

Record the time at the start, and at every even volt as the output voltage droops. If you need better specs, there are the more expensive LF and LF which mainly give an extended temperature rangeand the LFA, with tightened specs.

For most normal uses, a value of 0. After the hold command, the aperture time is the time after which changes of the input voltage no longer affect the output voltage. It is obvious that the capacitor should fl398 small leakage, so all electrolytics, whether aluminum or tantalum, are excluded. We also measure the leakage currents that exist in these circuits.

When the control is changed to “hold,” below 1. The type of capacitor used is important.


Selection of the hold capacitor is an important matter. The next most important characteristic is “dielectric absorption” or hysteresis in the dielectric constant. I found that it lc398 28 minutes to decline to 0. Polypropylene has the least hysteresis of all, and is the best choice for a hold capacitor, but any of the three will give adequate results.

There is, therefore, a tradeoff in selection of hold capacitor size. Secondly, there is a finite jump in the output voltage called the hold step when the hold command is issued. The acquisition time is the time for the internal nodes to settle, and the output to be within, say, 0. The rise rate of the logic control should be greater than 1. Bipolar op-amps are not suitable, because the input base currents are too large.

If you plot the output voltage versus time, you will find a straight line with a slope of The acquisition time depends on the size of the hold capacitor. This corresponds to a leakage current of only 33 pA, an excellent result. What happens is this: The voltage kept on decreasing, until it reached some internal saturation value at For the LF, with a 0.

These diodes then require the 30k resistor to avoid overloading the output amplifier. This is not a very convenient way to control the JFET, but it works for a demonstration.

The similarity to our test circuit is obvious. Buffer a slow signal with an LS The LF is connected as shown at the left. For us, we want to refer the logic to ground by connecting pin 7 to ground, and apply a positive input to pin 8 for the sample state. This droop is caused mainly by a constant leakage current, and can be predicted fairly well, so that corrections can be made for it if desired.


Measure the time required for it to fall by 1 V.

Apply an input voltage with a potentiometer, and watch the output voltage track it while the gate is connected to the drain. The specifications of the LF allow it to be up to pA.

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I found about 5 minutes, so the droop rate is 3. For the LF, it is lff398. Times from the hold command are measured from the 1. The circuit is basically two unity-gain buffers, with a hold capacitor between them, and a switch to disconnect the input.

The capacitor voltage changes as the dielectric “relaxes,” as well as when charge is supplied or taken away. A more practical discrete sample-and-hold circuit is given in Signal Switching. In fact, if the input ld398 to be digitized is varying, a sample-and-hold circuit is mandatory. LS or HC logic will do very well. Finally, there is droop as the hold capacitor voltage declines steadily in the “hold” state.

The smaller the hold capacitor, the more quickly it can be charged and the smaller the acquisition time.