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 switch is made from a JFET, which does very well.

Secondly, there is a finite jump in the output voltage called the hold step when the hold command is issued. To sample, the gate is connected to the drain or the sourceand to hold, the gate is connected to -V. If you plot the output voltage versus time, you will find a straight line with a slope of For the LF, this is about 0.

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The capacitor voltage changes as the dielectric “relaxes,” as well as when charge is supplied or taken away. Finally, there is droop as the hold capacitor voltage declines steadily in the “hold” state. The leakage current is found to be 30 pA. When the gate is connected to -V, the output will freeze, and you will note that it droops slowly but steadily.

The smaller the hold capacitor, the more quickly it can be charged and the smaller the acquisition time. The voltage kept on decreasing, until it reached some internal saturation value at I found that it took 28 minutes to decline to 0.


Measure the time required for it to fall by 1 V. The acquisition time depends on the size of the hold capacitor.

For the LF, with a 0. When the control is changed to “hold,” below 1. The circuit is basically two unity-gain buffers, with a hold capacitor between them, and lf938 switch to disconnect the input. I found about 5 minutes, so the droop rate is 3. The next most important characteristic is “dielectric absorption” or hysteresis in the dielectric constant. In this page, the principle of a sample-and-hold circuit is explained and illustrated, and the practical use of the LF monolithic sample-and-hold circuit is described.

Apply an input voltage with a potentiometer, and watch the output voltage track it while the gate is connected to the drain.

Calvert Created 29 July Last revised 30 July This gain error is less than 0. There is a settling time after the hold command until the output is within 1 mV of its steady value. This requires the opposed diodes that “catch” the output of the first op-amp when the feedback loop is broken in the “hold” state. This corresponds to a leakage current of only 33 pA, an excellent result. The acquisition time is the time for the internal nodes to settle, and the output to be within, say, 0.

The Of398 is connected as shown at the left. Times from the hold command are measured from the 1. We also measure the leakage currents that exist in these circuits. For the LF, it is ns. One change is that the feedback loop is llf398 from input to output. After the hold command, the aperture time is the time after which changes of the input voltage no longer affect the output voltage. Even if the times are taken into account, the accuracy of the output depends on several more parameters.


The transition voltage is 1. This is not a very convenient way to control the JFET, but it works for a demonstration. There is, therefore, a tradeoff in selection of hold capacitor size. If you need better specs, there are the more expensive LF and Lf938 which mainly give an extended temperature rangeand the LFA, with tightened specs.

These diodes then require the 30k resistor to avoid overloading the output amplifier. This droop is caused mainly by a constant leakage lf3398, and can be predicted fairly well, so that corrections can be made for it if desired. In the test, I used my debounced pushbutton for the logic signal, choosing the normally-low output. LS or HC logic will do very well.

It is obvious that the capacitor should have small leakage, so all electrolytics, whether aluminum or tantalum, are excluded. The LF is, however, an excellent circuit suitable for most peaceful requirements. The rise rate of the logic control should of398 greater than 1. This demonstrates that the droop is caused by a constant leakage current, and is not an exponential RC decay.

Buffer a slow signal with an LS The control logic lf938 is applied to a differential amplifier to allow interfacing with various logic families. Note that we did not do much worse than this with our discrete circuit. Bipolar op-amps are not suitable, because the input base currents are too large.