This article describes how to measure values of a variable resistor or a variable capacitor using a microcontroller to measure the RC charge time to a constant threshold voltage. A variety of these "one-shot" circuits are described and the accuracy of the various methods is measured. Sample programs for the 87C51 family are presented at the end of the article.

Measure resistance and capacitance without an A/D

The old "one-shot" monostable multivibrator circuit has been around since the discovery of the electron. Popular IC versions of this variable pulse width generator include the 74123 and the famous (Philips) Signetics NE555 timer. These circuits use the time to charge a capacitor to a predefined voltage as their time delay.

If the threshold voltage is 1-1/e (e is the natural log base), then the time delay in seconds is exactly the product of the resistance in ohms and the capacitance in Farads (t=RC). This is a linear equation and it can be used to measure an unknown value of R if C and t are known. The purpose of this article is to describe ways to measure a RESISTANCE by using this technique with an 80C51 type of microcontroller.

by Don Sherman AN449 - NXP Philips Application Notes

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## Saturday, November 12, 2016

### Active Filter Design with OpAmps

This is yet another Application Note of Exar found in- "Applications Data Book" June 1981. The Seven Scanned Pages as attachments can be viewed and studied at …

The availability of low cost dual or quad operational amplifier IC's have made the operational amplifier based active filter techniques cost effective over conventional passive filters. The recent availability of programmable quad operational amplifiers such as the XR-4202 or the XR-346 have provided the active filter designer with the flexibility to externally program gain-bandwidth product, supply current, input bias current, input offset current, input noise and the slew rate.

Application Note - AN-03 - Active Filter Design with IC Op-Amps

This application note is intended to familiarize the filter designer with the fundamentals of active filter design, using monolithic IC op amps. It presents a table of transfer functions and network equations for high-pass, low-pass, band-pass and band-reject filters. Several design examples are given to illustrate the respective merits and limitations of various filter configurations. Particular emphasis is given to applications of programmable quad operational amplifiers, such as the XR-4202, as an active filter element in FSK Modems.

The availability of low cost dual or quad operational amplifier IC's have made the operational amplifier based active filter techniques cost effective over conventional passive filters. The recent availability of programmable quad operational amplifiers such as the XR-4202 or the XR-346 have provided the active filter designer with the flexibility to externally program gain-bandwidth product, supply current, input bias current, input offset current, input noise and the slew rate.

Application Note - AN-03 - Active Filter Design with IC Op-Amps

This application note is intended to familiarize the filter designer with the fundamentals of active filter design, using monolithic IC op amps. It presents a table of transfer functions and network equations for high-pass, low-pass, band-pass and band-reject filters. Several design examples are given to illustrate the respective merits and limitations of various filter configurations. Particular emphasis is given to applications of programmable quad operational amplifiers, such as the XR-4202, as an active filter element in FSK Modems.

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