RC OpAmp Circuits 6 4 Dr Holbert April

RC Op-Amp Circuits (6. 4) Dr. Holbert April 10, 2006 ECE 201 Lect-18 1

Digital Meters and Oscilloscopes • Most multimeters and oscilloscopes are now digital. • A digital multimeter or a digital oscilloscope has an analog-to-digital (A/D) converter. • Most digital meters and all digital oscilloscopes have one or more processors. ECE 201 Lect-18 2

Data Acquisition Systems • In many applications, digital meters and scopes are being replaced by data acquisition cards that fit into a computer. • The data acquisition cards have A/D converters. • The computer provides processing and storage for the data. ECE 201 Lect-18 3

A Generic Digital Meter Input Switching and Ranging A/D Converter Amplifier Display Processor ECE 201 Lect-18 4

Voltage Measurements 100 V 1 V Hi ECE 201 Lect-18 Com 5

Model for Meter Hi 10 MW Ideal Meter Com The ideal meter measures the voltage across its inputs. No current flows into it; it has infinite input resistance. ECE 201 Lect-18 6

Meter Loading Hi R 10 MW Ideal Meter Com The 10 MW meter resistance in parallel with R may change the voltage that you measure. ECE 201 Lect-18 7

Loading • When measuring the voltage across R, we need to make sure that R is much less than 10 MW. • If R is close to 10 MW, significant current flows through the meter, changing the voltage across R. ECE 201 Lect-18 8

Loading Example Hi 50 m. A 2 MW 10 MW Ideal Meter Com • Without Meter: voltage is 100 V • With Meter: measured voltage is 83. 3 V ECE 201 Lect-18 9

Current Measurements 100 V 1 V Com Amp ECE 201 Lect-18 10

Measuring Large Currents (> 100 m. A) • The current to be measured is passed through a small resistor (called a shunt resistor) and the resulting voltage across the shunt resistor is measured. • From the voltage, the current can be computed. ECE 201 Lect-18 11

Meter Loading Amp R Com Rs Ideal Meter The Rs shunt resistance in series with R may change the current that you measure. ECE 201 Lect-18 12

The Voltage Follower + – vin + – + vout – ECE 201 Lect-18 13

Without a Voltage Follower Rs Sensor vs + – + v. A/D RA/D Converter – v. A/D is not equal to vs ECE 201 Lect-18 14

Op-Amp Review • The ideal op-amp model leads to the following conditions: i+ = i- = 0 v+ = v • The op amp will set the output voltage to whatever value results in the same voltages at the inputs. ECE 201 Lect-18 15

Op-Amp Review • To solve an op-amp circuit, we usually apply KCL (nodal analysis) at one or both of the inputs. • We then invoke the consequences of the ideal model. • We solve for the op-amp output voltage. ECE 201 Lect-18 16