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RC Op-Amp Circuits (6.4). Dr. Holbert April 10, 2006. 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.

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rc op amp circuits 6 4

RC Op-Amp Circuits (6.4)

Dr. Holbert

April 10, 2006

ECE201 Lect-18

digital meters and oscilloscopes
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.

ECE201 Lect-18

data acquisition systems
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.

ECE201 Lect-18

a generic digital meter
A Generic Digital Meter

Input Switching

and Ranging

A/D Converter

Amplifier

Display

Processor

ECE201 Lect-18

voltage measurements
Voltage Measurements

100V

10V

1V

Hi

Com

ECE201 Lect-18

model for meter

Ideal Meter

Hi

10MW

Com

Model for Meter

The ideal meter measures the voltage across its inputs. No current flows into it; it has infinite input resistance.

ECE201 Lect-18

meter loading

Ideal Meter

Hi

R

10MW

Com

Meter Loading

The 10MW meter resistance in parallel with R may change the voltage that you measure.

ECE201 Lect-18

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

ECE201 Lect-18

loading example
Loading Example
  • Without Meter: voltage is 100V
  • With Meter: measured voltage is 83.3V

Ideal Meter

Hi

50mA

2MW

10MW

Com

ECE201 Lect-18

current measurements
Current Measurements

100V

10V

1V

Com

Amp

ECE201 Lect-18

measuring large currents 100ma
Measuring Large Currents (> 100mA)
  • 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.

ECE201 Lect-18

meter loading1
Meter Loading

The Rs shunt resistance in series with R may change the current that you measure.

Ideal Meter

Amp

Rs

R

Com

ECE201 Lect-18

the voltage follower

+

The Voltage Follower

+

+

vin

vout

ECE201 Lect-18

without a voltage follower

RA/D

Without a Voltage Follower

Rs

vA/D is not equal to vs

+

A/D Converter

+

Sensor

vs

vA/D

ECE201 Lect-18

op amp review
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.

ECE201 Lect-18

op amp review1
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.

ECE201 Lect-18

with a voltage follower

RA/D

With a Voltage Follower

vA/D is equal to vs

+

+

+

Rs

vs

vA/D

Sensor

A/D Converter

ECE201 Lect-18

an integrator
An Integrator

C

R

+

+

+

Vin

Vout

ECE201 Lect-18

kcl at the inverting input
KCL at the Inverting Input

C

iC(t)

R

iR(t)

i-

+

+

+

vin(t)

vout(t)

ECE201 Lect-18

slide20
KCL

ECE201 Lect-18

solve for v out t
Solve for vout(t)

ECE201 Lect-18

slide22

Class Example

  • Learning Extension E6.9

ECE201 Lect-18

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