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RC Op-Amp Circuits (6.4)PowerPoint Presentation

RC Op-Amp Circuits (6.4)

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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

- 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

Input Switching

and Ranging

A/D Converter

Amplifier

Display

Processor

ECE201 Lect-18

Hi

10MW

Com

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

ECE201 Lect-18

Hi

R

10MW

Com

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

ECE201 Lect-18

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

- Without Meter: voltage is 100V
- With Meter: measured voltage is 83.3V

Ideal Meter

Hi

50mA

2MW

10MW

Com

ECE201 Lect-18

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 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

RA/D

Without a Voltage FollowerRs

vA/D is not equal to vs

+

A/D Converter

+

–

Sensor

vs

vA/D

–

ECE201 Lect-18

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 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

RA/D

With a Voltage FollowervA/D is equal to vs

+

+

+

–

–

Rs

vs

vA/D

–

Sensor

A/D Converter

ECE201 Lect-18

KCL

ECE201 Lect-18

Solve for vout(t)

ECE201 Lect-18

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