A/D Conversion and Interfacing

Physics 270

Voltmeters

Analog Voltmeter

Digital Voltmeter

• Signal Scaler (attenuator amplifier)

• Digital conditioner (reduces scaled input signal to a DC voltage within range of A/D converter)

• A/D Converter• Control Logic

Analog versus Digital

• Analog is continuous.• Digital is discrete.

• A/D = analog to digital• D/A = digital to analog

• Digital… base 2 system• Analog… base 10 system

Digital Representation

Digital Representation

Digital Representation

How many bits would you need to divide 10 V into 0.01 V intervals?• If you divide 10 V into 0.01 V intervals you need 1000

intervals.• If you need 1000 intervals you need to think about a

power of 2 that is larger than 1000.• The smallest power of 2 that is larger than 1000 is 210

which is equal to 1024.• That means that you need 10 bits in the converter, and

then the count in the counter/register will run from 0 to 1023.

• And that leads us to observe that real converters often go to 10.23 V, not 10V, because that gives perfect 0.01 V increments between resolvable voltages.• And another converter might run from -5.12v to

+5.11v for the same reason.

Digital Representation

Digital Representation

Quantization Error

Quantization Error

Offset Error

Gain Error

Other sources of errorCome from non-linearity, Missing bits, etc.

Noise

Voltage-Measurement Error

Current-Measurement Error

To minimize the percentage error, an ammeter's input resistance should be less than the Thevenin resistance of the original circuit by 20 times or more. Conversely, a voltmeter should have an input resistance that is larger than the Thevenin resistance of the original circuit by more than 20 times. The same goes for the ohmmeter; it should have an input resistance that is at least 20 times the Thevenin resistance of the original circuit. By following these simple rules, it is possible to reduce the error to below 5 per cent.

LabView

• Number One Data Acquisition and Control Environment – used everywhere

• Learning Curve• Labview Introduction Video

– http://zone.ni.com/wv/app/doc/p/id/wv-411/nextonly/y