Fundamentals of Digital Electronics - Lab 8 potx

However, when the test level becomes greater than—or in this case, equal to—the input level, the comparator will change sign and stop.. LabVIEW VI to Simulate an 8-Bit Ramp ADC The last

Analog-to-Digital Converters,

Part I

The analog-to-digital converter, known as the A/D converter (read as A-to-D converter) or the ADC, is the second key component to bridging the analog and digital worlds The ADC is the basis of digital voltmeters, digital multimeters, multichannel analyzers, oscilloscopes, and many other instruments There are many different ADC designs, of which the ramp, tracking, and successive approximation converters are common This lab looks at the ramp and tracking A/D converters

Purpose of the Analog-to-Digital Converter

The purpose of an ADC converter is to produce a digital binary number that

is proportional to an analog input signal The fundamental conversion process is shown in the following diagram

Figure 8-1 Symbolic Design for an 8-Bit Analog-to-Digital Converter

A counter creates a test binary sequence, and its digital output is converted into an analog voltage using a digital-to-analog converter The DAC is a basic element of many ADC circuits and was discussed in Lab 7 (This is a

Input Voltage

Test Voltage

+ – C

b7 b0

Reset Counter

DAC

process continues until the comparator changes sign, at which time the test level will be within one count of the input level Increasing the number of bits of the counter and DAC increases the conversion resolution

The Ramp ADC

The ramp ADC uses a binary counter and digital-to-analog converter to generate a ramp test waveform In this demonstration, an 8-bit binary up

counter, Binary Counter.vi, together with the 8-bit DAC, DAC.vi

(introduced in the last lab), generate the test waveform The test level will rise from 0 to 255 and repeat if left in the free running mode However, when the test level becomes greater than—or in this case, equal to—the input level, the comparator will change sign and stop

Figure 8-2 LabVIEW VI to Simulate an 8-Bit Ramp ADC

The last value on the binary bits (b7-b0) is the digitized value of the input voltage level In the LabVIEW simulation, a wait time of 60 ms is chosen so that the eye can follow the action The comparator function is simulated with

the LabVIEW Equal function.

Load and run the simulation VI Ramp.vi and follow the action on the front

panel Try other values of the input level and note that the conversion time depends on the input voltage level

Figure 8-3 LabVIEW Front Panel of 8-Bit ADC Converter The Comparator LED Indicator Changes State When the Test Waveform Numeric Value Exceeds the Voltage Input

In the next simulation, Ramp4.vi, the binary counter is allowed to free run

Whenever the test signal is greater than the input level, the comparator changes sign This intersection of the ramp waveform with the input level can be seen on a chart display The binary value of the counter at the intersection point is the digitized signal The transition of the comparator indicates this event

If the changing state of the comparator resets the binary counter, a true ramp ADC is simulated In this case, the binary counter is replaced with the binary

counter with reset, featured in Lab 6 Load the VI Ramp2.vi and observe

the action Note that as soon as the test level reaches the input level, the binary counter resets, and the ramp cycle starts all over again In the display below, the input level was changed three times

Figure 8-4 Chart Display of the Ramp ADC in Operation

An interesting feature, unique to the ramp ADC, is that the conversion time depends on the magnitude of the input signal Small input levels are digitized faster than large input levels The conversion time is thus

The ramp ADC works equally well with a down counter that runs from 255-0 The change of state of the comparator again signals the binary count that generates a test level equal to the input level

LabVIEW Challenge

Design a ramp ADC that uses a down counter to generate the test waveform

Could you use an up/down counter to track the input level?

Yes, such a conversion technique is called a tracking ADC, and it has the fastest conversion time

Tracking ADC

The first task for the tracking ADC is to use some technique such as a ramp waveform to catch up to the input level At that point, shown by the intersection of the ramp waveform with the input level, the tracking algorithm takes over

Figure 8-5 Tracking ADC Ramps Up to the Input Level Before Tracking Begins

The tracking algorithm is simply, if

test level is greater than the signal level, decrease the count by one else if

test level is less than the signal level, increase the count by one

and repeat forever

In the following example, a positive ramp ADC technique is used to initially catch up to the input level of 150.2 Once the input level is reached, the tracking algorithm takes over

By expanding the vertical scale, you can see the tracking algorithm in action

Figure 8-6 Tracking ADC Output when Input is Constant

However, if the input level changes, the ADC must revert to a ramp waveform to catch up to the input level Provided the clock is fast enough, the tracking can keep pace But if the signal changes too quickly, the digitized signal is lost until the test level catches up again In practice, it is the slewing speed of the DAC that limits the maximum input frequency that the tracking ADC can follow

Figure 8-7 A Sudden Change in the Input Level Causes the Test Level to Ramp Up to the New Level

Because the tracking ADC uses an up/down counter, the algorithm has the same problem when the input signal suddenly falls below the test level The tracking ADC reverts to a down ramp (Figure 8-8) until the test level reaches the input signal level

Figure 8-8 A Negative Change in the Input Level Causes the Test Level to Ramp Down to the New Level

The VI called Tracking ADC.vi is used to demonstrate this technique and

to generate all the above charts The algorithm shown on the block diagram

is quite simple A LabVIEW Select function and the shift register on the

While Loop implements the algorithm

Figure 8-9 LabVIEW VI for the Tracking ADC

The Wait function is set to 0.10 second so that the user can observe the

action on the front panel You can also use the Operating tool to redefine the vertical axis scale to zoom in on the action as the simulation is in progress

To observe the tracker catching up to a varying input, reduce the input

constant for the Wait function in Figure 8-9 to 1 ms.

Lab 8 Library VIs (Listed in the Order Presented)

• Ramp.vi (8-bit ramp ADC, conversion slowed for easy viewing)

• Ramp4.vi (ramp ADC with no feedback from comparator)

• Ramp2.vi (8-bit ramp ADC with chart output)

• Tracking ADC1.vi

• Binary Counter.vi (subVI 8-bit binary counter)

• BIN_RST.vi (subVI 8-bit binary counter with external reset)

• DAC.vi (subVI 8-bit DAC)