ECE3161 - [L1] Simple Op Amps and Scopy PDF

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Lab 1 Simple Op Amps and Scopy 1 Learning Objectives and Introduction The operational amplifier (op-amp) is a key component for manipulating analogue signals, as it can perform mathematical ‘operations’ on signals, such as multiplication, addition and subtraction using only a few extra resistors. It is also very easy to design with, as its high voltage and current gains allow many approximations, so that voltage gain (multiplication) can be set by the ratio of two external resistors. The op-amp will be used throughout the ECE3161 labs, but first you need some basic knowledge. This will also introduce you to the ADALM2000 instrumentation and Scopy software. In this lab, we introduce the operational amplifier (op amp), an active circuit that is designed with certain characteristics (high input resistance, low output resistance, and a large differential voltage gain) that make it a nearly ideal amplifier and useful building-block in many circuit applications. In this lab, you will learn about DC biasing for active circuits and explore a few of the basic functional op-amp circuits. We will also use this lab to continue developing skills with the lab hardware.

2 Components   

 

ADALM2000 Active Learning Module Solder-less breadboard, and jumper wire Resistors (We provide 5-band resistors in the lab: the breadboard pictures use 4-band – look up their ‘color codes’ to understand what the mean. Hint: 0 1 2 3 4 5 6 7 8 9) o o

2-off 2-off

1 kΩ 4.7 kΩ

100 × 101 470 × 101

o

2-off

10 kΩ

100 × 102

1% tolerance

o 1-off 47 kΩ 470 × 102 OP97 amplifier (a precision, if not slow, op-amp from Analog Devices). Note, the LTspice model is the LT1097, which is almost identical). 2-off 0.1uF Capacitors (to stabilise the power supplies – recommended for all circuits)

All components must be kept by you, for future use (including the miniproject).

3 ADALM2000 and Scopy The ADALM2000 Active Learning Module is an affordable USB-powered waveform-source and measurement unit. With 12-bit ADCs and DACs running at 100 MSPS, the ADALM2000 brings the power of high-performance lab equipment to the palm of your hand, enabling electrical

engineering students and hobbyists to explore signals and systems into the tens of MHz without the cost and bulk associated with traditional lab gear. The ADALM2000, when coupled with Analog Devices' Scopy™ graphical application software running on a computer, provides the user with the following high-performance instrumentation:           

Two-channel oscilloscope with differential inputs Two-channel arbitrary function generator 16-channel digital logic analyser (3.3V CMOS and 1.8V or 5V tolerant, 100MS/s) 16-channel pattern generator (3.3V CMOS, 100MS/s) 16-channel virtual digital I/O Two input/output digital trigger signals for linking multiple instruments (3.3V CMOS) Two-channel voltmeter (AC, DC, ±20V) Network analyser – Bode, Nyquist, Nichols transfer diagrams of a circuit. Range: 1Hz to 10MHz (Note, the plots are very slow when set to 1 Hz min. >100 Hz recommended.) Spectrum Analyser – power spectrum and spectral measurements (noise floor, SFDR, SNR, THD, etc.) Digital Bus Analysers (SPI, I²C, UART, Parallel) Two programmable power supplies (0…+5V , 0…-5V). 50 mA max.

ADALM2000 users normally interact with real-world analogue signals using Scopy. Scopy is a multi-functional software toolset with strong capabilities for signal analysis. Now you can run Scopy from the “Start Menu” on your computer. Click on the following Hyperlinks to complete the user guides on how to use each Scopy instrument: 

What is a solder-less Breadboard?



Getting Start (How to connect ADALM2000 to Scopy)



Scopy Power Supply



Scopy Signal Generator



Scopy Oscilloscope

4 Experimental Work Lab Report: Please answer the lab questions in the answer box in this lab sheet (you can do this electronically). Grab Screenshots of necessary graphs from Scopy and paste them into a Word file. Show your answered lab sheet and Word file to your demonstrators after completing all lab questions. If you are unsure about what is expected, ask lab demonstrators who will grade your report.

4.1

OP-AMP BASICS

4.1.1

First Step: Connecting DC Power

Op-amps must always be supplied with DC power (usually stable voltage sources) and therefore it is best to configure these connections first before adding any other circuit components. Figure 1 shows one possible power arrangement on your solder-less breadboard. We use two of the long

rails for the positive and negative supply voltages, and two others for any ground connections that may be required. It is good practice (we will award marks, and you will make fewer mistakes) for using:   

red wire for any connections to the positive supply, blue wire for anything connected to the negative supply (this is the white wire from ADALM2000) black wire for anything connected to zero volts (ground return or mid-rail of power supply)

Other colours can be use for signal carrying wires. Fig 1 shows the so-called “supply de-coupling” capacitors (0.1 uF) connected between the powersupply and ground rails. These are used to reduce noise (fluctuations) on the supply lines and avoid parasitic oscillations in circuits. They supply current to the op-amp, when needed, without hesitation. It is good practice in any circuit design to always include bypass capacitors very close to the supply pins of each op-amp in your circuit and with short wires to the ground (GND) rail, otherwise stray inductance will cause voltage fluctuations in response to changes in the current demanded by the op-amp. Fig. 1 is not ideal in this respect, and would not work at very high frequencies. A black wire connecting the upper and lower GND rails would help somewhat.

Figure 1: Power Connections. Note, we are providing double-ended header strips in the lab. These interface between the SCOPY female leads and the female breadboard sockets. Some rearrangement of the circuit will make it possible to use a line of header pins. A line of 5 pins make a useful set of ‘ground’ connections. Note, we are providing double-ended header strips in the lab. These interface between the SCOPY female leads and the female breadboard sockets. Some rearrangement of the circuit will make it possible to use a line of header pins.

Figure 2: ADALM2000 Hardware Pinout and wire colours. Only 10 pins are used in these labs. Insert the op-amp into your breadboard and add the wires and supply capacitors as shown in Figure 1. To avoid problems later you may want to attach a small label to the breadboard to indicate which rails correspond to Vp, Vn, and ground. It’s a good idea to use the upper rails for positive, and the lower rails for ground and negative. Also, in circuit diagrams and prototype layouts, signal inputs are on the left and outputs on the right. That is, signals pass from left to right. Next, attach the supply and GND connections from the ADALM2000 board to the terminals on your breadboard. Figure 2 shows the pinout of ADALM2000. Use jumper wires to power the rails as shown in Figure 1. Remember, the power-supply GND terminal will be our circuit “ground” (zero volts) reference for all the measurements. Once you have your supply connections, and have ENABLED and set the supplies from the Scopy interface, You can use a multimeter to probe the IC pins directly to ensure that Pin 7 is at +5V and Pin 4 is at -5V.

4.1.2

Unity-Gain Amplifier (Voltage Follower)

Background: Our first op-amp circuit is a simple one, shown in Figure 3. This is called a unity-gain buffer, or sometimes a voltage follower, defined by the transfer function Vout = Vin. At first glance it may seem like a useless device, but as we will show later, it finds use, because of its high input resistance and low output resistance. That is, it takes very little current from the input (sometimes picoamps!), but can supply a reasonable current (usually a few tens of mA) to the output. Note that, generally in linear circuits, op-amps aim to keep their input pins at the same voltage, by altering their output voltage, which is usually connected to the inverting (-ve signal) input via a feedback path. This is a good way to start analysing the function of a circuit.

Figure 3: Unity Gain Follower schematic circuit. Hardware Setup: Using your breadboard and the ADALM2000 power supplies, construct the circuit shown in Figure 3. Note that the power connections are not shown here; it is assumed that those connections must be made in any real circuit (as you did in the previous step), so it is unnecessary to show them in the schematics from this point on. Use jumper wires to connect input and output to the waveform generator and oscilloscope leads. Don’t forget to ground the scope negative input leads Ch1- and Ch2- (ground connections are not shown in the schematic), so they act as 0V references. Procedure: Use the first waveform generator as source Vin to provide a 1V amplitude (peak-to-peak), 1 kHz and 0V-offset sinewave excitation to the circuit. Configure the scope so that the input signal is displayed on Channel 1 and the output signal is displayed on Channel 2. Report: (1 mark) Export a plot of the two resulting waveforms and include it your lab report, noting the parameters of the waveforms (amplitude values and the fundamental time-period or frequency). This can be done by the “Measure” control located in the lower right section of the Scopy display. Your waveforms should confirm the description of this as a “unity-gain” or “voltage follower” circuit. 4.1.3

Slew Rate Limitations

For an ideal op-amp the output will follow the input signal precisely for any input signals, but in a real amplifier, the output signal can never respond instantaneously to the input signal. Thus, the designed function of the op-amp will be invalid during ‘slewing’ transitions, which could cause nasty errors. In music systems, parts of the waveform are missed during slewing. This non-ideality can be observed when the input signal is a rapidly changing function of time. For large-amplitude signals, this limitation is quantified by the slew rate, which is the maximum rateof-change (slope) of the output voltage that the op-amp is capable of delivering. The units of slewrate are usually expressed as V/μs.

Figure 4: Slew Rate definitions for a large-signal output of an op-amp. (The op-amp has a square-wave input signal). Procedure: Set the signal generator to a square-wave signal with a 2V amplitude (peak-to-peak) with 0V offset and increase the frequency until you see a significant departure from ideal behaviour, that is, when the output starts looking more like a trapezoid than a square wave. You will likely need to adjust the time scale (Sec/Div) on the scope display to see this. Report: (1 mark) Export a plot of the output waveforms at this point and measure its 10-90% rise time (and 90-10% fall time) as defined in Figure 4. Also note the peak-to-peak voltage of the output signal. Compute and record the slew rate for both rising and falling outputs according to your measurements. Comment on why the response to rising and falling edges might be different.

4.2

SIMPLE AMPLIFIER CONFIGURATIONS

4.2.1

Inverting Amplifier

Background: Figure 4 shows the conventional inverting amplifier configuration with a 10 kΩ “load” resistor at the output.

Note – the resistors in the lab are colour-coded with 5 bands. Find the colour codes on the web. If in doubt, use a multimeter to check the value of the resistor. Beware the resistance of you body in parallel with the resistor!

Figure 4: Inverting amplifier configuration. Hardware Setup: Now assemble the inverting amplifier circuit shown in Figure 4 using R2 = 4.7kΩ. Remember to disable the power supplies before assembling a new circuit (from Scopy). Cut and bend the resistor leads as needed to keep them flat against the board surface, and use the shortest jumper wires for each connection (as in Figure 1). Remember, the breadboard gives you a lot of flexibility in how you lay-out your circuit. For example, the leads of resistor R2 do not necessarily have to bridge over the op-amp from pin 2 to pin 6; you could use an intermediate node and a jumper wire to go around the device instead. Good practice is to keep wires short to increase speed and reduce stray inductance, and to minimise the number of connections, to improve reliability and reduce cost. Procedure: Enable the power supplies and observe the current draw (check the indicated voltage is close to the set voltage, in Scopy) to be sure there are no accidental shorts. Use the first waveform generator as source Vin to provide a 1V amplitude (peak-to-peak), 1 kHz and 0V offset sinewave excitation to the circuit. Configure the scope so that the input signal is displayed on channel 1 and the output signal is displayed on channel 2. Report: (1 mark) Calculate the theoretical voltage gain of the amplifier and export a plot of the input/output waveforms to be included in your lab report. Measure and record the voltage gain of this circuit, and compare to the theory that was discussed in class. Estimate the input impedance of this circuit.

Output Saturation: (2 marks) Slowly increase the amplitude of the input signal to 3 volts (peak-to-peak), and export the waveforms into your lab report. The output voltage of any op-amp is ultimately limited (‘clipped’) by the supply voltages, and in many cases, the actual limits are much smaller than the supply voltages due to internal voltage drops in the circuitry. Quantify the internal voltage drops in the OP97 based on your measurements above.

4.2.2

Non-Inverting Amplifier

Background: The non-inverting amplifier configuration is shown in Figure 5. Like the unity-gain buffer, this circuit has the (usually) desirable property of high input resistance, but also has some voltage gain:

Figure 5: Non-inverting Amplifier with voltage gain. Hardware Setup: Assemble the non-inverting amplifier circuit shown in Figure 5. Remember to disable the power supplies before assembling the new circuit. Start with R2 = 1 kΩ. Procedure:

Use the first waveform generator as source Vin to provide a 1V amplitude (peak-to-peak), 1 kHz and 0V offset sine wave excitation to the circuit. Configure the scope so that the input signal is displayed on channel 1 and the output signal is displayed on channel 2. Report: Calculate the theoretical voltage gain of the amplifier and export a plot of the input/output waveforms and include it in your lab report. Measure the voltage gain of this circuit, and compare to the theoretical result. (1 mark)

Increase the feedback resistor (R2) from 1 kΩ to 4.7 kΩ. What is the gain? (1 mark)

Now increase the feedback resistance (R2) to 47 kΩ. Describe and export a plot of the input/output waveforms in your lab report. What is the theoretical gain at this point? How small would the input signal have to be in order to keep the output level to less than 5V given this gain? Try to adjust the waveform generator to this value. Describe the output achieved. (3 marks)

The last step underscores an important consideration for high-gain amplifiers. High-gain necessarily implies a large output for a small input level. Sometimes this can lead to inadvertent saturation due to the amplification of some low-level noise or interference, for example, the amplification of stray 50Hz signals from power-lines that can sometimes be picked up. Amplifiers will amplify any signal at the input terminals - whether you want it or not!

4.2.3

Summing Amplifier Circuit (optional)

Background: The circuit of Figure 6 is a basic inverting amplifier with an additional input, called a “summing” amplifier. Using superposition we can show that Vout is a linear sum of Vin1 and Vin2, each with their own unique gain or scale factor. Such a circuit might be used to combine musical instruments into a common amplifier (‘PA’).

Figure 6: Summing Amplifier configuration. Hardware Setup:

With the power turned off, modify your inverting amplifier circuit as shown in Figure 7. Use the second waveform generator output for Vin2. Turn the amplitude all the way down to zero so that you can adjust up from zero during the experiment. Now apply a 2-volt amplitude (peak-to-peak) sinewave for Vin1 and 1-volt DC for Vin2. Observe and record the input/output waveforms on the oscilloscope screen. Pay close attention to the ground signal level of the output channel on the oscilloscope screen. When used in this way, such a circuit could be called a level shifter. It might be used to get a signal from a sensor in the correct range for an Arduino’s analog-to-digital converter.

Figure 7: Summing Amplifier Breadboard Circuit. Adjust the DC offset of waveform generator W1 (Vin1) until Vout has zero DC component. Estimate the required DC offset by observing the input waveform on the scope (note: it is not Vin2, be sure to understand why). Procedure: Use the first waveform generator as source Vin to provide a 2V amplitude (peak-to-peak), 1 kHz sine wave excitation to the circuit. The second waveform generator is used to generate a constant 1V. Configure the scope so that the input signal is displayed on Channel 1 and the output signal is displayed on Channel 2. Report: Reset the offset of waveform generator W1 to zero. With Channel 2 of the scope (the channel connected to the op-amp output) set for 2V/div, increase the offset voltage of waveform generator W2, Vin2 slowly. What happens to Vout? Record the DC voltage of the output.

Return the offset voltage of the waveform generator W2 to approximately +1V. Set the scope to 1V/div, and adjust the scope offset so you can see the complete Vout waveform. Turn Vin2 back up to the value you increased it to in the previous step. What does the oscilloscope trace for Vout look like? Does the amplifier appear to be amplifying?

Congratulations! You have now completed Lab 1. This is a good point to comment on circuit debugging. At some point in this class, you are likely to have trouble getting your circuit to work. That is not unexpected, nobody is perfect. However, you should not simply assume that a non-working circuit must imply a malfunctioning part or lab instrument. That is almost never true; 99% of all circuit problems are simple wiring or power supply errors. Even experienced engineers will make mistakes from time to time, and consequently, learning how to “debug” circuit problems is a very important part of the learning process. A good way to debug a circuit is to check the waveforms (using an oscilloscope) from input to output. It’s also a good idea to check all the voltages on the power supply rails (red, blue and black) to be sure they are getting to where they should get. Colour coding helps a lot! It is NOT the demonstrators’ responsibility to diagnose errors for you, and if you find yourself relying on others in this way then you are missing a key point of the lab and you will be unlikely to succeed in later coursework. Unless smoke is issuing from your opamp or there are brown burn marks on your resistors or your capacitor has exploded, your components are probably fine, in fact, most of them can tolerate a little abuse before significant damage is done. The best thing to do when things aren’t working is to just turn off the power supplies and look for a simple explanation before blaming parts or equipment.

Acknowledgement This labsheet is based on the Educational Programme of Analog Devices Inc. The original source can be found at https://wiki.analog.com/university/courses/electronics/electronics-lab-1 . Pleas...