A brief description of OP97-senpai's characteristics

Input voltage range: ± 14V

・Current consumption: 600μA Max

Input offset voltage: 20μV Max

Input offset voltage temperature drift: 0.6 μV/°C Max

・CMRR：114dB Min

Low input bias current: 25°C: 100pA Max

Extended industrial temperature range: -40 °C to + 85°C

The OP97 is positioned as a low-power successor to the OP07, a high-precision amplifier that once led the industry. For more details, please see the product page here.

OP97 Datasheet and Product Information |Analog Devices

For this experiment, I chose the OP97, but this device is quite old, having been released in 1997. When the OP97 was released, my "Power-chan" didn't even exist yet! So, the OP97 is the senior one. Anyway, Analog Devices is constantly releasing upgraded new products, so if you're designing something new, please choose a newer product instead of the OP97!

What is a unity-gain circuit?

Unity-gain circuit using OP97

First, some of you might be wondering, "What is a unity-gain circuit?" So, let me briefly explain. A unity-gain circuit consists of a single operational amplifier without any resistors in the feedback-backward (FB) path, and the output terminal of the operational amplifier is directly connected to the input terminal Vin-. As a result, in an ideal operational amplifier, negative feedback makes Vin ≈ Vout, and the output is adjusted so that the inverting input voltage and the non-inverting input voltage are equal. In other words, it has the characteristic of outputting the input signal as is.

When I first learned about this circuit, a lot of questions popped into my head. I thought amplifiers were used to amplify signals. If it outputs without amplification, then this amplifier isn't doing anything, is it? Is it even necessary? That's when I first learned that amplifiers have many roles besides signal amplification.

One advantage of building a unity-gain circuit is that it allows for high input impedance and low output impedance. Because of this, it doesn't draw much load from the input side, allowing the signal source to receive signals without burdening it, and providing a stable signal to subsequent circuits. The unity-gain circuit plays a crucial role as a "buffer."

Let's try building a unity-gain circuit on a breadboard.

Analog Devices' reference article also included information on how to assemble a breadboard, so even with limited circuit knowledge, I was able to create the circuit without any problems!

Breadboard connection example of a unity-gain circuit using OP97

The unity-gain circuit that was actually created

This is the pinout for the ADALM2000.

In this experiment,

・1+/1- (Oscilloscope channel 1/​ ​GND of oscilloscope channel 1)

・2+/2- (Oscilloscope channel 2/ Oscilloscope channel 2​ ​GND)

・GND

・V+ /V-(Positive power supply / Negative power supply)

• W1 (Signal Input 1)

We will use the following pins. Connect ch1 of the oscilloscope to the input side and ch2 to the output side, and observe the waveforms of each.

After connecting the included jumper wires to their respective locations, you can start taking measurements by connecting the ADALM2000 to your PC via USB!

Furthermore, this measurement was,

Input signal frequency: 1kHz (sine wave)

Input signal amplitude: 2Vp-p

Power supply voltage: ± 5V

The test is performed under these conditions, and the frequencies and Vp-p values of both the input voltage waveform and the output voltage waveform are checked.

Let's start Scopy and begin the setup! You can start the setup by selecting the tab on the left side of the screen.

This time, we will configure two things: "Signal Generator" and "Power Supply". The "Signal Generator" acts as the signal source, so you can set the input signal. On the other hand, the "Power Supply" acts as the power supply that powers the operational amplifier, so you can set the power supply voltage.

You can change the settings for each by selecting the tabs on the left side of the screen. For the input signal, ① open the "Signal Generator" tab, ② select CH1, ③ select Waveform, and ④ set Amplitude=2Vp-p/Frequency=1kHz to enable input of a signal with a frequency of 1kHz and an amplitude of 2Vp-p.

To set the power supply voltage, ① open the "Power Supply" tab and ② set Positive output=5V/Negative output=-5V. This will input 5V from V+ and-5V from V-. The Negative output at the bottom of the screen (in purple text) displays 5.000VDC Set, but this actually means-5V, so don't be fooled. This notation is very confusing.

Actual measurement! Unity gain circuit!

Once the setup is complete, we move on to the much-anticipated measurement with the "Oscilloscope"! Clicking "Run" yielded the following waveform.

Input/output voltage waveforms of a unity-gain circuit

The image on the left has been shifted for clarity. The orange waveform at the top is the input signal, and the purple waveform at the bottom is the output voltage waveform. As you can see in the image on the right, the output voltage waveform perfectly overlaps with the input voltage waveform. The frequency of these input and output voltage waveforms was 1kHz, and the Vp-p was 2.0V.

When I checked the waveform in LTspice, it output a waveform with a frequency of 1kHz and a Vp-p of 2V, similar to the waveform obtained this time.

As can be seen from the LTspice waveform, the unity-gain circuit outputs a waveform with a gain of 1, so it can be said that the desired waveform was obtained.

*Since LTspice does not have a component model for OP97, evaluation is being performed using the substitute OP07.

bonus

I wondered what kind of changes would occur if I tried tweaking the settings, so I gradually increased the frequency of the input signal. When I changed the frequency from 20kHz to 50kHz, the output voltage waveform began to become blunt and no longer overlapped with the input signal.

Looking at the datasheet, the OP97 has a slew rate of 0.2V/µs(typ). Slew rate is a value that indicates how many volts per microsecond the output voltage can follow a change in input voltage. In other words, the OP97 can follow changes at a rate of 0.2V per microsecond.

The slew rate (SR) is,

This can be calculated using the following formula. Using this formula, if we determine the maximum frequency (fmax) that an OP97 with a slew rate of 0.2V/µs can track,

These were the values we obtained. In other words, the 50kHz signal we set this time exceeded the trackable frequency of 31.8kHz, which caused the output voltage waveform to become slewn. This was a good opportunity to learn about the relationship between slew rate and output voltage waveform!

Analog Devices Manufacturer Information Top

Analog Devices Manufacturer Information If you would like to return to the top page, please click below.