Vos and Ib - Lab
This is the second of two videos in the TI Precision Labs – Op Amps curriculum that addresses operational amplifier input offset voltage and output limitations. In this training lab we walk through detailed calculations, SPICE simulations, and real-world measurements that greatly help to reinforce the concepts established in the op amp VOS and IB lecture.
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Hello, and welcome to the TI Precision Labs supplement for Op Amp Input Offset Voltage, VOS, and Input Bias Current, IB. This lab will walk through detailed calculations, SPICE simulations, and real world measurements that greatly help to reinforce the concepts established in the Op Amp VOS and IB lecture.
The detailed calculation portion of this lab can be done by hand, but calculation tools such as MathCAD or Excel can help greatly. The simulation exercises can be performed in any SPICE simulator, since Texas Instruments provides generic SPICE models of the op amps used in this lab. However, the simulations are most conveniently done in TINA-TI, which is a free SPICE simulator available from the TI website. TINA simulation schematics are embedded in the presentation.
Finally, the real-world measurements are made using a printed circuit board, or a PCB, provided by Texas Instruments.
If you have access to standard lab equipment, you can make the necessary measurements with any oscilloscope and plus or minus 12-volt power supply. However, we highly recommend the VirtualBench from National Instruments. The VirtualBench is an all-in-one test equipment solution which connects to a computer over USB or Wi-Fi and provides power supply rails, analog signal generator and oscilloscope channels, and a 5 and 1/2 digit multimeter for convenient and accurate measurements. This lab is optimized for use with the VirtualBench.
In experiment 1, we'll determine the effects of VOS and IB in a circuit where the input resistance, RIN, is equal to 0 ohms.
First calculate the expected total output voltage due to VOS and IB for the circuit shown here using the techniques and equations given in the VOS and IB lecture. Take note of the jumper positions in the table. JMP13 and JMP14 are open, and JMP15 is closed. JMP15 shorts the non-inverting input of U5 to ground causing RIN to be 0 ohms.
Calculate the output voltage twice. First with the OPA211 selected for U5 and U6, then with the OPA188. The different parameters of these op amps will give you different results.
In order to perform the calculations, you need to know the typical and maximum values of VOS and IB for each op amp. Those values are given here. Enter your answers in the table at the bottom of the slide. The solutions are already provided to allow you to check your work.
One important point to note is that VOS and IB can be positive or negative. This means that there are different possibilities for the output voltage due to VOS and IB which must all be considered.
First calculate Req, the equivalent input resistance, then multiply Req by IB to determine the input voltage due to IB. Next, use the equation Vout equals G1, gain of the first stage, times G2, gain of the second stage, times the sum of VOS VIB to calculate the total output. Again, there are four possibilities. Pick the largest value.
Repeat the same steps using the maximum values instead of the typical values. The plot on the bottom shows the Gaussian distribution of possible output values.
Repeat the same procedure for the OPA188. The different specifications of the OPA188 will give a different output voltage result in both the typical and maximum case.
The next step is to run a SPICE simulation analysis for the total DC output voltage. The necessary TINA-TI simulation schematics are embedded in this slide set. Simply double-click the icons to open them.
Ensure that the jumpers are set correctly. In the OPA211 circuit, JMP13 and JMP14 are open, and JMP15 is closed. In the OPA188 circuit, JMP9 and JMP10 are open, and JMP11 is closed.
To simulate the output voltage, click Analysis, DC Analysis, Calculate nodal voltages. For the OPA211, you should get a result of about 133.28 millivolts. For the OPA188, you should get a result of about minus 10.73 millivolts.
Make sure to disable the DC power supply before setting up the test PCB. In the Virtual Bench software, click the power button in the DC Power Supply area to turn off the power. Check the front panel of the VirtualBench unit to make sure the LEDs are off. Also make sure that the function generator is off.
To prepare the test board for measurement, install the jumpers and devices on circuit 3 and circuit 4 as shown here. On circuit 3, install JMP9, JMP11, and JMP12, as well as the OPA188 in sockets U3 and U4. On circuit 4, install JMP13, JMP15, and JMP16, as well as the OPA211 in sockets U5 and U6.
This slide shows the full schematic for circuit 3 on the TI Precision Labs test board. You will use this circuit to measure the effects of VOS and IB on the OPA188.
This slide shows the full schematic for circuit 4 on the TI Precision Labs test board. You will use this circuit to measure the effects of VOS and IB on the OPA211.
For the test board to function properly, it is important that you only install jumpers and devices in circuits 3 and 4. Do not install any jumpers or devices in any other circuits on the PCB. Remove any jumpers or devices from the unused circuits and store them in the storage area at the bottom of the test board.
This is the connection diagram between the TI Precision Labs test board and National Instruments' VirtualBench. Connect the provided power cable to the DC power supply of the VirtualBench and power connector J4 on the test board. Connect Vout 1 on the test board to VirtualBench oscilloscope channel 1, and Vout 2 on the test board to VirtualBench oscilloscope channel 2 using BNC cables.
Next, apply power to your National Instruments VirtualBench and connect it to your computer with a USB cable. The hardware should be detected as a virtual CD drive, and you can run the VirtualBench software directly from the drive.
Once the software opens, configure the software as follows. Set the time scale to 100 milliseconds per division with the acquisition mode set to Auto. Enable channels 1 and 2 on the oscilloscope, and set them to 1x DC coupled mode. Adjust the vertical scale as needed from 10 millivolts per division to 1 volt per division. Set the plus 25-volt power supply to plus 12 volts, 0.5 amps. Set the minus 25-volt power supply to minus 12 volts, 0.5 amps.
Press the power button to turn on the power supply rails. Enable mean measurements on both channels in order to read the output voltage of each circuit.
You must also set the mode of the VirtualBench oscilloscope. Click the button shown in the front panel, then set Acquisition to Sample and Persistence to Disabled.
The expected output voltage results from the measurement are shown here. The OPA211 has a measured output voltage of 74.7 millivolts, and the OPA188 has a measured output voltage of 4.6 millivolts. You may have different results in your experiment.
How did the measured and simulated results compare to the typical hand-calculated results. Take a moment to look over the previous results and draw your own conclusions.
For the next part of the lab, we'll repeat the same procedure as experiment 1, but this time with 5 kilohms of input resistance. This will emphasize the effects of input bias current IB.
As shown in the schematic, jumper JMP15, shorting the positive input of U5 to ground, is now removed. Jumper JMP14 is now installed in order to connect the positive input of U5 to a 5-kilohm resistor. The IB of U5 will now flow through this resistor developing a DC voltage due to Ohm's law and increasing the amount of offset voltage. As before, calculate the total output voltage due to VOS and IB for this circuit using both the OPA211 and OPA188.
The data sheet parameters for both devices are provided again for reference. Enter your calculated results in the lower table. The answers have been provided so that you can check your work.
With RIN equal to 5k, the calculations change slightly since the voltage caused by IB is now affected by RIN. Use the new equation VOS equals IB times Req plus IB times RIN. Otherwise, the steps are the same as in experiment 1. Repeat the calculations for the maximum values.
Repeat both sets of calculations for the OPA188 again using typical and maximum values. As before, the different electrical characteristics of the OPA188 will result in different output voltages calculations.
Rerun the simulated DC nodal voltage analysis, making sure to use the proper settings. In the OPA211 circuit, JMP13 and JMP15 are open, and JMP14 is closed. In the OPA188 circuit, JMP9 and JMP11 are open, and JMP10 is closed. You should see a result for the OPA211 of about minus 209.19 millivolts. You should see a result for the OPA188 of about minus 9.03 millivolts.
The jumper settings on the test board must be modified before rerunning the bench measurement. Remove jumpers JMP11 and JMP15, and install jumpers JMP10 and JMP14. All other jumpers and devices remain the same from the previous experiment.
In experiment 2, the OPA188 has a measured output voltage of 3.49 millivolts and the OPA211 has a measured output voltage of 375 millivolts. You may have different results in your experiment.
How did the measured and simulated results compare to the hand-calculated results? In this example, the OPA211 output was greater than the calculated and simulated typical values, but less than the calculated maximum value. The OPA188 output was less than both the calculated and simulated typical values.
Let's now compare the results of both experiments. How did the change in input resistance affect the output voltage? In the OPA211, increasing the input resistance caused a dramatic increase in output voltage. However, the OPA188 did not see such a large increase. This is because the OPA211 has a much larger input bias current than the OPA188.
That concludes this lab. Thank you for your time.
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Precision labs series: Op amps
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