EE501 Lab 7 Opamp Measurement Report due: Nov. 6, 2014 Objective: 1. Understand basic opamp measurement circuits. 2. Build testbench circuits for opamp measurement. Tasks: Op amps are very high gain amplifiers with differential inputs and single-ended outputs. They are often used in high precision analog circuits, so it is important to measure their performance accurately. But in open-loop measurements their high open-loop gain, which may be as great as 10 7 or more, makes it very hard to avoid errors from very small voltages at the amplifier input due to pickup, stray currents, or the Seebeck (thermocouple) effect. The measurement process can be greatly simplified by using a servo loop to force a null at the amplifier input, thus allowing the amplifier under test to essentially measure its own errors. For this lab, you need to build your own testbench to test your own designed opamp in Lab2 and Lab3. You should refer to the references to figure out which circuits should be selected and why. Then use the test results to compare with your simulation results, try to figure out whether there were difference and why. The auxiliary amplifier could be downloaded from the webpage. 1. DC measurement: offset voltage, DC gain, CMRR, PSRR and total quiescent current Build one testbench to measure all DC parameters. a. Offset voltage & quiescent current An ideal op amp has zero offset voltage (Vos); that is, if both inputs are joined together and held at a voltage midway between the supplies, the output voltage should also be midway between the supplies. In real life, op amps have offsets ranging from a few microvolts to a few millivolts so a voltage in this range must be applied to the input to bring the output to the midway potential. The example circuit is shown in Fig.1. Try to figure out how to measure the offset voltage and total quiescent current.
TP1 Vdd V1 Vss Fig. 1 b. DC gain The testbench could be the same as in Fig.1 or your own designed testbench. The method is shown as below. 1) Force V1 to 1V.(Vout(+)=1V) 2) Measure the voltage of TP1. Then calculate the Vin(+): Vin(+) = TP1(1) 1001 Vos 3) Force V1 to -1V.(Vout(-)=-1V) 4) Measure the voltage of TP1. Then calculate the Vin(-): Vin( ) = TP1(2) 1001 Vos 5) Calculate DC gain: Vout(+) Vout( ) Adc = 20 log ( Vin(+) Vin( ) c. CMRR & PSRR Refer to the materials from EE501 homepage to figure out how to test CMRR and PSRR with the same testbench you designed above. (Hint: Read thoroughly both slides, then try to figure out how to measure CMRR by changing supply voltage) 2. AC measurement: bode plot(ac gain and GBW), AC PSRR, AC CMRR Build one testbench to measure AC parameters a. Bode plot The sample circuit is shown in Fig.2.
Fig 2 To measure AC gain, apply a sine wave signal in the input node and the input frequency should be in the range from 500Hz~10MHz, measure the voltage in TP2, then calculate the AC gain at certain frequency. Collect the data to draw the bode plot and calculate the GBW of opamp. To stabilize the test circuit, the compensation capacitor (C1) should be calculated as shown in slides (Testing Op Amps and Comparators). You should select the input amplitude and resistance value to make sure that the amplitude of TP2 is not out of range. The sample schematic is shown below. Notice that you should put the proper load at the output node of opamp. b. AC CMRR & AC PSRR
To test AC CMRR and AC PSRR, the testbench could be the same as above, only difference is that the ac input is adding to supply. As shown below (Not exactly same as above) AC CMRR measurement 3. Transient mearsurement AC PSRR measurement
The testbench for measuring slew rate and settling time is the same as simulation testbench you built in Lab2. For testing settling time, just make the Vstep 100mV. The sample simulation result is shown below. To test THD (Total harmonic distortion), add a sine wave signal at the input node (you can select the input frequency and amplitude), then run transient simulation. Go to Measurements -> Spectrum, as shown below.
Harmonic number Window type Harmonic number
Test THD with different input level (different output level) and different input frequency, tabulate your results. (You can choose 3 to 5 input level begin with small signal to out of range and 3-5 different frequency from DC to GBW). Outputs