Using the setup shown in Figure 1, the first step to measure PSR+ is to calibrate the source by connecting the FET probe to node A. R1, R2, and C1 form a passive network that couples the network analyzerís output signal to the amplifierís positive power rail, while providing the necessary DC level shift (~+VS). To match impedances to 50Ω, set R1 to 50Ω and R2 to a large value (~100kΩ). C1 should be a nonpolar capacitor large enough to maintain the measurementís low-frequency corner below any frequency of interest.
After calibrating the source, connect the FET probe to node C and measure the frequency response at the positive output (VOP). Store the results as a complex number data stream. Then, measure the frequency response at the negative output by connecting the FET probe to node B. Store the new results as a complex number data stream.
Figure 1. PSR+ vs. frequency measurement
Table 1. Positive PSRR results for the amplifier
Power-supply rejection is calculated by subtracting the measured complex frequency response of the negative output from that of the positive output. This difference is equivalent to a differential measurement. As a last step, compute the magnitude of the resulting stream of complex numbers to obtain the PSR.
Table 1 shows selected data points of the measured results for the amplifier used in the example.
Processing the Measured Data
One challenge with the complex algebra method is that more than one measured data stream must be combined to calculate the desired result. Calculations can be performed using a spreadsheet. For example, Excel can compute the PSR+ from the measured data as follows:
- Import the measured data into Excel and arrange it as follows: Frequency data in column A, real and imaginary data from the positive output (VOP) in columns B and C, and real and imaginary data from the negative output (VON) in columns D and E.
- Combine real and imaginary parts for VOP and store the result in column F. Formula: F=COMPLEX(B,C)
- Combine real and imaginary parts for VON and store the result in column G. Formula: G=COMPLEX(D,E)
- Calculate the difference VOP Ė VON, and store the result in column H. Formula: H=IMSUB(F,G)
- Separate the real and imaginary components of the values in column H and store the new results in columns I and J. Formulas: I=IMREAL(H), J=IMAGINARY (H)
- Calculate PSR by computing the magnitude of the values in columns I and J and storing the result in column K. Formula: K=20*LOG(SQRT(I^2+J^2))
Figure 2 shows the magnitude of the PSR (column K) after processing the data in Excel.
Figure 2. PSR vs. frequency for the amplifier measured using the complex Algebra method