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 analyzer’s output signal to the amplifier’s positive power rail, while providing the necessary DC level shift (~+V_{S}). 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 measurement’s 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 (V
_{OP}) in columns B and C, and real and imaginary data from the negative output (V_{ON}) 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 V
_{OP} – V_{ON}, 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