The Picotest J2123A Negative Voltage Line Injector , like the J2120A positive voltage line injector, is used to superimpose an AC modulation signal on a DC source voltage for Power Supply Rejection Ratio (PSRR), Power Supply Modulation Ratio (PSMR), and Audio Amplifier Common-Mode Rejection Ratio (CMRR) testing. These applications require very low output noise, as well as a low output impedance over a wide bandwidth.