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Usage of Linear Injector

The linear injector can directly superimpose AC ripple and DC voltage from the power supply as the input to measure the Power Supply Rejection Ratio (PSRR) of the LDO. This article will elaborate on the PICOTEST J2120A linear injector.

The linear injector is similar to a broadband injection transformer, but the broadband injection transformer cannot be used to measure the PSRR of the LDO because it is not tolerant to DC current. Even a very small DC current (5mA or less) will significantly reduce the signal capacity and even saturate the transformer. Therefore, a linear injector is required for measurement.

Basic parameters of the J2120A linear injector:

  • Maximum DC input voltage: 50VDC
  • Maximum continuous current: 5A
  • Maximum voltage drop: 3.25VDC
  • 3dB frequency response: 15Hz~5MHz
  • Available frequency response: 10Hz~10MHz
  • Recommended injection signal: -20~10dBm
  • Operating temperature range: 0~50℃

Test scenario:

It should be noted that the input decoupling capacitor should not be added during the test because the output of J2120A has a certain impedance. Adding a capacitor will form a low-pass network, which will affect the PSRR result.

Parameters setting of the network analyzer:

  • Start frequency: 10Hz
  • Stop frequency: 10MHz
  • Sweep mode: Logarithmic
  • Number of sweep points: ≥401
  • Output level: -10dBm
  • 1/2 channel attenuation: 10dB
  • Receiver bandwidth: 100Hz

References and Acknowledgements

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