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LF156: DC CMRR vs AC CMRR

Mike Woodard
Prodigy 245 points

Part Number: LF156

When does the DC Characteristic CMRR apply and when does the AC Performance CMRR apply?

  • 0
    TI__Mastermind 30985 points

    Hi Mike, 

    1. DC Characteristic CMRR
    The DC CMRR applies to static or very low-frequency common-mode voltages (near 0 Hz).
    • When to use it: Use this value to calculate the steady-state output offset error caused by a fixed common-mode voltage.
    • LF156 Specification: Typically 100 dB. This ensures high precision in buffers or integrators where the input has a large, constant DC offset.
    2. AC Performance CMRR
    The AC CMRR applies as soon as the common-mode signal has a time-varying or frequency component (such as 60 Hz hum or high-speed noise).
    • When to use it: Use this to determine how well the amplifier rejects dynamic interference or "ripple" appearing on both inputs.
    • Frequency Dependency: Unlike the DC value, CMRR degrades as frequency increases. For the LF156, you must refer to the "CMRR vs. Frequency" plot in the datasheet (Figure 28) to find the specific rejection value for your operating frequency.
    Summary Table
    Feature DC CMRR AC CMRR
    Signal Type Static / 0 Hz Dynamic / > 0 Hz
    Primary Goal Minimize output offset shift Reject AC noise and "hum"
    LF156 Value 100 dB (Typical) Variable (Decreases with frequency)

    I hope this helps. 
    Best Regards, 
    Chris Featherstone