Suppression Capability
Performance metric quantifying the ability of an instrument to suppress signals that appear simultaneously and in-phase on both input terminals. A high differential common mode rejection ratio ensures that electrical noise from the environment does not corrupt the small differential signal from the sensor. Logarithmic decibels typically express this value.
Most instrumentation amplifiers provide values exceeding 100 decibels at low frequencies, allowing the detection of microvolt-level changes amidst volt-level interference.
Impedance Balance
Mismatches in the source impedance or the internal resistors of the amplifier degrade the suppression. Even a small imbalance of a few ohms lowers the differential common mode rejection ratio. Precise resistor matching is required during the manufacturing process to maintain high performance.
Frequency Dependency
Rejection effectiveness decreases as the frequency of the common mode signal increases due to internal parasitic capacitances. A differential common mode rejection ratio might be excellent at 60 hertz but poor at 10 kilohertz. Shielded twisted pair cabling helps reduce the high-frequency noise reaching the amplifier inputs.
Metrological Verification
Testing involves applying a known voltage to both inputs and measuring the resulting output change. This differential common mode rejection ratio is verified at both the minimum and maximum power supply voltages. Lab standards define the specific test conditions for these measurements.
