ZHCSQG1H November 2009 – May 2022 LMP8645 , LMP8645HV
PRODUCTION DATA
While the sense amplifier is depicted as a conventional operational amplifier, it really is based on a current-differencing topology. The input stage uses precision 5-kΩ resistors internally to convert the voltage on the input pin onto a current, so any resistance added in series with the input pins will change this resistance, and thus the resulting current, causing an error. TI recommends that the total path resistance be less than 10 Ω and equal to both inputs.
If a resistance is added in series with an input, the gain of that input will not track that of the other input, causing a constant gain error.
TI does not recommend using external resistance to alter the gain, as external resistors do not have the same thermal matching and tracking as the internal thin film resistors. Any added resistance will severely degrade the offset and CMRR specifications.
If resistors are purposely added for filtering, resistance should be added equally to both inputs and be less than 10 Ω, and the user should be aware that the gain will change slightly.