SBOA571 august 2023 OPA2387 , OPA387 , OPA4387
The preconditioning requirements can be targeted with a shelving filter. The easiest approach is a first order circuit. As we are gaining up low frequencies and are dealing with small signals, a low noise OPAMP with auto-zero technology was selected, OPA387. Due to the auto zero feature, the OPAMP removes the 1/f noise and has a flat noise power spectral density of only 8 nV/sqrt(Hz). The circuit further incorporates a matching resistor divider at the non-inverting input to reduce the Ib current based offset error. The shown filter structure is an inverting structure. The output polarity is reversed.
Looking at the group delay of such filter, one can see that the response time for detecting a DC failure is about 160 ms. For many applications, 160 ms is not acceptable.