Several Causes of noise:
. Poor circuit board layout. Ensure that the connections to the
. Op amp inputs are short and far from noise sources.
.
. The Op amp is inherently noisy. Look at the noise specs. There
. are several low noise op-amps, such as the OP-07 available.
.
. High input resistor values make the circuit susceptible to
. input current noise.
.
. Poor power supply decoupling (already mentioned in other posts)
.
. Microphonic capacitors. Ceramics are particularly bad.
.
. One trick that you can use in a pcb layout is to surroundt the summing
. junction (inverting input in an inverting amplifier) with track that is
. connected to signal ground. This is referred to as a "guard" band. This
. prevents noise current from reaching th NI input.
Regards,
Jon