Please, can you describe your question in more detail (supported by a drawing or circuit diagram). It is not clear to me what your problem really is.

Mr Lutz,

Actually i am supposed to read data from 30 strain gauges, for that i am using ac bridge circuits but i need to excite the bridges at some frequency, as we know that the output of the strain gauges is very small so i need to amplify it also. So i am looking for a relationship between frequency and bandwidth that can be used to get the noise free exact values.

As a general rule, to improve the rejection of line frequency and its harmonics, you should use either a sub-harmonic or an integral number of harmonics.

PSD techniques may help you in such application

Please note that high-accuracy (better than 20 ppb) resistance bridges use 25 Hz and 75 Hz excitation frequency. 4-wire (or 6-wire) kelvin is of course needed to insure the measurement accuracy.

In general, active devices that you would use to sense the bridge output have white noise properties (independent of frequency) above a certain frequency but below that frequency their noise increases with decreasing frequency at a rate of 1/f. This frequency is often referred to as the flicker noise corner, and is often found to be in the range of 1 kHz to 20 kHz depending on the device. If possible to operate your bridge circuits above that flicker corner frequency, you should observe improved SNR in your sensors.

Try reading this paper about carrier frequency technique and its advantages, it could help with your question:

http://www.hbm.com/fileadmin/mediapool/techarticles/2001/High_Precision.pdf

Probably the simplest answer is: the more narrow the bandwidth is, the lower noise you will have in the output signal.

But (there are several "buts") you will never get totally noise-free, exact values at least because the bandwidth is always finite (greater than zero), which means that you will always have some noise. Theoretically you could go down with the bandwidth very low (to a fraction of hertz), but you must take into consideration accuracy and stability of the excitation frequency source and the filter located in the output signal path (its center frequency must be adjusted to the excitation frequency and even slight deviation from this frequency will attenuate the signal thus resulting in measurement errors). In addition, as already indicated in the earlier posts, the bandwidth should not be the only concern in such an application.

DearAwais

Good question.. Please share me the best answer might you trust...

Regards…

Yes, as higher is the exitation freq. as better is the output accurancy