hi i am using a lockin amplifier for the 1st time other than a canned college lab demo i have a 5mV signal from a strain gage that is driven with 10Vpp at 1kHz or whatever i choose. the 5mV is increased to 5.01mV when i add some weight to the arm the gage is weighing. i wanted to increase sensitivity of this reading with the LIA since there is noise in the next digit, i.e. i read 5.01mV +- 0.001mV.
But i seem to go out of range (overload) before reading any weight change. do i need to get a 0.000V average signal first, and put this into the LIA?
The 4 strain gages are in a full wheatstone bridge configuration. The 10V 1kHz signal is sent to the bridge and to the LIA 'reference' input,
5mV 1kHz bridge output sent to to LIA signal in. I adjust the LIA phase for max output, zero the signal with the internal LIA offset, and increase sensitivity until i overload. then i back off the sensitivity so theres no overload. at this point i see no change in output with change in weight. the LIA sensitivity is generally 30mV at this point, whereas the signal I want to measure is 0.01mV or less, so it seems natural i wont get a change in output. the only thing i can think of is i must 'balance the bridge' to get 0.00V output, and amplify this to higher levels e.g. at the 0.01mV level. is this right?Finally, having read a primer that suggests using 'offset' and 'expand' for low-noise small-change signals such as mine, do I need an LIA with 'expand', which seems to be an output amplifier (after the lockin stage)? My old analog lockins have only offset and input amplification (sensitivity), but no 'expand' function.