He said if there is no Vin connected, there will be interference coupled on
10Mohm R1~R4. He would like to know that whether we have a solution to avoid
the interference when no connect at the input.2. Another problem he encountered
is as below.
When he uses a signal source which does not have any connection to earth ground
to input to figure 17, the measurement is OK.
If one of the terminal of the signal source output is connected to the earth
ground and this terminal is connected to COM of figure 17, the measurement is
OK. However, if the terminal which is connected to earth ground is connected to
Vin and the other output of the signal source is connected to COM, the circuit
can not work. Do you have any comment?
1.It will depend a lot on the application, if the circuit is used in an a high
interference environment a solution might be to provide some metallic shielding
around the circuit. If the interference is from a high frequency source, some
filtering might be a workable solution. Such filtering might be in the form of
a serial inductor or ferrite bead around the input circuitry. 2.It sounds as
though the signal source may have been single-ended, not differential, and the
low side of the source was connected somewhere to ground. If the circuit in
figure 17 were also referenced to ground, the input would have been shorted
circuited. Note that the circuit of figure 17 is single-ended, not
differential. The input impedance would not be reversible necessarily.