LTC1860L is true differential inputs or pseudo differential

1) The difference between IN+ and IN- of the LTC1860L is captured by the track and hold circuit when the CONV command is given. Therefore the LTC1860L has true differential inputs. The LTC1860L is best used in circuits where IN- is held constant and IN+ goes above and below IN-. For this reason LTC1860L is referred to as pseudo-differential.

2) Because IN- has a track-and-hold (T/H) circuit, there will be a glitch on IN- when the T/H goes into the track mode. This glitch must settle completely before the ADC can perform an accurate conversion. If you look at the equivalent RLC model of the magnetic bead you are using, you will begin to understand why it can cause performance problems with the ADC. The T/H circuit of the ADC can be modeled as a capacitor charged to Vref/2 that is switched onto the RLC model of the magnetic bead. It takes some time before the voltage at the junction of the T/H and the magnetic bead settles. Until the junction settles, a CONV pulse should not occur.

1)The T/H circuit captures the IN- and IN+ at the same time or not.
2)Can you offer the model of the T/H circuit with picture?Does the performance problem with the ADC only occur with the change of the system noise?

1) Yes, the T/H circuit captures both IN+ and IN- at the same time.

2) LTC1860L T/H looks something like this

If the T/H is not allowed to settle it can effect the linearity, THD and noise performance of the ADC.