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I have Used AD598 for many years but as the LVDT coils are not linear I have switched to AD698 in conjunction with the data sheet. I have not fitted any components for Lead/Lag and get not DC output Will the IC work with these open circuit? The data sheet gives no info are this, what is the basic values if I require no compensation? I have made provision on the board for either configuration but have left all parts open circuit.
The values used for the R and C of the phase compensation circuit depend entirely on the phase shift introduced by your LVDT, at the frequency of operation. This is not listed in the LVDT datasheet and should be checked using a scope to observe the inputs and outputs of the LVDT. I can however explain to you the reason for the phase compensation circuit and this should assist you in choosing your values when you have determined the phase shift. To explain the phase compensation circuit I will refer you to the block diagram on page 5, fig 6, of the datasheet. If you look at both A and B channels you can see that there is a V/I block and a comparator block on each. In both cases what is happening is that the input sinewave is being full wave rectified, using the comparator to find the zero crossings of the input sinewave and using this information to either pass or invert the output of the V/I block. Both channels are then filtered, using the external C2 & C3 caps and a duty cycle based on the ratio of the amplitudes of the rectified inputs is set up. However, when using a series opposed LVDT the amplitude of the A channel input reduces to zero at the null point of the LVDT. The A channel input itself should not be used to rectify itself as it will be prone to errors. Because of this we include the Acomp inputs, which are taken either directly from the B inputs, which are constant amplitude, or which are phase adjusted versions of the B inputs. The phase should be adjusted such that the phase of the Acomp inputs matches that of the A inputs. In many cases setting the values of the phase lead/lag components does come down to some trial and error when initially setting up the AD698. This is simply done using a scope to overlay the Acomp and A channels, usually when the LVDT is at either end point, of the linear range, not the null point. If there is a phase difference between the channels simply adjust either one of the cap or the resistor values until the signal match up. Using a pot as suggested in the diagram does speed this process up and the pot can then be replaced by the equivalent resistor value in a final setup.