Using a signal generator to simulate the LVDT secondary coil output to the AD698, the AD698 output jumps.

Question

The above circuit, remove the LVDT, the primary side of the LVDT is disconnected, use a signal generator to simulate the secondary LVDT, output a fixed AC voltage, to the AD698, measure the output of the AD698, not a stable sine wave, the jump is relatively large. Whether the signal generator can not be used directly for simulation, or the circuit needs to be modified, thank you for your help in advance!

Achim · Accepted Answer

Hello, so you want to use EXC to drive ACOMP. And you want to trigger your signal generator with this signal to generate the sinewave for AIN? I'm afraid but I don't think that will work satisfactory. The problem is: you can never set the frequencies of EXC and of the 33500B to really identical values (i.e. synchronize them). Both sources have individual deviations and drifts. The triggering will initially define the phase relation. But this defined phase relationship will drift away over time and the synchronous demodulation of the AD698 will sooner or later deliver unstable results. Do the test I suggested above with the dual channel oscilloscope. Can you really make both signals stand still by triggering only on one of them? If yes, then the signals are in synch. But this synchronization will be lost over time due to termperature drifts etc. The signals on ACOMP and AIN must not only start with a defined phase, they must also maintain the defined phase relationship over time. This can only be achieved, if both signals are derived from the same source. They must be kept at the really identical frequency over time (like in a phase locked loop). It's not enough, if the frequencies are nominally identical, and a start trigger for the second signal source will not be sufficient. If you can disconnect ACOMP from EXC, the simplest approach should be as described yesterday: drive both from the same source (33500B). For the ACOMP the digital synch output should be sufficient, for AIN you can use the analog sine signal of the 33500B. If you cannot disconnet ACOMP from EXC (e.g. due to limited access to the signals on your board), you must derive AIN from EXC. The easiest way to do this is to use a real LVDT (turning your simulation into a real experiment). You may replace the LVDT by other transformers or build something with a PLL. A kind of "handmade PLL" would be to try to keep the signals in synch based on the oscilloscope measurement suggested above. The frequency resolution of the 33500B is quite fine, so you could try to manually adjust the 33500B frequency so that if follows the EXC frequency exactly over time (controled with the parallel oscilloscope measurement). best regards Achim)

Achim · Answer

Hello, 
 AD698 said: Whether the signal generator can not be used directly for simulation, or the circuit needs to be modified, thank you for your help in advance! 
 the AD698 uses the ACOMP signal to synchonuously demodulate the AIN signal. That works, when ACOMP and AIN are synchronized. Synchronized means: if you monitor ACOMP and AIN with a two channel oscilloscope and trigger on ACOMP, also AIN has to stand still on the scope. 
 In your setup you use different signal sources to drive ACOMP (driven by EXC) and AIN (driven by signal generator). If you do not actively ensure the synchronization of both sources, the output of the AD698 will be unstable. 
 The easiest way to run this circuit successfull would be: use a real LVDT indstead of the signal generator. The output voltage of the LVDT is always in synch to its input. 
 If you absolutely need a simulation with signal generators: use two signal generators, which can be synchronized (or a two-channel generator, where both outputs are synched). Take one of them to drive ACOMP with a constant sinewave. Take the other one to drive AIN with a sinewave of identical frequency, but varying amplitude/phase. 
 best regards 
 Achim

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Using a signal generator to simulate the LVDT secondary coil output to the AD698, the AD698 output jumps.