LVDT excitation circuit ideas?

[Styx]

Afternoon,

I am after some inspiration with regards to generating a 2kHz (10k for resolver) 3-7Vrms sinwave of low thd

At present I typically do one of two schemes

uP generates high freq PWM of the desired sinwave. This is followed by a 4th order bandpass (cascaded low pass and high pass) to then finally drive a class AB to provide short-circuit capability and a bit of drove strength

The other method used is a DAC and then followed by a 4th order bandpass and the same class AB. The DAC should permit dropping the filter order


Besides directly driving an LVDT with PWM (at 10MHz) and relying on its impedance to filter, any other neat methods?

 

[Nigel Goodwin]

You mention 'low THD' but don't give any suggestion how low it needs to be?

But anyhow, for an alternative method how about an AD9833 frequency generator chip/module?.

 

[ronsimpson]

Something like the AD9833 is very good. Then you want a low pass filter to strip off some of the high frequency noise.

 

[Styx]

Ahh yes, low is all relative. What's needed isn't audiophile low, I believe 1-2% is acceptable BUT that is frequency dependant (ie all the distortion in the 2nd would be very bad)

A waveform chip is a good idea. This would need to be amplified before going into a class AB
Waveform - LPF - Opamp - class ab.

The Opamp feedback can be closed around the AB to provide gain.

This is a possibility .... One MSOP, one sot23-5 OPAMP, two sot223 (and two sot23) BJT for the push pull stage + foldback and rice

 

[rjenkinsgb]

How about a phase shift oscillator to produce a reasonable sine wave to start with, then filter that? It's probably simpler than a digital source in that application.

Phase-shift oscillator - Wikipedia

en.wikipedia.org

 

[Beau Schwabe]

For the input resolver coil you can simply use a square wave. The resonant frequency only needs to match the secondary coils. In my case about 30kHz


EDIT: One thing to mention. If the micro can handle the overhead and generating the excite signal and also read the sine and cosine values from the secondary coils, then you can "PHASE LOCK" exactly when you read the magnitude value of the sine/cosine coils and save on time reading the ADC.
I used two 4-Channel ADC's because I needed to read both sine and cosine at the exact same moment, and also that I needed 3 other resolver channels. One channel was a spare.

Resolver Test #1: (One output coil and input signal)

Resolver Test #2: (Both Sine/Cosine output coils)

Resolver Test #3: (Graphical interpretation of Sine/Cosine coils using a Parallax Propeller IC and two MCP3204 ADC's )