Digital Lock-In amplifier implementation into stereo codec with DSP (like the ADAU 1761) ?
Dear All,
I need to build an accurate sensor processing system for our laboratory and I consider to build an digital Lock - In amplifier based on DSP technology.
I am new in the field of the DSP and before dealing with the learning curve of new processor ( SHARC or other ),
I wonder myself if an stereo codec with intergrated mini DSP will not be a solution ?
My problem is that I dont know the capacity of the internal DSP of this kind of codecs :
- is there a way to build an internal sin wave generator for external and internal reference ?
- is there a way to shift a sinus signal by 90° ?
- is there a way to shift dynamicaly a signal (+ - 180 °) ?
So, here after is a slightly modified shematic found on web.
Thank you for your opinion and advices,
Jean
Hello,
- SigmaStudio has sine tone generators (as well as other function generators) included in the library.
- To answer the question about phase shift...
Please check these three discussions for more information on this topic:
https://ez.analog.com/message/12996#12996
https://ez.analog.com/message/18805#18805
https://ez.analog.com/message/11254#11254
The simple answer is that SigmaDSP can implement group delay (that is, delay for all frequencies). This can also be used as a variable phase delay for a single frequency. However, since you're using a range of input frequencies, the phase delay for a single delay cell will be frequency-dependent.
So, as far as I know, what you're trying to do is not possible in SigmaDSP.
Actually, I have an update. It seems like the Hilbert Transform can be used to implement a 90 degree phase shift. Please take a look at this post for details:
Dear Brett,
I wish to thank you infinitely for your kind and valuable answer and link.
In the meantime, I have buy the starter kit for the ADAU 1761 and downloaded Sigma Studio .
So, I have implemented a lock – in design into the ADAU and the first tests are very encouraging.
The phase shift was implemented simply by delay ( since the sinus generator was always constant in amplitude ).
The big problem was the SNR of the ADAU but averaging seems to be a good solution.
My last problem is a little amplitude drift in time at the output of the ref signal DAC ( +- 40 µV ) and consequently at the measured signal but I think that all will be fixed the next week by some tricks.
So, I know that the ADAU is not intended to do what I make but I must say that this wonderful little chip is incredible and that I am verry pleased to use it.
Thank you again Brett
Thank you Jean. Feel free to post here if you need additional help.
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EZ Admin
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