AD9954: Need to generate a quadrature signal synchronized to the output of the AD9954

Hello.

I have put/found my self in a bad situation, while designing a LCRZ-meter I researched several techniques and originally I was going to use 2 ADCs precisely timed to the DDS output of the AD9954. But since the design calls for a excitation signal with a frequency up to 1MHz I tried to avoid having to deal with a 10MSPS speed system by using a Lock-In Amplifier(LIA), I only learned about LIAs when I where confronted by the possibility to use them to solve the complex impedance measurement and I'm sure you could do it in other way's but to the best of my knowledge I will have to use a Dual PSD(Phase Sensitive Detector) LIA where the 2 PSD's is driven by a square-wave reference signal separated by a 90deg phase difference.

I explored generating the reference signals using the AD9954 sine-wave as a starting point and then by using comparators/XOR-gates/D-type Flip-Flops I generated two square-waves one of which had a phase difference of 90deg referenced from the first square-wave. But a LIA will not tolerate such a circuit due to it adding delays and such to the reference signal's so it is no longer synchronized to the sine-wave excitation signal...

I've talked to people whom have done this sort of design but they where using FPGA's and as such could generate all the required signals from the same source at the same time, I am restricted to microcontrollers and I need 12-bit output DAC in the AD9954 DDS.

I have tried to find another AD DDS IC to be synchronized to the AD9954 to be used to output a reference signal with a 90deg phase shift but I haven't found a suitable one, it would feel weird to be forced to use a second AD9954 to generate the phase-shifted reference signal, they aren't cheap.

Do anyone know of a solution for this situation?

Or have I run into a brick wall and need to turn around and go back to the high-speed synchronized ADC sampling solution?

I would really appreciate any input on this what so ever.

Regards

  • you can use AD9854 instead as it has quadrature signal output.

  • 0
    •  Analog Employees 
    on Jul 29, 2016 4:31 PM

    Other possibilities as well:

    Use an AD9859, for the quadrature signal; it's got much the same architecture (register map, circuitry) but only a 10 bit DAC.

    The AD9958 is a good solution that can be set up in prefect quadrature as well, but if you had cause to need to vary from exact 90 degrees to compensate for any external variance, the two channels have independent phase tuning so that can be easily implemented.  Unfortunately, this is only a 10 bit DAC.

  • I like to looks of the AD9854, if there are no down side in choosing that one I will go with that.

    I am however unsure about one thing, in the excitation sine-wave's signal chain there a at least 3 opamps and probably something like 5 in the end as well as some switchable passive components to adjust gain and switch the current limiting resistor to different values.

    Do I need to design a signal chain for the reference signal that looks just like the signal chain of the excitation signal in terms of opamps or would you think that I may simply measure/observe the excitation signal channel in action on a scope and then adjust the AD9854's phase-shift control register to account for the propagation delays that the excitation signal is experiencing?

    I don't see anything wrong with talking about a LT part since it will before soon be a part of ADI, but LT has a dual comparator named LT1715. The cool thing about it is that it has separate input/output supplies so I can run the input from my ±5,5V supply and then run the output from a 3,3V supply which is a suitable voltage for the switches I have thought to use for the PSD's.

    By the way, I think it's very exciting that ADI is going to buy Linear Technology.

  • 0
    •  Analog Employees 
    on Aug 10, 2016 9:54 PM

    I would you think that you could 'simply measure/observe the excitation signal channel in action on a scope and then adjust the AD9854's phase-shift control register to account for the propagation delays that the excitation signal is experiencing.'

  • 0
    •  Analog Employees 
    on Aug 2, 2018 3:32 PM
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