AD8302 is not working

Hello everybody,

i am building a QCM (Quarz crystall microbalance). The concept is similiar to this: I am using a AD9850 DDS Signalgenerator, who generate a sinuidal signal. This Signal should actuate the crystall and then the signal from the crystall and the signal from the dds goes to the AD8032, to measure gain and phase. In place of the crystal i am using a branch of a resistor, capacitor, and inductivity like the Butterworth-Van-*** modell. the supply voltage (3,3V) for the DDS and the AD8302 comes from a teensy 3.6 microcontroller. I upload a picture of my test Enviroment and the signals i can measure with an oscilloscope. I build the circuit like it says in the Datasheet of the AD8302. Unfortunately it does not work correctly. I measure a voltage of 900mV on both outputs (Vmag and Vphs) independend of wether i connect the signals to the AD8302 or not. When i connet the PFLT and MFLT to ground, i measure 15mV on the output pins. The Vref-Pin shows 1.8 Volt. I do not understand what i do wrong, can you guys help me please?

kind regards,


  • 0
    •  Analog Employees 
    on Dec 11, 2020 2:20 AM

    Hello Danil,

    According to the Tek scope screen capture shown, the input signals are 6dB different amplitude, but phase-quadrature. Under that condition, VPHS should be 0.9V, and VMAG should be 0.9V plus or minus 0.18V, depending on which input port has the larger input signal.

    When both inputs are disconnected, the two outputs can often go to 0.9V, because the input amplitudes are relatively equal, and the phase tends to float to the middle of the measurement range when only noise at the inputs.

    To help further test the circuit, the inputs could be connected to two separate signal generators. Make sure the VMAG and VPHS respond as shown in datasheet graphs on pages 6 and 10. For this test, make sure the input amplitudes are always within the specified voltage range, -73 to -13 dBV, which is 224uV to 224mV rms.   -Bruce H.