I ve got a signal that consists of 2 square waves at 2 different frequencies at the kHz range, about 10 Hz apart.I need to determine the relative amplitudes of 2 different known frequencies and I was thinking to build a module based around the AD630 modulator. The ultimate objective would then be to get the signal through 2 AD630s and harvest the 2 distinct DCs, but before doing that I would like to test it with a single AD630 and two frequencies. The circuit is the one from the data sheet (with a different low pass filter, in my case a simple RC, with R=10kΩ and C=3.3μF, and no opamp/noise/attenuator at the input) and +-12V power supply.
As a proof of concept, I am trying to test it using square waves from two arduinos. I pass the arduino pin output through an capacitor to remove the DC component, since my understanding is that AD630 prefers AC coupled signals for both reference and the actual input. My problem is that I am not 100% sure as to what to expect at the output of the low pass filter for the different scenarios, so I would really appreciate if someone told me if the following behaviour is expected.

1. No input & No ref & Out= 9V DC (weirdly it's -9V after an input is introduced)
2. input=0V (actually 10mV) & No ref & Out=10mV DC
3. No input & ref=0V (again 10mV) & Out=200mV DC
4. input=0V (again 10mV) & ref=0V (again 10mV) & Out=400mV DC
5. input=4.4V (+-2.2V),f=100Hz & ref=4.4V (+-2.2V),f=100Hz & Out=-4V DC (same arduino)
6. input=4.4V (+-2.2V),f=100Hz & ref=4.4V (+-2.2V),f=95.2Hz & Out=250mV DC
7. input=4.4V (+-2.2V),f=100Hz & ref=4.4V (+-2.2V),f=100Hz & Out=+-4 AC (~20sec beat) (different arduinos)

Is this behaviour the expected? Especially at cases 3 and 4, the dc output seems to be quite high. Finally, since I am interested only in the relative amplitudes, does my reference need to be a sin wave?

• Do you have a 'scope? Maybe the first thing to do is send a sine wave into the input and a phase shifted sine wave (of the same frequency) into the reference. And look at the output as you change the relative phase. This is the output before any low pass filter. (you could make the phase shiftier from an RC or all-pass opamp circuit with variable R.) Feb 8 '18 at 17:16
• Are the two signals essentially added together and guaranteed to be of different amplitudes? If so, the input voltage could only be a handful of voltages as the two waves went positive and negative. Might a solution involving a digital controller work? Is a start-up time of a few cycles permitted? Oct 10 '18 at 0:42

The ultimate objective would then be to get the signal through 2 AD630s and harvest the 2 distinct DCs

The problem you have is that you won't necessarily have phase coherence between your arduino output square wave and your reference frequency so, as the two drift apart then back together (almost randomly sometimes I expect) you will get a vast variation in the output of each AD630 multiplier.

Some folk solve this by using two multipliers for a single channel and adding the two output signals like so: -

X = $\sqrt{A^2+B^2}$

Now this would produce a constant output signal if your frequencies matched but the phases were not locked-in.

My understanding is that AD630 prefers AC coupled signals for both reference and the actual input

No, that is incorrect. You must use DC coupling or a pull-down resistor to 0 volts after the input capacitor to leach away the input bias currents.

• Phase shift is indeed one of my concerns, but I had the feeling that since the reference and the input signals are essentially the same signal they would be coherent. The input signal path though, has an LED and a photo-diode so I am not sure if this could potentially introduce a phase shift. Feb 8 '18 at 17:11
• I've added to my answer - I didn't notice that you'd AC coupled the inputs. If I were you I'd draw exactly what you are testing so as not to confuse people. Nobody cares about a circuit that is similar to etc... Feb 8 '18 at 17:30