If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

# How to design synchronous detection circuit to measure a small signal

Other Parts Discussed in Thread: TIEVM-HV-1PH-DCAC

To eliminate the voltage signal with a phase difference of 90 degrees produced by AC sensing in signal transmission line, we want to design a synchronous detection circuit to pick original signal. Do we have any reference circuit or guide?

• Hello Ruixin,

Please provide more details such as the signal waveform and voltage, the suspect interfering signal, any other details about the signal.  A block diagram would be helpful.

Sounds like an interesting project.

~Leonard

• Hi Leonard,

The case is AC Internal Resistance Tester for battery. Through testing the ac voltage level when apply ac current to both sides of battery is the way to test internal resistance. As we know that the internal resistance of battery is usually mΩ level, the max peak-to-peak value of ac voltage we need to measure is 4mV, the min value could be 0.1uV.

As the below picture showed, each line has a ac voltage test line and a ac current line to apply to cathode and anode of battery. The electromagnetic field caused by ac current will cause a sensing voltage with a phase difference of 90 degrees on ac voltage test line.

To eliminate this sensing voltage, I think synchronous detection circuit or Lock-in Amplifier may solve the problem. So I'd like to know if we have any solution or reference design.

Thanks a lot~

Rayna

• Thanks very much for the details, Rayna.  I am routing this to the Battery Group to get their suggestions.

Regards,

~Leonard

• Thanks Leonard! Looking forward to reply!

• Hi Ruixin,

at what frequency do you want to perform the synchronous detection? Doing this is rather simple, provided the frequency is not too high

Kai

• Ruixin,

Unfortunately, CSPS doesn't have much in the way of guides regarding synchronous detection, but the methodology of such a circuit is pretty straightforward as Kai mentions. Do you know what frequency these AC waveforms will operate at?

• Hi Kai,

The signal frequency is 1kHz. And the sensing signal is also 1kHz, but 90 degree phase shift compared to it.

Rayna

• Hi Ruixin,

as the battery impedance is complex or, by other words, the equivalent circuit contains resistances, inductances and capacitances, the phase shift isn't constant. So you would need a dual phase lock-in amplifier. Correct?

Kai

• Hi Kai,

The dual phase you mentioned means that lock-in amplifier has a range of lock-in phase based on reference signal, right?

Rayna

• Hi Ruixin,

dual phase lock-in means that the excitation signal and measuring signal need not to be in phase. If you measure the voltage drop across a pure resistance, excitation signal and measuring signal are in phase. But when you add some inductance in series to the pure resistance, the voltage drop across this series cirucit will no longer be in phase with the excitation signal. This the more the higher the frequency of excitation signal is, because then the inductance will play a more and more dominant role over the pure resistance.

In such applications a dual phase lock-in is used. It has two different signal paths with switching signals which are 90° out of phase. Finally, the geometrical sum of the two output signals is formed SQRT(a^2 + b^2).

Kai

• Hi Kai,

Got it! So we have any reference circuit or solution for dual-phase lock-in ?

Rayna

• Hi Kai,

This is a pre-research issue of our customer, Neware. I'm trying to find some solutions.

Rayna

• Rayna,

One of our targeted end equipments in C2000 MCU is Grid Tied Solar Inverters, where we use a SW PLL to match the grid frequency.  A good example is TIEVM-HV-1PH-DCAC .

Even though the full soln may not be applicable, perhaps the SW PLL portion would be applicable here?  The Code examples are on TIREX here https://dev.ti.com/tirex/explore/node?node=ADdFS7XvzyN061svQtDw.g__5C6SEVO__LATEST

I can pull in some others from the systems team if we think this is the right path on how to adapt this for battery testing.

Best,
Matthew

• Hi Ruixin,

a synchronous demodulator can be built this way:

Kai