Instrumentation amplifier and Crystal Microphones

[cal222]

I am trying to amplify the singals from two crystal microphones. I need the signals to be matched as exactly as possible. My employer purchaced the LT1101 and I am unable to get that to work, and I cannot find good application notes. Are there any other chips/amplifiers etc that will work more effectively with an Xtal mike?
Thanks

 

[Lewish]

How about a little more information about the mike. What brand? What model number? Does it require phantom power?

 

[cal222]

Theicrophone is Mouser # 25LM037 and is made by Kobitone. There is no power source for the microphone. You can just hook up the two leads to the scope and you see the waveform. Of course, the signal is centered about zero volts.

 

[Lewish]

OK, my next question is, why are you trying to use a crystal mic? It has poor noise performance and is very sensitive to mechanical stress, including temperature. Also, it will be very difficult to match them.

If you insist on using it, instead of an electret, what is your supply voltage to the LT1101? How do you have it wired? Do you have bias resistors on the inputs to the amp?
Why would you use an instrumentation amp? They are very noisy.

 

[cal222]

I need a very sensitive microphone for my application. Basically I am comparing the amplitudes of the two microphone signals and then using the amplitude of the signal to determine which is nearer to a sound source. We chose an instrumentation amplifier because the signals are small and because they can be matched well. I need the two signals to be as similar as possible. If you know of a better route I would really appreciate it--we have been stumped for quite a long time. The LT1101 has datasheets at linear technology. I am hooking the minus lead of the microphone to the V- and the positive lead of the microphone to the V+. I am powering off of a 9V battery.

 

[Lewish]

Hi CAL, the matching between any 2 crystal mic elements is not going to be very good. You will have to calibrate out the differences. But, you should be able to make them work.
Any Opamp needs some source of bias current on the input pins. In your case, that could be a resistor to ground. Without looking at the datasheet, I can't give you a correct value for the resistor, but try a 1megohm for starts. Put a 1meg resistor between each amp input pin and ground. If that doesn't bring the output up, try a 100Kohm instead.

How are you connecting the output of the inst. amp.? What are you using to determine the output signal level? Are you overloading the amp output? Check the datasheet for available output current. Some inst. amps are quite weak.

A 9V battery is a fine power source, but you are running a single ended supply, and the inst. amp is trying to swing about ground. Connect another 9V battery, such that the negative terminal goes to the V- pin on the amp, the one you currently have grounded, and the positive terminal of the battery goes to ground. This will allow the output signal of the amp to swing about ground.