Problem with electret mic & INA217 combination

[David Gold]

Problem with electret mic & INA217 - attenuation instead of gain

Hello everyone,


I'm having problems with the combination of the two.

My goal is to record audio speech at low frequencies ( up to about 4kHz ).

I'm using the schematics below to connect my electret mic to a Texas Instruments INA217 Instrumentation amplifier:



The gain equation for this amplifier is : GAIN = 1 + 10,000/Rg, where Rg is the value of the resistor between pins 1 & 8.

I tried to use all sorts of values for the gain. Theoretical gains of 50,100,200 all gave me a smaller peak to peak voltage while recording( attenuation! ).

I didn't have these problems when I connected the mic with a simple operational amplifier like the Texas Instruments TL071. With the
TL071 I recieved very large gains at the output and it was very clean and good.


I have 2 questions:


1)What do you think is the problem with this circuit?
2)In general, are the bypass capacitor values O.K for frequencies as low as 4kHz? Should I use larger capacitance capacitors?

 

[crutschow]

1) You need a dc bias path to ground from the + input of the instrumentation amp. This is true for all amplifier inputs. The capacitor is blocking that path. Try a 10kΩ resistor to ground.

2) The bypass capacitor values you show should be fine.

 

[David Gold]

I've connected the following circuit and it simply doesn't work
It brings out very high frequency oscillations at the output :

 

[crutschow]

Then you likely have either a layout problem or a decoupling problem.

Is the output line located well away from the inputs? Are all leads as short as possible?

Are C3 and C4 connected close the pins on the amp with short leads directly to ground? Do you have a good, low impedance ground (ground plane, large wire or wide trace)?