I am using mic preamplifier MAX4063 (see attached schematics). Differential outputs are connected to MIC+ and MIC- pads of GSM. Amplification coeffisient of amplifier is 20. My questions are:
1. Are MIC+ and MIC- real differential inputs? And is this connection right? In open line only noise is present.
2. If microphone is connected directly to MIC+ and MIC- (amplifier is missing) noise is limited but the voice is low – can I increase amplification. Currently it is set to AT#HSMICG=7 and AT#PSEL=1, AT#PSET=6. Also biquad filters are set as notch AT#BIQUADIN=31784,-31769,31784,-31769,30802,31784,-31769,31784,-31769,30802, AT#BIQUADINEX=31784,-31769,31784,-31769,30802,31784,-31769,31784,-31769,30802.
I suppose you don’t need an external amplifier with 26dB gain, since mic gain can be set u to way higher value trough AT#HSMICG.
MIC+ and MIC- are true differential amplifier, and you can connect them a microphone as per attached suggestion schematic.
BTW: are you sure you really need to modify BIQUAD?
Without external amplifier, received soung on remote side is too low for my customer. Max value of AT#HSMICG is 7. How to increase amplification at microphone path additionally? Microphone is connected according to HW users guide – see attachemet.
With external amplifier I have too high noise level that is over voice level. Maxim support engineer claims that amplifier schematics and layout are OK.
Biquad filter values are changed to achieve noise reduction, but without any success.
OK, I will try with your schematics, but advice how to increase amplification and related command.
micg 7 does it means 42dB gain, way higher than your 26dB external gain.
Please try my suggestion ( very low noise sensitivity).
I can tell you that less than 42dB has been used in a automitive solution with a succesfully audio level on microphone, so: how much gain do you need?
Are you sure your microphone is good enough?
Please avoid notch filter with BIQUAD , your gain in noise reduction will be negligible
Obviously I will check your schematics. The problem is that customer provides microphones as “very sensitive” but without any datasheet, so I do not know what amplification is needed.
OK, I will remove from my code commands for biquad filters.
Does AT#PSET=6 give additional digital amplification?
PSET=6 give 6 more dB.
But I strongly recommend to check the microphone behaviour
Yes, suggested schematics works perfect! No any noise from bursts.
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