TLV320ADC6140: Connect -5V/+5V preamp output to 3.3V ADC

Hi Christophe,

Please wait until our TI engineers return from work holiday Tues. Sep, 6th to answer your question.

Thank you for your patience

Why do you need to connect a preamp to the ADC input. It already is a very sensitive input and can handle very small signal levels. Page 26 shows the connections

The microphone I use is a sound level meter microphone and cannot be used without the associated preamp. It can be installed with a 50m cable or more so that is why there is a preamp.

MIcrophone can accept symmetric +/-6V power. Maybe I can power it so that mic voltages levels -6V / GND / + 6V corresponds to ADC levels GND / +6V / +12V. As it is preamp output with low impedance, maybe we can divide voltages by 4 to adapt to 0/+3V range ?

You can choose a single ended AC Coupled scheme as shown in Fig 27 above.

Connect INM to GND with a 1uf Capacitor. 

If Mic Preamp for your mike uses a +/-6v Supply then Max sine wave it can make is 12vpp. This is a 4vrms signal. In single ended mode the ADC can accept 1vrms signal maximum. A n AC Coupled divider should work.

Use an op-amp configured with a gain of less than unity to reduce the level and apply a DC offset to your input voltage.

Yes that is the idea. Can you propose a reference design for this, as it is not so obvious for me ?

Hi Christophe,

Apologies for the delay, Sanjay or Andy will respond as soon as possible. *For record keeping purposed please wait to respond until either Sanjay or Andy has continued the thread.

Kind regards,

You should be able to couple the signal capacitively into the chip as shown in the scan The DC level is established by the ADC PIN

About TLV320ADC6140 input channel settings, can you confirm what would be the best setup ? single ended, AC coupled, mic input or line input ? 2.5/10/20k impedance ?

It looks that the Microphone preamplifier is an already existing arrangement. This means that it has a established DC Bias Level and has a 12vpp maximum signal that is output  from the pin as a single ended output.

If this is the case then the signal can be capacitively coupled as shown by the diagram i include.

I do not know the current carrying capacity of the Mic Buffer but to linit the load on the Buffer we can speicfy a 10k Load.A 39k resistor in series with the Capacitor should be enough to divide the signal by 4.

Thanks a lot !

In the datasheet, I see that TLV320ADC6140 has a mic input/line input setting, but I found no information on what it does really... Does somebody know how it works ?

Please refer to the diagram. The spec for AC coupled signals should wok for both Mic and line inputs

Yes. The fact is that I don't understand what the registry bit "mic/line input" changes in the behaviour of the ADC, and it seems undocumented...

Thats a valid question. let me check this out and get back on the exact difference.