ADS8920BEVM-PDK: ADS8920BEVM-PDK - single ended to differential input

Dejana Vukovic

Intellectual 845 points

Part Number: ADS8920BEVM-PDK

Hello everyone,

I am working with ADS8920BEVM-PDK evaluation board.

Link to evaluation board:

http://www.ti.com/tool/ADS8920BEVM-PDK?jktype=tools_software#technicaldocuments

Inputs of this board are differential. However, output of mycrophone that I need to connect to this piece of equipment will be single ended.

Therefore, I would probably need some kind of differential amplifier to convert my signal from single ended to differential.

I would very much appreciate it if someone could advise me with which piece of equipment should I go in order to make this conversion?

Thank you very much in advance!

Kind regards,

Dejana

over 4 years ago

0 Keith Nicholas over 4 years ago

TI__Mastermind 36240 points

Hello Dejana,

You are correct; the ADS8920EVM only accepts differential inputs. Please take a look at this Analog Engineer's Circuit that shows how to go from a unipolar single ended signal to a fully differential signal.

http://www.ti.com/lit/an/sbaa264a/sbaa264a.pdf

This specific circuit will take a 0V to 3.3V input and convert it to a +/-3.3V differential signal, but you can use the same circuit to convert a 0V to 5V up to +/-5V differential by changing Vocm from 1.65V to 2.5V.

Regards,
Keith Nicholas
Precision ADC Applications

0 Dejana Vukovic over 4 years ago in reply to Keith Nicholas

Intellectual 845 points

Hello Keith,

I have noticed that this example suggests schematics for conversion of unipolar single ended to unipolar differential signals.

I have also read this document http://www.ti.com/lit/an/sbaa246a/sbaa246a.pdf which suggests schematics for conversion of bipolar single ended to unipolar differential signals.

Is it possible to find schematics that does suggest schematics for conversion of bipolar single ended to bipolar differential signals?

Kind regards,

Dejana

0 Keith Nicholas over 4 years ago in reply to Dejana Vukovic

TI__Mastermind 36240 points

Hello Dejana,

The same circuit that you referenced above can be configured for bipolar differential outputs as well. You would connect the Vocm to ground, VS+=+2.5V, and VS-=-2.5V. However, you could not drive the inputs of the ADS8920 in this configuration, since it requires unipolar differential inputs.

0 Dejana Vukovic over 4 years ago in reply to Keith Nicholas

Intellectual 845 points

Hello Keith,

In the Quick user guide for this evaluation board I have found the following:

It says - Jumper-selectable 0V and 2.5V input common-mode options allow unipolar and bipolar inputs.

Shouldn't that mean that the evaluation board allows both unipolar and bipolar inputs?

Kind regards,

Dejana

0 Keith Nicholas over 4 years ago in reply to Dejana Vukovic

TI__Mastermind 36240 points

Hi Dejana,

Yes, the evaluation board supports bipolar inputs, but these must be balanced differential bipolar inputs. The board does not support bipolar single ended inputs.

Regards,
Keith

0 Dejana Vukovic over 4 years ago in reply to Keith Nicholas

Intellectual 845 points

Hi Keith,

Thank you so much for your help!

Would the same values for resistors and capacitors work for my ADC?

Kind regards,

Dejana

0 Keith Nicholas over 4 years ago in reply to Dejana Vukovic

TI__Mastermind 36240 points

Hello Dejana,

What is the output voltage range of your microphone? The analog engineer's circuit that I had recommended will take a 0V to 5V input and drive a differential, +/-2.5V output suitable for the ADS8920 when using a 2.5V reference.

If the microphone output is smaller than 5V full scale, then an additional gain stage will be needed.

0 Dejana Vukovic over 4 years ago in reply to Keith Nicholas

Intellectual 845 points

Hello Keith,

This is the specification of microphone that I want to connect:

https://www.benthowave.com/products/Specs/BII-7186Datasheet.pdf

According to it, seems that maximum output voltage is (supply voltage-4)Vpp. Since supply is 9V, it seems that output voltage of it goes up to 5Vpp.

What kind of gain stage could be an option of the received signal is too low?

Thank you for your help.

Kind regards,

Dejana

0 Keith Nicholas over 4 years ago in reply to Dejana Vukovic

TI__Mastermind 36240 points

Hello Dejana,

The original circuit suggestion will work since the peak-to-peak input voltage is 5V. However, the datasheets for the microphones do not state what the DC bias, or common mode, voltage is set to.

You can always force a DC level set by using a DC blocking capacitor on the input to the OPA320. The following circuit should work well to interface the microphone to the ADS8920EVM.

Regards,
Keith

0 Dejana Vukovic over 4 years ago in reply to Keith Nicholas

Intellectual 845 points

Dear Keith,

Thank you very much for your help.

Kind regards,

Dejana

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ADS8920BEVM-PDK: ADS8920BEVM-PDK - single ended to differential input