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Thread: DC offset on ADC readings on Teensy 3.2

  1. #1
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    DC offset on ADC readings on Teensy 3.2

    We are using Teensy 3.2 to record data from a contact microphone. The microphone is connected to a customized pre-amp, which outputs the voltage between 0- 3.3V.

    What we found weird is , we regularly found a 2.8 V DC offset on the readings from ADC. The signal looks fine except this weird offset at 2.8 V, which made the signal saturated. We also tested the output from the pre-amp with the oscilloscope without connecting to Teensy, which did not have the DC offset.

    Any thought on what may cuase this issue?

    Thank you very much for any comments .

  2. #2
    Senior Member+ Theremingenieur's Avatar
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    Please show the preamp schematic and your wiring diagram. The Teensy ADCs are switching delta-sigma converters, which means that the input impedance varies during a conversion cycle and thus, source impedance and slew rate matter.

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    Here is the Schematic of the preamp and the wiring

    Quote Originally Posted by Theremingenieur View Post
    Please show the preamp schematic and your wiring diagram. The Teensy ADCs are switching delta-sigma converters, which means that the input impedance varies during a conversion cycle and thus, source impedance and slew rate matter.
    Here are the schematic of the preamp and wiring. Thanks a lot for offering help.

    Click image for larger version. 

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    Senior Member+ Theremingenieur's Avatar
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    As I suspected, the source impedance of your Vmid is ways too high! In an application note, NXP states that for precise conversion, the source impedance seen by the ADC has to be smaller than 10kOhm. Thus, make your R8=10k, and R2 and R4 1k each and you will be fine.

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    Quote Originally Posted by Theremingenieur View Post
    As I suspected, the source impedance of your Vmid is ways too high! In an application note, NXP states that for precise conversion, the source impedance seen by the ADC has to be smaller than 10kOhm. Thus, make your R8=10k, and R2 and R4 1k each and you will be fine.
    Thanks a lot for your input. Your suggestion solves our problem. Now we do not have the DC offset. Another issue we found on our data recorded by Teensy was the noise floor was too high compared to the data we collected using ADC via audio card from a mac book. The preamp is the same. We suspect the noise came from the resistors. We are going to put an op-amp as the buffer between ADC on Teensy and the preamp. We think this should address the noise issue. Just in case any one else also runs into similar issues.

  6. #6
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    Basically, higher resistor values are prone to generate more thermal noise than lower values. Thus, a lower source impedance tends rather to lower the noise floor. That’s why I would try first to lower R3 to 22k and use low noise metal film (instead of carbon) resistors everywhere.
    But you can’t naturally compare the Teensy’s relaitevely simple 12bit ADCs to an USB sound card which samples 16 bits and does perhaps still some oversampling to further reduce noise. There’s at least a 12.24dB difference in S/N for the quantization noise.

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