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TLV320ADC6140: TLV320ADC6140 input DC current

Hardware Engineer
Prodigy 30 points
Part Number: TLV320ADC6140


Regarding the TLV320ADC6140 https://www.ti.com/lit/ds/symlink/tlv320adc6140.pdf :

  1. Fig. 36 in the datasheet shows that the ADC can be DC-coupled. What is the input DC current when the ADC is DC-coupled?
  2. How does the input DC current change with the input DC voltage when DC-coupling configuration is enabled?
  3. What is the ADC's analog highpass corner frequency when DC-coupling configuration is enabled?

  • 0
    TI__Mastermind 27710 points

    Hi,

    Im not sure im fully understanding question but my comments:

    • The input impedance of the device is programmable from 2.5kΩ to 20kΩ (10kΩ-20kΩ for DC) , the max analog input voltage rating is 18V relative to AVSS; you can use this to roughly estimate the threshold for overcurrent protection/degradation. MICBIAS also supplies a load current as well (up to 20mA with overcurrent protection threshold of 30mA) but ultimately dependent on the resistors and microphone(s) used for your input. The current thresholds will be the same irrespective of input configuration and your system will determine the current input. In addition, the recommended operating conditions for your analog input voltage are below. 

    • The cutoff frequency or the HPF at the input is programmable (all pass or up to 0.008*fs) irrespective of AC or DC coupled input configuration.

    Regards,

  • 0
    Prodigy 30 points

    Thank you for your help. Regarding your response:

    "The input impedance of the device is programmable from ... (10kΩ-20kΩ for DC)" <= is it 10 kΩ to ground? Or is it 10 kΩ to some common-mode voltage?

  • 0 in reply to Hardware Engineer
    TI__Mastermind 27710 points

    Hi,

    The d/s states in section 8.3.3 "The value of the coupling capacitor in AC-coupled mode must be chosen so that the high-pass filter formed by the coupling capacitor and the input impedance do not affect the signal content. Before proper recording can begin, this coupling capacitor must be charged up to the common-mode voltage at power-up".

    To answer your question, the input impedance is likely referenced to GND to form the high pass filter.

    Regards,

  • 0 in reply to Daveon Douglas
    Prodigy 30 points

    Sorry, I am not sure if I was clear. I am using the DC-coupling mode. I want to know if the 10 kΩ input impedance you wrote in your reply is 10 kΩ to ground?

  • +1 in reply to Hardware Engineer
    TI__Mastermind 27710 points

    The 10 kΩ impedance is still referenced to GND

  • 0 in reply to Daveon Douglas
    Prodigy 30 points

    I measured the open-circuit voltage at the input and it is at 1.46 V (see schematic below). This means the 10 kΩ input resistance is referenced to 1.46 V, not ground. The 1.46 V makes sense since the op amp usually has common-mode feedback to set its input bias. This voltage is 1.46 V whether the ADC is configured as AC-coupled or DC-coupled. Can you please check with your team?

  • 0 in reply to Hardware Engineer
    TI__Mastermind 27710 points

    Hi,

    I will continue thread within 2 days, allow me some time to double check on internal circuitry.

    Regards,

  • +1 in reply to Daveon Douglas
    TI__Mastermind 27710 points

    Hi,

    While the internal circuitry is not explicitly detailed in the d/s, the analog input may be designed similarly to TLV320ADCx120. In this device the input pin is referenced to (VREF/2) as shown in Working With Analog Inputs in the TLV320ADCX120 and PCMX120-Q1 Family

    Nonetheless, both configurations result in creating a high pass filter for the analog input when AC coupled. For all intensive purposes and DC analyzing the circuit, (VREF/2) virtually acts a GND from a gain perspective. The difference between GND and (VREF/2) is the latter would DC offset the input signal, essentially raising the common mode voltage.

    Hope this helps and thanks for the catch!

    Regards,