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PCM1802: Modify 0dBFS input sensitivity

Mochi
Genius 3865 points
Part Number: PCM1802

Hello,

Our customer is trying to replace A-company ADC with minimum modification of existing EE.

PCM1802 0dBFS input sensibility is 3.0Vpp at VCC=5V but previous ADC has 2.1Vpp.

On the specification input sensitivity is varied by VREF2 voltage, and on application circuit VCC pin connect to VREF2 pin.

Then VREF2 voltage range is limited 4.5V to 5.5V by VCC operating voltage.

Can we connect VREF2 pin to other voltage to adjust input sensitivity?

 

We know this configuration is not recommended on datasheet, it is own risk circuit design but PCM1802 first stage amplifier voltage gain can be simply function of VCC within absolute max spec?

 

Regards,

Mochizuki

  • 0
    TI__Guru 62085 points

    Hello Mochizuki-san,

    You're correct about the operation of VREF2 and the interaction with the full-scale voltage.  Figure 20 shows VREF2 pin as an input to the reference generator block and lowering the input voltage (e.g. with a series 1k resistor as suggested in 10.1.5) lowers the full-scale input voltage slightly.  All indications in the datasheet suggest that VREF2 should be in the range of 4.5V to 5.5V so while in that range there should not be issues.  With a 4.5V regulated reference on that pin the full-scale range would be 2.7Vpp.  

    Another, and more common option is to add a series resistor with the input impedance of 20kΩ to lower the effective input range.  There will be a +/-5% typical variability in the input impedance but adding a 9kΩ in series with the input should reduce the effective gain by 20kΩ/(20kΩ+9kΩ) = 0.689V/V which is similar to the desired 2.1V/3V = 0.7V/V attenuation desired.

  • 0 in reply to Collin Wells
    Genius 3865 points

    Hello Collin,

    Thank you for your prompt suggestion.

     

    Our customer well understood your proposal.

    He will reduce Vref2 to 4.5V keeping within recommended operation range.

    And additional 9Kohms will be increased input signal voltage for 0dBFS, then he will try to increase frontend amplifier gain without series resistor.

     

    Regards,

    Mochizuki