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ADS7865: Digital conversion influence when AVDD accuracy is rough compared with VREFIN

Hirotaka Matsumoto
Guru 27585 points
Part Number: ADS7865

Hi all

Would you mind if we ask ADS7865?

Could you let us know the relationship between AVDD and REFIN.
-Analog inputs are single end.
-REFIN is sapplied by external referecence voltage. The customer doesn't use REFOUT.
In case of follows condtion, how will be digital conversion?(Does it effect for digital conversion?)

①-1　AVDD=5.2V，VREF=2.5V，Analog input=5.0V
①-2　AVDD=4.8V，VREF=2.5V，Analog input=5.0V

②-1　AVDD=5.2V，VREF=2.5V，Analog input=4.8V
②-2　AVDD=4.8V，VREF=2.5V，Analog input=4.8V

③-1　AVDD=5.2V，VREF=2.5V，Analog input=3.0V
③-2　AVDD=4.8V，VREF=2.5V，Analog input=3.0V

As this qusestion's background, on the customer's ciruit, AVDD accuracy is rough compared with VREFIN.
Therefore, it may occur above conditions ①-1 to ③-2.

Kind regards,

Hirotaka Matsumoto

  • 0
    TI__Guru 54576 points

    Hello Hirotaka-san,

    The ADS7865 specifies 70dB PSRR using the internal reference; and unfortunately, I have no specification when using external reference.  However, if the external reference is a constant 2.5V per the description above, where the external reference and amplifier driver circuit have a high PSRR and remain constant,  it is safe to assume using the 70dB PSRR ADC spec would be worst case.  Also, the change in LSBs codes for a 12-B device is very small in this case.

    Assuming the AVDD +/-200mV change is a DC error in the supply voltage,  the change in ADC output in voltage or codes can be estimated using the PSRR specification,  An approximate estimate shows the code change to be less than +/-1 LSB for the ADS7865 (12-B ADC).  Please see below.

    Thank you and Best Regards,

    Luis

  • 0 in reply to Luis Chioye
    Guru 27585 points

    Luis san

    Thank you so much for your reply!
    We would like to know more detailed information.

    ①-1　AVDD=5.2V，VREF=2.5V，Analog input=5.0V
    ①-2　AVDD=4.8V，VREF=2.5V，Analog input=5.0V
    ->In case pf ①-1, because of REFIN×2=2.5V ＜ AVDD=5.2V, we assume that FS increases.(+FS)
    ->In case pf ①-2, we assume that it wille be saturated

    ②-1　AVDD=5.2V，VREF=2.5V，Analog input=4.8V
    ②-2　AVDD=4.8V，VREF=2.5V，Analog input=4.8V
    ③-1　AVDD=5.2V，VREF=2.5V，Analog input=3.0V
    ③-2　AVDD=4.8V，VREF=2.5V，Analog input=3.0V
    ->In these case, we assume FS will fluctuate.
       Or, if REFIN is fixed value, is there no effect by AVDD voltage?

    We are sorry to ask you again.
    However, our customer would like to know more specific content.

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Guru 54576 points

    Hi Hirotaka-san,

    The full-scale in the conversion result is defined as ±VREF;  where the ADC performs conversions comparing the voltage difference (CHxx+)- (CHxx-) against the reference voltage (VREF).  VREF defines the full-scale voltage and the code result is primarily a function of the reference voltage.  

    However, the device valid input linear range delimited by the AVDD supply, and will only provide valid conversions while VIN does not exceed the condition VIN=< AVDD+0.1V.    Per the datasheet, the device will accept input voltages up to AVDD + 0.1V; providing the expected linear conversion results. Voltages exceeding AVDD+0.1V will be outside the valid linear range of the ADC device.    Please refer to the datasheet Full-Scale differential input range and absolute input voltage specification on page 3 of the datasheet.  If the input voltage exceeds, the AVDD+0.1V condition, the internal ESD protection cells may start to turn on affecting the linearity of the results, where the input voltage is outside the linear range of the device.

    In the cases above, the difference in voltage o between CHxx+ and CHxx- is converted, where the full-scale is always ±VREF=±2.5, and the device works properly for all cases except case 1-2, where the input voltage is outside the linear range, where CHxx+ exceeds the AVDD+0.1V limit.

    Best Regards,

    Luis

  • 0 in reply to Luis Chioye
    Guru 27585 points

    Luis san

    Thank you for your reply.
    OK we got it.

    Kind regards,

    Hirotaka Matsumoto