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Hi Team,

My customer and I are a little confused on the way the ADS5474 datasheet describes the full-scale resolution (FSR) with respect to the voltage reference. In our case, we will be driving the Vref pin with an external reference voltage of 2.5V.

Typically, the FSR is directly proportional to the reference voltage; however, Figure 38 leads us to believe that it's inversely proportional. With that said, can you please help clarify the following questions:

1) What’s the common mode level?

2) What’s the full scale amplitude that I can expect?

Thanks,
Mitchell

• Hi Mitchell
One of our ADC experts is reviewing your question and will provide a detailed response soon.
Best regards,
Jim B
• Hi Jim,

Any update here?

Thanks,
Mitchell
• Hi Jim,

Any update here?

Thanks,
Mitchell
• Hi Mitchell,

According to the Figure 42, on the page 23 of the datasheet. When you apply 2.5V as an external ref the Vcm pin's output should be around 3.13V. To get the exact Vcm for your device I would recommend to apply 2.5V as your external ref and measure the output voltage on Vcm pin.

Regards,

Neeraj

• Hi Neeraj,

Thanks. I was under the assumption that FSR is directly proportional to the reference voltage, however, the plot and print in the datasheet express that it's actually inversely proportional. Could you explain?

Thanks,
Mitchell
• Hi Mitchell,

The figure 38 is showing the normalized gain with respect to internal reference 2.4V. This can be used for systems that require the analog signal gain to be adjusted or calibrated, this can be performed by using an external reference. So going from 2.4V reference to 2.7V reference will change your gain by ~1dB. So you can use External reference to change the effective gain instead of change the input signal level.

The negative value means if input signal same amplitude is applied to the ADC's input with 2.4V reference versus 2.7V external reference the ADC will read the input signal as 1 dB lower when using higher external reference(2.7V).

Regards,
Neeraj