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# What is the safe common mode voltage range for the LTC2208 ADC to avoid damage, while maximizing SFDR?

Category: Hardware
Product Number: LTC2208
Software Version: NA

The LTC2208 has an output voltage that can be used for providing the common mode voltage to the differential input signal, named Vcm on pin 3. There are some descrepancies in the datasheet regarding what voltage can be used to bias the common mode voltages of the input pin:
https://www.analog.com/media/en/technical-documentation/data-sheets/2208fc.pdf

This table shows that the Vcm output range is 1.15 to 1.35v

This table shows the Analog Input Common Mode voltage range is 1 to 1.5v

Then this figure shows performance data taken with a common mode range of 0.5 to 2.0v.

I'm most interested in maximizing SFDR and the figure shows that SFDR can be maximized with a common mode voltage near 1.6v, which is higher than the maximum shown in the table above. Is it safe to operate with an external supply that provides 1.6v common mode voltage?  If so, can someone provide guidance on the requirements (noise, impedance, etc.) for an external voltage source that would maintain performance equal to that when using pin 3 Vcm.

added question about common mode voltage supply requirements
[edited by: sanjosanjo at 3:43 PM (GMT -4) on 26 Jul 2023]
• Hi sanjosanjo,

Thanks for your interest in LTC2208.

The 1.25 V is the optimize Analog Input Common Mode Voltage. On the other hand the maximum 1.5 V is guaranteed by design, and not subject to test.

As long as the absolute maximum of the Analog Input Voltage is within the range of –0.3V to (VDD + 0.3V) the device will be safe.

Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime.

Regards,

Adrian

• So is the third plot showing that the optimal common mode voltage depends on frequency of operation?  It looks like 1.25v is optimum at 70 MHz, but 1.6v is optimum at 10 MHz.