ADS7851: Can I use a different VCM than the 2.5V specified in the datasheet

user5018898

Part Number: ADS7851

Hi,

I am using ADS7851 and noticed that in the datasheet the maximum input is 2* Vref or AVdd. I then noticed the VCM is spec'd very tight around 2.5V. If I need to make my common mode voltage 3.3V (or some other voltage other than 2.5V) would I be able to with this device as long as I don't exceed the input voltage range limits? I see that the internal reference is set to 2.5V, is this the limiting factor, as with a different VCM then you reduce your full scale range?

Thanks,

Jose

over 4 years ago

0 Keith Nicholas over 4 years ago

TI__Mastermind 48180 points

Hello Jose,

In order to meet the datasheet specifications, the common mode input voltage must be 2.5V+/-0.1V.

If you drive the inputs outside of the common mode range, the INL, gain, THD, and other specs will be degraded. I'm not sure exactly how much the specs are degraded in this part, but I have seen other 14-16b ADC's perform closer to an 8b ADC level when this specification is not met.

Driving the inputs with a fully differential amplifier, similar to Figure 45, will shift the input common mode to the correct 2.5V output common mode needed by the ADC. The ADS7851EVM user's guide includes the schematics, which can be copied to achieve datasheet results.

https://www.ti.com/lit/ug/sbau217a/sbau217a.pdf?ts=1608562530071&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FADS7851

Regards,
Keith Nicholas
Precision ADC Applications

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ADS7851: Can I use a different VCM than the 2.5V specified in the datasheet