Differential ADC voltage lower than VREF but VINP/VINN higher than VREF

[sben]

Hello,

I need to use the ADC with 600mV internal voltage, ADC Gain stage = 1.

The input signal on VINP vary from 750mV to 1750mV, and input signal on VINN is fixed at 1250mV.

VINP and VINN are both higher than VREF but the resulted differential voltage is +/-500mV.

is the ADC can work in this condition ?

Best regards, 

[Karl Ylvisaker]

Hello,

I need to use the ADC with 600mV internal voltage, ADC Gain stage = 1.

This will not give you the described SAADC range. Please see the Input Range calculation.
With internal reference 600 mV, and gain = 1, your input range will be only 0 - 600 mV.
You will need your range to cover at least 0 - 1750 mV, so you will need to use at least gain = 1/3.

As pointed out by hmolesworth - this configuration should indeed work for this project, since the +-600 mV covers the entire range of VINP, relative to VINN.

The input signal on VINP vary from 750mV to 1750mV, and input signal on VINN is fixed at 1250mV.

VINP and VINN are both higher than VREF but the resulted differential voltage is +/-500mV.

is the ADC can work in this condition ?

Yes, the SAADC will work for this - but you will get a negative value when VINP goes below VINN.

Which internal reference you use for this does not affect the SAADC's ability to measure these currents.
The internal reference is primarily added as a way to avoid the skew of using VDD/4 as the internal reference when measuring the VDD battery voltage.

Could you elaborate more on the requirements of your application? Do you need to monitor for primarily when VINP crosses VINN, or do you need to monitor them continuously?

Best regards,
Karl

[hmolesworth]

I think I disagree, although stand to be corrected. In single-ended mode the input range is indeed limited from 0-600mV as described above but in differential mode not so:

Input range = (+- 0.6 V or +-VDD/4)/Gain

In differential mode the difference is applied to the true differential ADC and so can range from Vin+ 600mV below Vin- to Vin+ 600mV above Vin-, a true span of 1200mV centered about the differential mid point (1250 mV in the question) provided neither input goes outside the GND and VDD limits.

So yes the SAADC should work correctly in the original question.

[Karl Ylvisaker]

Keenly spotted, hmolesworth!
I stand corrected - the original SAADC configuration should indeed work for this project, since the +- 600mV range covers the entire span of VINP, relative to VINN.

I will update my previous comment accordingly.

Well caught! :)

Best regards,
Karl

[sben]

Thank you for your reply, it is a good new for my project

Best regards