# LL2263-14 ADC input range question

I'm using LTC2263-14 14 bit pipeline ADC in my application. However, I'm completely confused regarding the input voltage range, reference voltage etc. It has a differential input with an internally generated voltage reference of 1.25V and the ADC generate a common mode voltage of VDD/2 which is 0.9V in my case. As far as I know, I'll should be able to digitize a differential voltage upto 1.25V, right?

However, from page 22 of the datasheet: From the table it seems that when the differential input is 1V or -1V, the ADC hits its limit(2^14-1, 0 respectively). So what's the point of a 1.25V voltage reference if I can only measure up to 1V differential. From my understanding, the ADC should give a full scale reading if the differential input is 1.25V. Also I'm actually using single ended input (grounding AIN- and feeding the signal at AIN+). I'm using 2V input range and from page 18, it says: From the above info, VCM-0.5V and VCM+0.5V, it doesn't seem to provide 2V range. Can someone clarify the above doubts?

Is it okay to drive the ADC with a single ended input, ignoring noise issues etc., and then do adjustments in software as necessary?

From my understanding, the ADC should give a full scale reading if the differential input is 1.25V.

Not for this device.

Also I'm actually using single ended input (grounding AIN- and feeding the signal at AIN+)

That would be incorrect - there are a few example circuits given in the data sheet that allow a single ended input to drive the two inputs differentially.

I'm using 2V input range

A 2Vp-p differential signal is the limit; one of the differential signals is 1Vp-p and the other should be the same but inverted - net effect is a 2Vp-p differential signal.

• Okay. So what's the point of 1.25V ref? – sdk Oct 17 '16 at 13:01
• The voltage reference does not have to match (in some clear numerical way) the range of the ADC. If you mean, "why is there a reference" - because every ADC needs one. – Andy aka Oct 17 '16 at 13:17
• Thanks for the info. So far I've only used ADCs with the full scale deflection matching the reference voltage. – sdk Oct 18 '16 at 3:40
• The point of the 1.25V reference is that a simple silicon circuit can produce a temperature-independent reference voltage, by a well-known mechanism, only if that voltage is set at 1.25V (or a small-integer multiple, because you can connect 'em in series). It's called a 'bandgap reference'; 1.25V is the bandgap (an electron configuration constant) for silicon. – Whit3rd Jan 22 '17 at 6:23