inverted input for adc

Question

I am using ADC122S625 (Datasheet: http://www.ti.com/lit/ds/symlink/adc122s625.pdf).

The ADC has a differential input per channel. My signal is from a RF devide with 3V peak to peak and 1.5V DC offset. I want to use the ADC in Single-Ended Input Operation. So I have choosen a Reference Voltage of 1.5V. Now my question is what should I use for the Inverted ADC input?

First I thought I will pull it to Ground as a normal Single input ADC would use 0 to Va input range. But after reading the Datasheet (page. 15) it seems that I need to feed the Vref (1.5V) to the inverted inputs.

From Datasheet (copied)::

"Single-Ended Input Operation For single-ended operation, the non-inverting inputs of the ADC122S625 can be driven with a signal that has a maximum to minimum value range that is equal to or less than twice the reference voltage. The inverting inputs should be biased at a stable voltage that is halfway between these maximum and minimum values. In order to utilize the entire dynamic range of the ADC122S625, the reference voltage is limited at VA / 2. This allows the non-inverting inputs the maximum swing range of ground to VA. Figure 35 shows the ADC122S625 being driven by a full-scale single-ended source. Even though the design of the ADC122S625 is optimized for a differential input, there is very little performance degradation while operating the ADC122S625 in single-ended fashion."

Please suggest to clear my understanding about the Inverted input.

Answer 1

On an ADC, the reference voltage Vref is used to establish the full scale voltage, not some middle-ish setpoint. If you're expecting an input voltage of greater than 1.5V, and you'd like the ADC to sample the difference between the input voltage and 1.5V, yes, you would use differential mode, and send 1.5 V to the negative input.

Vref may also need to be set, depending on the ADC (very often there is a default value and the use of the Vref terminal is set through configuration commands). If you want to get the best resolution out of your ADC, you would set Vref to the largest difference between your signal and 1.5V that you expect. If you're not concerned about resolution, you might just use Vcc or the default value (but you should do some napkin calculations to make sure your resolution is good enough)

Answer 2

You say your input signal has 1.5 V DC bias, and that you have chosen a reference voltage of 1.5 V. Connect the - input to this reference, and the + input to your signal and all should be fine.

Basically, it appears that this A/D always converts the difference between the inputs. If you don't actually have a differential signal, then set the - input to the fixed midpoint voltage of the single-ended signal.

Answer 3

Hello Olin Lathrop thanks for your reply. The input signal ranges from (0 -> 1.5 -> 3) so the minimum voltage expected is 0v and max voltage is 3v with the DC offset being 1.5v. Now my understanding was a mid range reference will help me to use the complete resolution. Is 1.5v not a correct choice? Please suggest to use the complete resolution of the ADC.

Also reading the definition of a differential ADC is think the negative input defines the lowest possible voltage level. So if I set the inverted pin to 1.5v does that mean values below 1.5v will not be sampled?