LSB and Quantizaion Noise for Differential Input

[justinsong]

I am studying ADC now and understood 1LSB=FS/2^n.

I got confused when input becomes single-ended to differential input.

for example, input common mode voltage is 0.5V and reference voltage is 1V.

then maximum input range which is full scale range is 1V for single-ended.

but when fully differential input is applied, I think input range becomes to 2V for differential input.

so, FS = 1V for single ended, FS=2V for differential input.

1LSB becomes 1/2^n for single ended and 2/2^n for differential input.

quantiation noise is (1/2^2n)/12 and (4/2^2n)/12

is it right??

 

[KlausST]

Hi,

unfortunately it is ADC dependent and therefore you have to look into the datasheet.

Often there is an equation for the expected digital output value depending on input voltage and reference voltage.

A differential input is not necessary +/- input voltage, sometimes it is used for compensating ground bounce (ADS8320).
A reference voltage not always specifies 1:1 the input voltage. Some use a fixed gain factor (fixed gain of 1.5 combined with variable PGA gain: MCP3911).

Always this booring reading of datasheets.....

Hope this helps
Klaus

 

[justinsong]

thanks KlausST.

Unfortunately, I am not looking for a specific ADC product. I am studying about principle. my question could be easy but not more than that.

I was figuring out whether quantization noise changes or not, which means 1LSB changes or not depending on an input type.

Single-ended input range is VFS (from 0 to REF) when input common mode is 1/2*REF. And 1LSB is VFS/2^n. this is written in every book for Data Converters but not mentioned about differential input.

So, I got confused because differential input range(Vin+ from 0 to REF, Vin- from REF to 0) is 2*VFS.
In this case, what is the 1LSB value for n-bit ADC??

 

[KlausST]

Hi,

quantisation noise is a relative value.

for example you have a 10 bit ADC with 0..5V input range
--> Fullscale is 5V and 1 LSB is 5V/1024.

now you have a 10 bitADC with -5V ... +5V input range
--> fullscale is 10V and 1 LSB is 10V / 1024.

So the realtive value between LSB / Fullscale is the same, although the absolute value of the stepsize (LSB) is different.

I see it as independent form input type.

****************
Another point: You have a single ended input ADC, but it is fed by an instrumentation amplifier with differential input. The relative quantisation noise stays the same, even if the amplifier has a gain of 10.


Klaus