Does your sine meet the following Vdda >=Vsine >= Vss, ie. there
is no negative portion of sine below ground ?
Also keep in mind the the transfer function is
Vout = Vref + (Vin – Vref) × Gain
What is the sine frequency ?
the sine work in 12hz with vpp=0.5 and average value = 2.9
Use the equation, Vout = Vref + (Vin – Vref) × Gain, measure the peak values of Vin and Vout.
and see if the equation predicts what you are seeing.
I set up something close to your case and found my board Vdda was causing a problem, at a
G = 4 the output was saturating because my Vdda was 4.3 V (I need to figure out why this,
So I cut the offset of the sinewave in to 2.00 V, and got the predicted output
for G = 4.
I try it. The problem about gain is form vref. I use internal vref (2.5 V) from PSoC and it wasn't constant if i connect directly on PGA pin.
is it normal?
The Vdd / 2 Vref internal is a R divider off of Vdd. Vdd can be noisy if you
do not have good decoupling, and that noise would appear on the Vdd / 2
reference. Also references tend to be hi Z, so you need to use an OpAmp
configed as a buffer to buffer Vref to the PGA gain setting R input.