Other Parts Discussed in Thread: LM358, OPA364
Hi all,
I am trying to design a programmable amplifier board. The intent was to be something small with the PGA113 at its heart, being controlled by a PSoC5LP. However, I have noticed that when the gain is set to anything other than unity on the PGA113, there is noticeable distortion on the INPUT signal to the PGA113.
Here is an image of the schematic being used. It is designed with am LM358 preamp with LPF and adding a DC bias at half voltage:
the signal Sig1_Preamp is the signal in question here. I have noticed that when channel 1 on the PGA113 is enabled and the gain is set to 5 or above, what was once a clean signal from the pre-amp circuit becomes noticeably distorted. Those distortions get amplified and are seen on the output of the PGA113. When I disable the channel or switch to the other channel on the PGA113, the signal returns to its normal, clean state. I have more or less followed all of the design recommendations for the PGA113, so I am unsure what exactly could becausing the problem here! It's like the PGA113 is loading the pre-amp circuit despite it having relatively high impedance inputs with very low input bias current!
Any assistance is appreciated.
Some extra info:
Signal input is capacitvely-coupled (ac-coupled) through a 3.3uF cap into the SIG1 input on the schematic. I am using a 100mVpp @ 5kHz sine wave test signal. If I shrink the input to be vanishingly small (so small that it's mostly noise), I can make the lower gain values (2x and 5x) lose the weird noise signal. If it is desired I can provide scope shots with specific input signals.