Hello,
An OPA2322AIDR is being using in the attached schematic as a charge amplifier for a piezoelectric receiver array.
With the following parameters:
- Input 50ohm-terminated to ground (i.e. no signal input)
- Powered by 3V from a bench power supply with 0.1uF bypass cap
- 1/2Vcc (1.5V) reference voltage at non-inverting input with 0.1uF bypass cap
- RC feedback network with 1pF 50V cap and 1Mohm resistor
- 10pF 50V input capacitor on opamp
- 1 pole RC filter between op amp and VGA of R=40.2ohm and C=1nF 50V
- VGA input jumper (J35) is open
- VGA (U9, AD8338) gain set to 0.8V (approx. 64dB)
When probing the differential output of the VGA at J34 the following FFT is observed:
It's believed that this is a single spur and a harmonic of that spur. Turning off superfluous lab equipment doesn't seem to have any effect.
When using a the OPA4322 in a nearly identical situation, this set of spurs also appears but are slightly shifted up in frequency to approximately 6.5 and 13.1MHz.
These spurs do not appear when the AD8338 is measured with its inputs shorted together and the charge amplifier configuration disconnected from it.
These spurs do not occur with similar devices such as the AD8648.
Any thoughts or ideas on what may be causing this unwanted distortion would be greatly appreciated!