Recently I designed a circuit to reduce an input signal by 10x. The design uses a resistor-divider network and then a unity op-amp. The signal is appropriately scaled but there appears to be a ton of ripple / noise in the output. The input signal is -1V to 1V @ 100 KHz and output is a motion control DAC.
I am using a TI OPA188 OpAmp with a +12V / -12V power supply. On both power supplies I have a 1uF ceramic, 100 uF electrolytic and 10 uF tantalum capacitor, and then directly next to the OPA188 I have 0.1uF decoupling capacitor. The schematic is this (note the part is OPA188 not OPA140):
- Do I need to do something with the power (filter?) from the switching power supplies (TDK-Lambda LS25-12)?
- I do not have a resistor or capacitor on the feedback from op amp output to negative. Does that cause instability? Ditto with the output - I drive the output directly to the remote ADC, no resistor. Error?
- Did I make a poor choice - is this op amp not suited to unity gain buffer?
The noise is periodic, at least 200 mV pk-pk (a deal-killer as noise like that is directly picked up by the ADC control circuit). The output cable is a typical BNC 50 Ohm cable, ~3' long, high impedance ADC controller at the other end.