THS3091: output distortion in signal generator application

Hi Will,

the following points come into my mind:

1. Are the relays performing "break" before "make"?

2. R2 and R16 are very high ohmic. This might work with a slow OPAmp but it's critical with the super fast THS3091. The THS3091 should see a much lower source impedance.

3. To have R17 is nice. But each THS3091 should have an individual R17, directly at its input.

4. Do never measure the signal directly at the output pin of THS3091! Even the very low probe's capacitance and the cable can make the THS3091 oscillate by eroding the phase margin due to too high capacitive load. Do always insert a resistance of about 50R, which helps to isolate the output of THS3091 from the capacitive load. Mount this resistance directly at the output pin.

5. Connecting several probes to a high frequency circuit can make trouble due to the additional ground connections. These can form loops and add complex load impedances. Be very careful when connecting more than one probe to a high frequency circuit.

6. Ferrite beads in the power supply decoupling can be critical if they introduce resonances. Add a small series resistance to each ferrite bead to eliminate these resonances.

7. Be very careful when paralleling unequal decoupling caps. If both of them have very low ESR a resonance can be formed which will ruin the decoupling ability at this frequency.

Kai