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ltc6409 oscillation

Category: Hardware
Product Number: LTC6409

I’ve used the LTC6409CUDB as single ended to differential converter for the AD9694 ADC.  Because of pcb space restrictions the LTC6409 is placed about 2200mil away from the ADC and connected with a 100Ω microstrip transmission line terminated with 100Ω resistor.  We’ve also added cutouts in the ground plane around the LTC6409 and in the ground plane directly under the device.  With this topology we find the LTC6409 oscillates around 1.2GHz (input grounded).

We know the oscillation is caused by either the cutouts or transmission line because of four LTC6409 electrically identical channels one is placed only 600mil from the AD9694 so has no transmission line or cutouts.  There are no oscillations on this channel.

We find the oscillation magnitude can be reduced from 400mVpp to around 100mVpp by increasing the source resistance into the transmission line from 10Ω to 50Ω.  Also increasing the LTC6409 gain to x3 (150Ω input with 450Ω feedback resistors) and changing the feedback cap from 1.3pF to 10pF helps.

In the schematic shown below the transmission line starts after R53/R54 and ends at R128.  All the ADC input components (R55, R56, C53, C54 and C55) are grouped together at the ADC input.  Any idea on route cause of this oscillation?

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  • Hi Mae

    The LTC6409 input is driven by a AD8336 VGA at R36.  The overall circuit amplifiers TIA pulses but for all this testing the TIA output was dc (no light).  We also opened the VGA output pin and grounded R36 -- this had no effect on the oscillation behaviour.

    We found in general the LTC6409 is very sensitive to any reflections on the transmission line.  The oscillations can be stopped as I mentioned above by pulling the 1.3pF capacitor and increasing the gain to x3.  But cooling the LTC6409 to around 10C causes the oscillations to start up again on some parts.   More testing found replacing the D4/D5 clamp diodes with 44pF capacitors stabilized the LTC6409 across all temperature.  This is not ideal however as it impacts high speed performance of the signal chain.