10MHz instrumentation amplifier oscillating.

[zoulzubazz]

Hey guys,

Attached is the figure of a instumentation amplifier which I am trying to get to work. I am trying to amplify a 10MHz signal received by a 2uH center-tapped aircoil. Its running off a 9V battery and a reference voltage of 2.5 V (Vref) is generated by a general purpose op amp. Also an LDO provides a regulated 5V supply to the opamps (LMH6629, 900MHz GBW). Now I am seeing spurious voltages reaching the 5V supply when nothing is connected to the input and also when the coil is connected to the input. Am i missing something here, what extra care should i take to ensure that the circuit does not oscillate. Any ideas thoughts will be very helpful. Thanks.

 

[FvM]

If build as sketched, the circuit doesn't work because the noninverting inputs are floating.

Apart from this point, to allow an estimation of circuit stability, you should tell the resistor values used in your circuit.

Don't know if this the schematic is complete, but there must be bypass capacitors for power supply and Vref.

 

[Audioguru]

Can't you connect the center-tap of the coil to the +2.5V reference and direct-couple the opamp inputs to the coil without the input coupling capacitors? Then the noninverting inputs will be biased properly.

 

[FvM]

Seriously speaking, I didn't yet hit on the idea to make an instrumentation amplifier of GHz OPs. A fully differential amplifier would be my first choice.

The design should work though, if using
- correct bias, either as suggested by audioguru or a resistor network
- state-of-the-art supply bypass
- a solid ground/ground plane
- suitable resistor values
- low inductance and low capacitance layout
- an output series resistor (e.g. 50 ohm) to isolate capacitive loads

 

[zoulzubazz]

Hi,

Thank you very much. As suggested the circuit has been modified as shown in the updated image (R values included). The design is on a two-layer board with the power being handled in the bottom layer and the two ground planes interconnected with several vias. The inamp OPs are decoupled with 100n and 10u caps as suggested in the data sheet, so is Vref with 100nF. I shall make the changes suggested an report back. Thanks again.

 

[FvM]

Did you notice the datasheet part about feedback resistor dimensioning? The favourite Rf value used for the specified gain configurations is 240 ohm. Values in the kohm range are likely creating severe gain peaking or oscillations.

You didn't yet remove the input capacitors in your schematic.

 

[Audioguru]

Make it like this: