[SOLVED] LM8272 - Capacitive Loading

[natmag]

I am trying to use the LM8272 Op-amp which is a high output current and unlimited capacitive load Op-amp to drive a 2 LM224 Op-amps and 3 analogue switches, AS1745. The LM8272 is used as a unity gain buffer whose output is used as the supply of the afore mentioned components. When the LM8272 is not connected to the rest of the circuitry it is seen to operate correctly and behave like a unity gain buffer. However, on connecting the rest of the circuitry to the output of the buffer, the LM8272 no longer behaves as a unity gain buffer but a voltage drop of 1.3V develops between the positive, negative and output terminals of the op-amp I've tried adding a series resistor between the output of the LM8272 and the rest of circuitry but to no avail .A voltage drop equal to 2V develops. (For resistor values Ive tried 1Ohm, 51Ohms, 100Ohms, 10kOhms.)

I think the components are somehow loading the LM8272 on connection. How can this be mitigated please help .

https://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=lm8272&fileType=pdf .

 

[FvM]

Did check voltage levels with an oscillocope or average values with a multimeter?
Did you verify correct supply voltages?

Would be better to see a schematic.

 

[natmag]

Voltage levels of supply to LM 8272 are correct ...
I am using a multimeter to check outputs

This is my schematic

 

[:Dark_Force]

I see that the outputs of the LMC6462 Opamps are connected to the outer SMA conductor as an active guard. Is the shielding of the SMA on the other side of the connected cable wired to GND so that the LMC6462 ouputs are effectively shorted to GND?

 

[natmag]

I see that the outputs of the LMC6462 Opamps are connected to the outer SMA conductor as an active guard. Is the shielding of the SMA on the other side of the connected cable wired to GND so that the LMC6462 ouputs are effectively shorted to GND?

On the other side they are connected to the reference electrode of a pH electrode...so 1 BNC is connected to the glass-electrode and the other is to the reference pH electrode

 

[:Dark_Force]

OK, so to wrap things up: With the LM8272 you want to provide a supply voltage of 3.072V to the rest of the opamps and switches (=VCC). If the LM8272 output is unloaded everything is fine. But when you connect the rest of the circuit, what happens exactly? I didn't get it from your first post!

 

[FvM]

I am using a multimeter to check outputs

The most likely explanation is a high frequent oscillation of the amplifier, e.g. due to unsuitable circuit layout. You should check with oscilloscope.

- - - Updated - - -

Although LM8272 is specified to operate with inifinite load capacitance, the datasheet reveals a rather small stability margin. Some parasitic circuit elements, e.g. a small amount of positive feedback generated by ground impedance can make it unstable. As the load current won't be very large, a small isolation resistor (e.g. 5 or 10 ohms) can help, otherwise an additional RC load ("zobel" network) near the LM8722.

 

[:Dark_Force]

The most likely explanation is a high frequent oscillation of the amplifier, e.g. due to unsuitable circuit layout. You should check with oscilloscope.

- - - Updated - - -

Although LM8272 is specified to operate with inifinite load capacitance, the datasheet reveals a rather small stability margin. Some parasitic circuit elements, e.g. a small amount of positive feedback generated by ground impedance can make it unstable. As the load current won't be very large, a small isolation resistor (e.g. 5 or 10 ohms) can help, otherwise an additional RC load ("zobel" network) near the LM8722.

In his first post, natmag stated that he already tried to improve the situation by adding a series resistor of different values, but to no avail.

 

[natmag]

There was a wrong connection with the rest of circuitry and hence LM8272 was working incorrectly. Thanks for all your help, it is now working correctly with the connection change.