After some feedback from the DIYaudio forum clearly the INA122 was a killer in this application as no differential signal was used at the output stage. Despite having an INA122 around, I looked at an alternative option with a good op amp I had at hand. The LM833 is a dual low noise op-amp that can do the trick here:
You could reduce the sensing resistor to 1Ω but after simulating it I wasn’t too happy with the response at low current. A 10Ω resistor will provide initial gain of 10. The LM833 has two op-amps which are really handy in this application. One op-amp is used as a buffer/driver to the oscilloscope vertical output. The other op-amp is used instead as a non-inverting amplifier of about x10. The preset (P1) will be used to calibrate the sensing circuit. A switch provides selection between 10mV or 100mV per mA.
The two extents when tracing valves are:
- Power valve: Ia=0..100mA. Y signal = 0..1V (x1 mode)
- DHT (e.g. CX301a): Ia=0..5mA Y signal = 0..0.5V (x10 mode)
We can also connect the input pins (IN+ and IN-) to either the screen or the grid input and hence measure the screen or grid current.
I’m not concerned about the offset in this application as generally the vertical offset of my oscilloscope will be manually adjusted to place zero the beam at the X axis with no signal.
The AD8479 is an 8 SOIC device. Quite tricky to work with (or at least for me). I soldered it on a PCB adapter:
Time for some breadboarding…
