Hi @Kravchenko_Audio, the noise is likely due to the small 0.02 ohm sense resistor used in the QA461. The QA461 can drive up to 1.5A, and for sizable currents a 1A load across the 0.02 ohms will deliver a sizeable 20 mV signal. That then goes through a gain of 50 high-side current sense amp (INA199). This is a zero drift amp with 1% accuracy. The aim here is to provide a simple way to sense current, with a single-ended output.
But, there are limitations. First, there’s the frequency response. The INA199 has bandwidth starts to run out of steam around 60 kHz or so.
Second, the noise is about 25 nV/rthz (RTI). In a 20 kHz bandwidth, this is about 3.5uVrms referenced to input if the noise was flat. So, on the output this would be 175uVrms = -75 dBV. But this is for 1 kHz. Take a look at the INA199 noise density versus frequency:
Note the INA199 noise rises very quickly after 5 kHz or so.
In your plot here, you can see the red trace (current) showing just this. At low frequency the current noise is fine, but at 20 kHz we can see it’s quite high. That’s the INA199
So, I think the takeaway is this: If you see the current trace showing fuzz at higher frequencies, then it will result in fuzz on the impedance plot.
Next, think about the currents being measured. In your case, you are driving at -40 dBV = 10mV. And that 10 mV drive is resulting in a 10mV/10 ohm = 1mA current. And in the 0.02 ohm sense resistor, that is resulting in a 20uA current, which is pretty small. If you used a 10 ohm series sense resistor (and a diff measurement into right channel across the 10 ohm sense resistor) and bypassed the internal current sense on the QA461, you’d see the noise and bandwidth improve considerably.
The QA461 current sense will work well for sensing higher currents–from 10’s of mA to 1.5A. But as currents drop below 10 mA, the circuit limitations will start to become apparent. When the QA461 first came out, component testing wasn’t a priority. I think a subsequent QA461 will want a few switchable sense resistors–maybe something like:
| Sense R | Range |
|---|---|
| 0.02 Ohms | 1.5A to 50mA |
| 0.2 Ohms | 150mA to 5 mA |
| 2 ohms | 15mA to 500uA |