AFAIR lean AFR will output voltage close to zero. So this can possibly affect sensor warm up with stopped engine when atmosphere air is in exhaust pipes.mck1117 wrote: ↑Mon Mar 28, 2022 9:08 amThe nernst sense is used for two things:
Try testing with a bench PSU set to 0.45 volt with a 300 ohm resistor (the target ESR for an LSU 4.9), and that should work. 100 ohms is also a bit low to test with - 100 ohms means we've massively overheated the sensor. 300-2000 works very well in my testing.
- The DC component is used as feedback for the pump controller. The pump is servoed to maintain 0.45v DC on the Nernst cell.
- The AC component is used for internal resistance measurement of the sensor, which is a proxy for temperature (the sensor is roughly an ideal voltage source in series with an NTC thermistor). The heater is controlled to maintain the set peak-to-peak AC on the nernst sense voltage, something like 50mV peak to peak, off the top of my head.
The difficulty with measuring ESR down near the bottom rail isn't a very big deal, since the sensor has to be very very rich (with an undriven pump - as soon as we drive the pump it pops up to 0.45v). I think this is actually the purpose of the 20uA bias current (copied from the CJ125, in our case a pullup resistor on Un to 3.3v) - when the sensor is high impedance (cold), they pull it up far enough that we can sense the ESR. Once the sensor is hot and the ESR low, we closed loop control the voltage in to the range we know we can sense.
BTW do you now where to get Ri vs t diagram for LSU4.9? I was able to find only for LSU4.2.
Anyway I just don't like diff amps couse it needs high precision resistors, input buffers/low output R sources and so on. For me ac-decoupled inverting or non-inverting opamp is much simpler. If this cascade is used for internal resistance measurement only - I will try non-diff variant.