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INA282: Output analog signal traverses below ground with single ended SAR input.

Genatco
Guru 55553 points
Part Number: INA282
Other Parts Discussed in Thread: TM4C1294NCPDT, LM5007

Hello again,

Have recently noticed the INA282 output signal at times traverses 200mv or more below ground at the single ended SAR input TM4C1294NCPDT. Doesn't seem possible the INA output signal can even travers below ground when the REF1/2 inputs are tied to ground. Yet the scope captures indicate that is occurring and seems very odd that occurs anywhere in the INA output circuit.

Past efforts to stop false PWM comparator fault trips may be due to current monitors signal traversing far below ground at times? 

https://e2e.ti.com/support/amplifiers/current-shunt-monitors/f/931/t/555537

Do you see anything in the INA output R divider that might cause the signal to traverse below ground?

Perhaps the INA (-14v) rail ability and shunt CMM is picking up FET body diode rectified forward current (di/dt) crossing the shunt? 

BTW: MCU software blanking period is concurrent with low side FET on times, allow sampling of INA output only during reverse shunt current (ground to B+). 

 

CH2: INA output at R8/R9.

  • 0
    TI__Genius 14825 points

    BP101,

    The INA device cannot drive below ground, so already this is an odd behavior.  I'm looking at your schematic and I see you have two different ground symbols.  I wonder if perhaps you are referencing against the wrong one for the output signal?  Are the grounds tied together somewhere?

    Another thing that came to mind is that maybe there is some kind of flyback happening.  A resistor divider, in my experience anyway, doesn't seem like the thing to cause a signal to shoot below ground.  I think it would rather attenuate the signal, so I am hesitant to think it is that.  Have you tried putting a small cap in parallel with R9 to try to filter out these effects?  You might also try removing R8 and directly probing the output of the INA to eliminate it as the single source of the dips, as it may in fact be the combination of the SAR and INA that is causing the behavior.

    Also, forgive if I'm misunderstanding your schematic but it looks like you have the inputs of the INA reversed across the sense resistor.  Is that the case?

  • 0 in reply to Jason Bridgmon
    Guru 55553 points

    Hi Jason

    Jason Bridgmon said:
    I wonder if perhaps you are referencing against the wrong one for the output signal?

    The grounds are common and tied together.

    Jason Bridgmon said:
    Another thing that came to mind is that maybe there is some kind of flyback happening.

    Looks like parasitic transient oscillations in the latest capture below. Much ringing when the FET turns on and large spikes shooting below ground when that event occurs. Some of INA 0vCMM standby output ringing (75-100mvPP roughly 837Khz) seems to be coming from the 3v3 LDO supply on the EK-TM4C1294NCPDT-XL Launch Pad. Perhaps some of the parasitic oscillation originates from the +5v switcher LM5007 buck down +24v prior to feeding 3v3 1 amp LDO TPS73733. There is bypass all over the inverter PCB and 10uf on 3v3 bus feeing 3-INA282's on the 3 phase inverter bridge. 

    Jason Bridgmon said:
    looks like you have the inputs of the INA reversed across the sense resistor.

    Exactly, in the opposite mode inverting input tied to ground there is no output (ratio metric) linear rise following a PWM cycle, 3.3ms or less depending on motor speed. Otherwise there is no ratio metric liner rise at all and only single pulses occur on the INA outputs. Only 2 phases are commutated at any given time and only 1 INA is analog input is triggered/sampled concurrently to low side FET turns on so all the other noise on the delta phase wire is ignored.   

  • 0 in reply to Jason Bridgmon
    Guru 55553 points

    Hi Jason,

    More review of the INA282 output ringing and capture below is same pattern, scope probe all 3 INA outputs into TM4C1294NCPDT ADC analog inputs X11 connector. Similar level ringing pattern is present 3v3 LDO regulator but it seems to originate from the INA amplifiers feeding the TM4C analog inputs.

    Perhaps something about the INA connection method to TM4C is causing an oscillator loop at the analog inputs? Does this 75mv ringing might be causing some internal issues with INA282 when FET turns on?

  • 0 in reply to Genatco
    Guru 55553 points

    This endpoint ringing INA output seems to originate from +24v 17 amp switching power supply and is filtered down by 3v3 LDO to 50mv ringing. These remnant artifacts exist on the PWM outputs indirectly being monitored by INA282.

    The IAN282 seems reactive to a switching power supply current limiting artifact or the electrolytic output capacitors have degraded less than a 100 hours of use. 

    There exists roughly 200mv PP random noise on the +24v switching supply and linear supplies never had such issues.

    Perhaps 50mv PP ring is industry acceptable standby noise these days but in linear power supply days it would have been rejected.