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INA240: Low side INA240 not producing linear output ramp similar to INA282 and produces large +/- ringing pulses.

Genatco
Guru 55553 points
Part Number: INA240
Other Parts Discussed in Thread: INA282,

Have installed 3 INA240 low side PWM inverter on 3 separate tiny PCB very close to 500 micro ohm shunt Kelvin connection (1cm or less) and resistively trimmed output amplitude same as INA282 used.

Most of the low level PWM shunt noise is now filtered out by the 240 but so is the ratio metric linear analog rise from ground completely missing. Both the INA282/240 when REF1/2 tied to ground the output is crossing zero when that is not a described characteristic in datasheet circuit analysis. Perhaps the output rail voltage should start at zero and progress up to positive rail voltage minus some amount for the swing ceiling.

1. Why is there so much output zero crossing (-500mv) or more when REF1/2 are tied to ground?

2. Why is the datasheet not showing a real time shunt signal in the test bench illustration which shows the shunt signal only above ground?

3. Have the +/- inputs of the INA240 been reversed from that of the INA282? 

4. How is the INA240 capture below different than what TI lab might have deemed acceptable analog signal for low side FET monitoring? 

CH2: INA282 has a peak avalanche current ramp (^) shortly after the FET turns back on. I suspect the ringing preceding current avalanche results from the previous FET turn off delta (dv/dt).

INA240 same current scenario but no FET avalanche current transient response is present. Not a big deal if the wire phase current is being measured minus the PWM. Ok but then where is the ratio metric linear rise (REF1/2=0V) off the signal floor?

 

CH2: INA240 zoomed out:

  • 0
    TI__Genius 14825 points
    BP101,
    Please forgive my confusion, are you saying that with increasing current across the shunt, the output of the INA240 is not increasing, like the part is just not working?
  • 0 in reply to Jason Bridgmon
    Guru 55553 points
    Hi Jason,
    Added captures and edited above post. I'm going to capture the shunt signal next but in the past signal crossed zero vector every PWM pulse cycle. The shunt 0.0005uohm full scale is 25/30mv or 50/60amps and the captures are INA240A2, 50v/v.
  • 0 in reply to Jason Bridgmon
    Guru 55553 points

    Hi Jason,

    Working on Saturday has become a norm of late.  Oddly the 3 shunt signals CMM looks exactly like the output of the INA240 ringing and all. Can't say what shunt signal looked like INA282 yet the output appeared much like the Hantek 65 current clamp. Figured must be doing something right seeing the INA282 output mimic the Hantek current probe.

    Have any thoughts?

  • 0 in reply to Jason Bridgmon
    Guru 55553 points

    Hi Jason,

    BP101 said:
    , 3. Have the +/- inputs of the INA240 been reversed from that of the INA282? 

    Yes the monitor inputs of INA240 are reversed relatively with current (load) direction detection with the non-inverted input of INA282 connected to ground.

    Have reversed shunt inputs so INA240 inverted (-) input connects to ground and non-inverted (+) faces the load or B+ supply. The INA282 datasheet said the same instruction but was reversed compared to the INA240 and that has obviously been corrected in INA240.

    Still confused why PWM delta (dv/dt) seems to inflict INA240 as the reverse direction of INA240 inputs captured FET reverse current artifact occurring 175ms intervals but not the forward current shown below. That 175ms FET dv/dt interval is still present even in the capture below notably feedback from other active phases between PWM cycles of this capture.