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INA169 and LOG112

jim9654
Prodigy 20 points
Other Parts Discussed in Thread: LOG112, INA169, OPA333

Hello,

Nice forum but please make more categories to post and publish "forum" link on front TI page. I think thousands of people can share info because if we solve our problems, we buy TI products. I had to search on google for this forum.

If somebody can give me a hint, appreciate:

I try to implement a current sensor like in LOG112 datasheet, page 10 figure 12, except I use INA169.
I use 10 milli ohm current sensor, so out current will be 10^-5 of input current (my target 1mA to 10A).

If using 1mA as reference current for LOG112 ( is ok? question #1 ) I will obtain -0.5V to -2.5V voltage out of log amplifier.
Sense is OK (for 1mA I have -0.5V and for 10A I have -0.5V) but I need positive values for output.

So ( question #2 ) can I use operational amplifier inside LOG112 as voltage adder, eg. adding +2.5V from the same reference will shift my output scale to 0... 2V which is  more convenient.

Thanks in advance,
Edi

 

 

 

  • TI__Expert 7185 points

    Hi Edi,

    You correct.  With Vlogout in the range of -.5V to -2.5V, if you ground +IN3 and sum these values into a unity gain resistor ratio (R2 and R1as feedback to -IN3) this will scale the output of A3 to the +.5 to +2.5V range. 

    One thing that is worth mentioning--you are using the INA169 to measure 5 decades of shunt current.  At your lowest current (1mA) you are sensing around 10uV (.01*1mA), which is 50x less the voltage offset of the INA169 (500uV), which means at these lower currents your error will be huge.  You will be better off using a very low offset OPA on the front end (like the OPA333) and coverting it to a current rather than using the INA169.  There is a circuit on page 10 (figure 23) of the OPA333 data sheet that is good for this.  Notice that even though the OPA333 is a 5V device this configuration references the negative rail to a potential such that the it only sees +5V from Vdd to Vss because it is essentially floating.  The Zener diode clamps the supplies to ensure that they never drift output range.  The output FET provides isolation from the higher voltages on the input side and level shifts everything down to whatever the load commands.  You can modulate the scaling on your current through the FET with RG.  Connect the source side of the FET to the input of the LOG112 in place of RL.

    I hope this helps.

    Matt

  • in reply to Matthew Hann
    Prodigy 20 points

    Thank you Nancy and Matt, I was begun to worry if there are some FAE here :)

    Ok, back to technically :

    First I was tempted to use OA to sense high current but I found INA169 and I like'it. In addition I payed a lot of money to Arrow NAC for shipment (I will call them to see why so expensive parts...abt 3x....and shipment than Digikey or Farnell) so I think I will test INA for the moment.

    My app doesn't require so low current sensing actually, it's a clamp protection unit for a high power radio frequency amp. I have a LDMOS offering something about 500W power. I plan to cut it's 48V supply when Id > 16..18A, of course, with some timeout standing in off state. But I thought I also can monitor it's dynamic current (A/D on some MCU) and also to see it's bias current (no power in) in range of 100~500mA more or less.

    Anyhow, interesting suggestion you have, I will consider them. I can post a little schematic with what I have in mind but I think this forum doesn't allow.

    Edi,

  • in reply to jim9654
    TI__Intellectual 1955 points

    Hello,

    Try this to post an image: http://community.ti.com/forums/t/2502.aspx

    Regards,

    nancy!