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Re: current monitor

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On 9/27/2016 2:34 PM, whit3rd wrote:
> On Tuesday, September 27, 2016 at 11:10:13 AM UTC-7, George Herold wrote:
>> On Tuesday, September 27, 2016 at 12:45:44 PM UTC-4, John Larkin wrote:
>>> I want to measure some power rail currents on a PC board, part of
>>> self-test, and it looks like I could use a shunt resistor and an
>>> INA199.
>>>
>>> http://www.ti.com/lit/gpn/ina199
>>>
>>> It's fairly cheap and second-sourced by ON.
>
>> So you can power it from 3V and sense currents on a 26V rail....
>> How's it do that?
>
> Unlike most  instrument ampllifiers, this one doesn't buffer the inputs;
> it takes circa 26 uA input current, through dropping resistors.
> In some sense, this gives it low input impedance (but not when applied
> across a resistor measured in milliohms).
>
> I presume they laser-trim the resistances, to get good common-mode
> rejection.  It would be difficult to match its performance with loose parts.

The dropping resistors don't drop much.  The amp has gain which means 
the feedback and reference resistor are much larger than the input 
resistors.  These values are nominally 5 k and 1 Meg.  So the common 
mode voltage on the inputs to the internal amp would be about 99.5% of 
the input voltage to the chip.  See fig 22 in the data sheet.

-- 

Rick C

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current monitor John Larkin <jjlarkin@highlandtechnology.com> - 2016-09-27 09:45 -0700
  Re: current monitor Syd Rumpo <usenet@nononono.co.uk> - 2016-09-27 18:09 +0100
  Re: current monitor Winfield Hill <hill@rowland.harvard.edu> - 2016-09-27 10:17 -0700
  Re: current monitor George Herold <gherold@teachspin.com> - 2016-09-27 11:10 -0700
    Re: current monitor whit3rd <whit3rd@gmail.com> - 2016-09-27 11:34 -0700
      Re: current monitor rickman <gnuarm@gmail.com> - 2016-09-27 18:49 -0400
      Re: current monitor Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2016-09-28 07:39 -0700
        Re: current monitor Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2016-09-28 07:42 -0700
          Re: current monitor George Herold <gherold@teachspin.com> - 2016-09-28 09:57 -0700
            Re: current monitor Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2016-09-28 10:01 -0700
              Re: current monitor Winfield Hill <hill@rowland.harvard.edu> - 2016-09-29 12:42 -0700
  Re: current monitor papabear546@gmail.com - 2016-09-27 14:17 -0700
    Re: current monitor John Larkin <jjlarkin@highlandtechnology.com> - 2016-09-27 16:36 -0700
  Re: current monitor Johann Klammer <klammerj@NOSPAM.a1.net> - 2016-09-28 00:38 +0200
  Re: current monitor Reinhardt Behm <rbehm@hushmail.com> - 2016-09-28 11:21 +0800
  Re: current monitor Winfield Hill <hill@rowland.harvard.edu> - 2016-09-28 09:52 -0700
    Re: current monitor John Larkin <jjlarkin@highlandtechnology.com> - 2016-09-28 20:44 -0700
  Re: current monitor Robert Roland <fake@ddress.no> - 2016-09-28 22:45 +0200
    Re: current monitor krw <krw@nowhere.com> - 2016-09-29 13:00 -0400
      Re: current monitor "P E Schoen" <paul@pstech-inc.com> - 2016-09-30 18:32 -0400
        Re: current monitor Winfield Hill <hill@rowland.harvard.edu> - 2016-09-30 18:53 -0700
          Re: current monitor Neon John <no@never.com> - 2016-10-01 10:30 -0400
            Re: current monitor Winfield Hill <hill@rowland.harvard.edu> - 2016-10-01 09:13 -0700
              Re: current monitor Klaus Kragelund <klauskvik@hotmail.com> - 2016-10-02 14:07 -0700
                Re: current monitor klaus.kragelund@gmail.com - 2016-10-02 14:54 -0700
          Re: current monitor rickman <gnuarm@gmail.com> - 2016-10-01 10:33 -0400
            Re: current monitor "P E Schoen" <paul@pstech-inc.com> - 2016-10-02 00:01 -0400

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