> From:         Mike Singer[SMTP:m_singer@POLUOSTROV.NET]
> Sent:         Monday, January 05, 2004 7:00 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [EE]: How to measure light? Phototransistor?

>> John N. Power wrote:
>>
>>       The maximum current into each input is 5mA max at 25C, and
>>       reduces to 3.8 mA at 70C. Within this range, the inputs can
>>       accomodate a perfectly reasonable amount of current.

> Have a look at the LM3900 datasheet, please.
> Mirror Gain is specified only for 0.02 ma and 0.2 ma input current. And
> the difference of Mirror Gain for these two input currents could be as
> big as 5%. For usual light intensity range 2^12=4096 one could suspect
> the difference of Mirror Gain as big as 20 times. Pretty linear, yeah.

> And you can't compress the signal, cause even National failed to
> specify temperature drift of Mirror Gain.

      First of all, the maximum input current is 6 mA, which is 30 times
      0.2mA. This is 1.5 decades. If one decade changes the gain by 5%,
      then the next 1.5 decades changes it 7.5%, not 20 times.
      Second, this amplifier is not meant for precision measurement of
      low currents. I suggested it only for use in a collision detector.
      The fact that there is a 5% maximum change in mirror gain at the low
      end of the scale does not mean that that rate is maintained over the
      entire range. I agree that a 20 times change in gain would be bad. I
      don't believe that it is that bad. The change in mirror gain is only
      given for the range 20uA to 200uA; that indicates that the effect is
      typical of the low current end of the scale.

John Power

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