🚀 go-pugleaf

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Hello Jeff and all others,

Here is a low-cost-solution to convert 0...38mV to 4mA...20mA.  All  you 
need is a quad-opamp and a little bit of passive elements.


Build an inverting amplifier:

     >--------R1--|---R2----| <---- 0V ... 1.6V
                  |         |
 from Source      |---|- \  |
 0mV ... 38mV         |op1--|------R3---|---R4----|
                  |---|+ /              |         | 
                  |    Gain =           |---|- \  |      to current-
                  |    - 42.11              |op2--|----> voltage conv.
                 GND                    |---|+ /         0.4V ... 2V
                                        |                      |
                      U-offset = 0.2V >--    Gain = -1         |
     op3/op4:                                                  |
     --------                                                  |
     Here you must take one of the many voltage-current-       |
     converters. The decition of the kind of the circuit       |
     is given by precision and temperatur-coefficient.   <-----
     You will need one ore two opamps. Therefore for the
     whole circuit you need a quad-opamp.

If  the sourceimpedance is high you must set a high resistance  for  R1. 
The offsetvoltage U-off can deviate because of the inherent offsetvolta-
ge  of the first opamp op1.  You can comprise both function of  op1  and 
op2 to one opamp. But this is not recommendable because the outputvolta-
ge  of  this opamp influences the setting of the  offsetvoltage  if  you 
make it additional to the gain-resistance-network.  Then the  inverting-
input  is  not the "flying" ground in case of  the  noninverting  ampli-
fication!!!!

I hope I can help you something.
Greeting from  Thomas              schaerer@isi.ethz.ch
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