I am trying to use this equation for difference amplifier to sense a max current of 75 mA. However in my circuit I have no R4 so which equation should I use? 
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- \$\begingroup\$ You have an R4, it's just infinite. \$\endgroup\$user16324– user163242017-03-18 13:06:10 +00:00Commented Mar 18, 2017 at 13:06
- \$\begingroup\$ Why does the first diagram have a non-infinite R4? Calculate your differential gain. Calculate your common mode gain. \$\endgroup\$Neil_UK– Neil_UK2017-03-18 13:55:37 +00:00Commented Mar 18, 2017 at 13:55
- \$\begingroup\$ So if I substitute with an infinite R4 I get Vout=1/R_1 ((R_1+R_4 ) V_2-R_2 V_1). Is this the right equation? After simplification I get Vout=106.6V2-V1. right? \$\endgroup\$Nemo– Nemo2017-03-18 14:00:30 +00:00Commented Mar 18, 2017 at 14:00
- \$\begingroup\$ Why don't you just follow the (tried and tested) circuit with the given equation? All it takes is for you to add an extra resistor in your circuit. \$\endgroup\$MCG– MCG2017-03-18 14:45:16 +00:00Commented Mar 18, 2017 at 14:45
- \$\begingroup\$ Actually your circuit works ok and the signal ground can be the leftmost ground symbol. I simply miscalculated it. sorry. Your load is floating (neither end at ground) but that is problem only if its exact voltage is needed elsewhere. Vout = I * R1 * (R5 + R4)/R4. Select R5 for right gain. R5 = 65,7 kOhm should be ok. You may need 2...3 resistors in parallel to get it. I deleted the bad answer. \$\endgroup\$user136077– user1360772017-03-18 23:37:22 +00:00Commented Mar 18, 2017 at 23:37
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