# Differential Op Amp Schematic (help)

Discussion in 'The Projects Forum' started by DC_Kid, May 25, 2009.

1. ### DC_Kid Thread Starter Distinguished Member

Feb 25, 2008
641
9
i need to build a diff op amp for two different setups (see attached schematics).

Q1 is driven by a PWM signal
R7 is 0.1ohm

i know the basics for diff amp gain as:
when R3=R4 and R2=R5
Vout = R2/R3*(V2-V1) where V2 is on +in and V1 is on -in of amp.

my question is if these two op amp circuits will function exactly the same way? i question the setup that has V1 tied to ground.

do i really need R4 and R5 (i dont really need to divide the +in voltage), just do a simple divided neative feedback like this one here http://www.allaboutcircuits.com/vol_3/chpt_8/5.html. perhaps the +in divider is needed if the voltage there +Vin is too high for the +in, etc.

also, in the 1st pic the load is inductive and Q1 will be protected by a 33v zener. so i guess i need to be careful choosing a amp and perhaps need R4 and R5 to knock that voltage down to the amp...?

do i have this right?

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• ###### op-amp2.jpg
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Last edited: May 25, 2009
2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Maybe it would help us to know why you cannot use a simple single-ended opamp stage to provide the gain you need.

hgmjr

3. ### DC_Kid Thread Starter Distinguished Member

Feb 25, 2008
641
9
if i understand it correctly, because in the high-side circuit the input will settle at 14Vdc. i need to amplify only the difference of voltage passing through the current sense resistor. in the high-side circuit when zero current flows both inputs would be at 14Vdc, diff of zero, and zero out.

single ended would work for the low-side circuit, but i need one amp configuration that can do both high and low side amplification.

Last edited: May 25, 2009
4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
If it's current through R7 you are measuring, and one end is tied to ground, then the voltage on R7 is single-ended.

5. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Ok, so your application is the monitoring of the current in a signal path using a sense resistor.

What is the maximum value of the current you wish to measure?

Maybe you could post the schematic of your circuit in case there are some easier ways to monitor the current.

hgmjr

6. ### DC_Kid Thread Starter Distinguished Member

Feb 25, 2008
641
9
4amps pulsed is about the max current. supply to the amp is +-9v, and i dont care if the output reaches the +rail as i will zener off the output to 5v. however, i do care about the gain because i dont want it to be too sensitive, just enough gain to get 5v output with a 1amp pulse.

the schematic is what i put in post #1. a transitor is pulsed, the load is a inductor, and there is a 0.1ohm resistor in series.

when its on the low side (after Q1) it is, but when its on the high side (before Q1) it is not exactly single ended.

Last edited: May 25, 2009
7. ### DC_Kid Thread Starter Distinguished Member

Feb 25, 2008
641
9
ah, think of it and it already exsists in completed form

the LMP8276 does exactly what i need and is about 4x less than the INA114 i was using. however, the 114 has 50% higher CMR, but in my app this is ok.