Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Please Help me to caliberate this Opamp Circuit!

Status
Not open for further replies.

picstudent

New Member
Hai

The circuit attached is the setup I am using to measure current through R15 and R9.(0 to 500ma) OP07 (U2) opamp Amplifies this voltage 10 times and U3 (Op07)offsets this voltage to my ADC's range. (.798 to 3.298) U4 acts as a low pass filter.

The ADC programming and all is ok at the MCU part and reading is Ok for my purpose. My problem Is I find it difficult or not possible to calibrate it for my range.
I have to adjust RV3 RV4 and RV5 to tune amplification and offset. But when I adjust for 0, it is not ok at 500ma Area.

How I should tackle the situation. What is the procedure here to fine tune gain and offset so that my reading is consistent.

Please suggest
 

Attachments

  • opamp.jpg
    opamp.jpg
    62.2 KB · Views: 158

Roff

Well-Known Member
The offset adjustment pins on the OP07 are to be used only for adjusting the internal input offset voltage (a few microvolts) of the op amp. To add an offset to the signal path, you need to sum in a reference voltage into the path.
As an aside, why do you have 2200 ohm load resistors on U2 and U3?
 

picstudent

New Member
The OP07 offset adjustment preset is provided, but it is not used, as not much effect or use in my case.
The reference voltage of 2.5V is summed in the second opamp.

2.2K load resistor is not needed?

thanks
 

Roff

Well-Known Member
The OP07 offset adjustment preset is provided, but it is not used, as not much effect or use in my case.
The reference voltage of 2.5V is summed in the second opamp.
I missed that.:eek:

2.2K load resistor is not needed?
No.

OK, I'll look at it again. What is the purpose of U2? How much gain does it need to have?
 

picstudent

New Member
Thanks for the reply.
I will brief it up.
I have to measure the current flowing through R9 and R15 (parallel). current expected is from 0 to 500ma. So voltage drop there is 0 to 250mVolts. U2 amplifies this 10 times so that the output is 0Volts to 2.5 Volts. My ADC is configured in pseudo differential mode with reference voltage 2.048Volts. So I need to spread out this 2.5volts swing of signal symmetrically on both sides of 2.048V reference- is 0 to 2.5V is to be offset to .798 to 3.298 volts. that offsetting is done by U3. U4 is low pass anti-aliasing filter.
The arrangement is working but can't align the gain and offset adjusting presets correctly so that gain remains 10 and offset is .798 volts.
Hope I am clear.
Thanks for your time.
 
Last edited:

Roff

Well-Known Member
As you have discovered, your gain and offset adjustments interact. Do you have the freedom to change the circuit, other than changing component values? We can use a different topology to make the adjustments independent. It will probably require another op amp.
BTW, the maximum gain on U2 is (R3/R1)+1=(2200/300)+1=8.33. You said it is supposed to be 10.
 

picstudent

New Member
As you have discovered, your gain and offset adjustments interact. Do you have the freedom to change the circuit, other than changing component values? We can use a different topology to make the adjustments independent. It will probably require another op amp.
I cant change circuit as PCB is already done. (Unless there is no other way). otherwise I have to redesign a PCB which is almost 1 sq feet in size. Even if the calibration is a tricky affair in this circuit, I have to stick on if it is possible.
BTW, the maximum gain on U2 is (R3/R1)+1=(2200/300)+1=8.33. You said it is supposed to be 10.
R3 is 2400 ohms and there is a series preset of 500Ohs.(RV4). so more than 10 is possible.

Thanks a lot
 

Roff

Well-Known Member
If you have probe access to pin 3 on U3, you can do the following:
1. Monitoring U1-6, set the gain of U1 to 10.000
2. With the input current=0, adjust RV5 for 0.6049V at U3-3 (You will first have to change R7 to 1800R).
3. With the input current=500mA, adjust RV3 for 3.298V at U3-6 (or at ADC_OUT).
4. With the input current=0, verify that ADC_OUT=0.798V.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top