# Building a Differential Amplifier PROBLEM need helps !!!

Thread Starter

#### Tài Dương

Joined Jun 15, 2017
10
i use this design to detach battery 1 voltage from 2 battery in series, but i cant get right Vout voltage. i use both 2 Vout from lm358p op amp but voltage from 2 Vout is not the same.
anyone experienced with this design please tell me what i do wrong

#### Ian0

Joined Aug 7, 2020
759
What output voltage do you get?
What is the power supply voltage for your LM358?

Thread Starter

#### Tài Dương

Joined Jun 15, 2017
10
What output voltage do you get?
What is the power supply voltage for your LM358?
output voltage i get with v2=8v, v1=4v is 4,2v and 4,4v for 2 output in lm358
supply voltage is 8v from the battery

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,292
8-=4 is ~4, Which, in theory is correct if ideal. The real OP amp isn't ideal.

What non-ideal issues are causing problems?

#### Ian0

Joined Aug 7, 2020
759
8-=4 is ~4, Which, in theory is correct if ideal. The real OP amp isn't ideal.

What non-ideal issues are causing problems?
That's a LOT of offset. 5mV would be the maximum for an LM358 unless the gain resistors very large

There are obviously two circuits here - how are they connected?
What are the real voltages on the battery? I would doubt that they are both 4.000V

Thread Starter

#### Tài Dương

Joined Jun 15, 2017
10
8-=4 is ~4, Which, in theory is correct if ideal. The real OP amp isn't ideal.

What non-ideal issues are causing problems?
i need to get accurate battery voltage to balance the battery pack

Thread Starter

#### Tài Dương

Joined Jun 15, 2017
10
That's a LOT of offset. 5mV would be the maximum for an LM358 unless the gain resistors very large

There are obviously two circuits here - how are they connected?
What are the real voltages on the battery? I would doubt that they are both 4.000V

i wired like this picture
bat 1 = 3.64v
bat 2 = 3.61v

Vout 1 = 3.62v
Vout 2 = 3.68v
this is excalty voltage in my circuit

#### Ian0

Joined Aug 7, 2020
759
So both circuits measure the output of the top battery, and neither measures the output of the lower one?
Those offsets are rather more realistic and about what you could expect for a LM358 and 1% resistors.
If you need better accuracy (why?) then use an OP07 or TLV07 and 0.1% resistors.

Thread Starter

#### Tài Dương

Joined Jun 15, 2017
10
So both circuits measure the output of the top battery, and neither measures the output of the lower one?
Those offsets are rather more realistic and about what you could expect for a LM358 and 1% resistors.
If you need better accuracy (why?) then use an OP07 or TLV07 and 0.1% resistors.
Yes you're correct, this circuits mesure the output of the top battery, and neither measures the output of the lower one.
I dont have OP07 to test right now, i googled this model, this also call Precision Operational Amplifiers, hope it what i need.
i need accuracy because i want to BMS this battery pack with 2 or 3 battery in series