[SOLVED] Opamp with good small signal gain?

[nyaknyan]

I'm trying to make hi-side current sensor.
I used circuit from https://www.intersil.com/content/dam/Intersil/documents/an18/an1827.pdf and modified it by using separate power supply, so no need to use rail2rail input opamp, because signal level is only about 1.4v greater than opamps's ground (this voltage is set by two diodes and resistor r3). Also I added zenner diode to protect opamp when there is short circuit in load, because i'll use it for lab power supply and output voltage can reach 60v

In ltspice it worked well, but in real life I used LM358 and found out that it has low gain when differential voltage is low. My self-made shunt resistor has a voltage drop about 10mv per 1A and with 100r and 1k resistors circuit should have gain 10. But it only reaches normal gain (8 because shunt is not exactly 10mv/A, but i calculated like 10mv) when differential input voltage is greater than 6-7mv and can't sense 0-5mv, but I want to have current regulation in 0-1A range too, and can't increase shunt resistance, because power supply will be atx smps based and i can get 50A from it on low voltages

Here is my circuit and results of measuring


What opamp i should use? And I haven't found gain on small differential signal in lm358 datasheet, which parameter I should look at? Can someone advise some cheap opamp with good small differential signal gain, which i could get on ebay?

 

[crutschow]

Your circuit diagram is really too convoluted and messy to readily follow. Please move and spread out the components and/or their designations so that they are all readily visible and not on top of each other. Also do not run wires through the middle of components.

Post your .asc file and I'll try to simulate it.

 

[chuckey]

I am unhappy about R3. V2 is the opamps supply, so the input and output currents have to flow through this. Consider the traditional opamp circuit, centre tapped PSU, load connected from output to "earth" (centre tap of PSU), inputs referenced to earth (centre tap). Now you have R3 connected in series with the input voltages and the output currents to the earth.
Frank

 

[nyaknyan]

Your circuit diagram is really too convoluted and messy to readily follow.

You're right, it's really messy. Here is better version. But there is no problem with circuit, it works in ltspice and in real world too. The problem is that real circuit doesn't work when voltage drop on shunt resistor is 0-6mv, with greater drop it works fine.

I am unhappy about R3. V2 is the opamps supply, so the input and output currents have to flow through this.

Yes, transistor's base current flows through it, but in real circuit i used 3k, and opamp output current trough transistor base shouldn't ever be greater than 1ma, so offset voltage created by that resistor and diodes is always stable


I figured out my problem - it's because of offset voltage. Difference between inverting and non inverting output is 1.8-2.5mv , lm358 doesn't make it 0v, there is offset voltage error. I was aware about offset voltage, but thought it is offset from gnd, and doesn't apply when one signal is for eg. 2.5v and other 2.499v. I had lack of understanding what offset voltage is, now i see that i just need low offset amplifier.

 

[schmitt trigger]

What you really require is a low offset input voltage opamp.
What is happening is that the LM358's Vos fights against the very low signal voltage.
TI has a good selection of low cost and low offset opamps. Go to their website and using the parametric filters choose a device with a Vos between 0.5 and 1 mV. And within your budget.

 

[nyaknyan]

Thanks. I'm going to use OP07 from TI, because it's cheap in my local electronics parts store, has offset only 50-80uV and pins for offset trimming.