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# opamp single supply input limitation !

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#### Sharagim

Hi,
Is there any possibility to use single supply OPAMP (rail-to-rail) to amplify 10-100mv signal? or for such a small range we have to use Dual Supply!

As long as the input signal range meets the common mode range, and its lowest value is at least an order of magnitude larger than the input offset voltage, yes it is feasible.

Some opamps like the LM324, LM358, TLE2141 and rail-to-rail opamps have inputs and an output that work down to 0V when the supply is only positive.

Is this AC below ground or biased ?
or DC coupled with bias to raise common mode in range?

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Some Rail-Rail types need CM range to be restricted CM range centered around V+/2 depending on type , Vin offset, noise , idle current, etc.

Hi,

First you need to specify your signal. Absolute voltages, and frequency and waveform..
Then you need to specify supply voltage (s),
And Opamp output voltage.

Then you can start to select an Opamp or a circuit to meet your specifucations.

Klaus

Thanks for you replies.
I tried to use LM358 in single supply mode(5V) with 1.25 as virtual ground.
When the input signal is more than 800mv I see it is working , but when the input signal is less than this or the output is more than 4V I see it is not working.
You can see schematic here.

Hi,

It´s the same as before: Specify your signals first! Then select the circuit and the OPAMP.

***

I imagine a input voltage rising from zero to 5V. (Just imagination, because it may be off the specified operating conditions)

0V input should give 7.5V output because of the gainsetting, but the output voltage is limited by Vcc-1.1V = 3.9V like the datasheet says (TI datasheet Fig. 10).
0.72V input is the point where the OPAMP operates correctely. Output voltage is 3.9V
1.25V input is the point where about no current is through both resistors. Output voltage is also 1.25V
1.6V input is the point where the OPAMP stops to work correctely. It´s output is slightely above 0V
above 1.6V input the output voltage is limited by the lower supply rail (GND)

The equation is: V_out = 1.25V + (1.25V - V_in) * 5kOhms / 1kOhms
Where V_out is within 0.01V ... 3.9V

Klaus

Points: 2

### Sharagim

Points: 2
Yes, the datasheet for the LM324 and LM358 opamps shows that their outputs cannot go higher than about 1.1V less than the positive supply with no load.

You might not want to use the very old LM324 or LM358 for anything because they are noisy, have crossover distortion and perform poorly above 2kHz (bad slew rate).
They have these problems because they were designed to be the first low power opamps (their quiescent current is typically only 0.5mA for two opamps with a 5V supply).

It became clear for me what is my problem.
I see here is one important items which I have to keep eyes on them.
Where I can find these "1.1V" from VCC and "0.1V" from GND in datasheet. I see figure 10 but could you please let me know exactly where these parameters are in datasheets.!

For better understanding op amp and application refer to Malvino Books

Hi,

I see figure 10 but could you please let me know exactly where these parameters are in datasheets.!
Figure 10. Y axis is the voltage (VCC - V_out). It never goes below 1.1V.

So: V_out_max = Vcc -1.1V = 5V -1.1V = 3.9V

Klaus

cant find another op amp able to not use rail to rail

It is figure 8 in the Texas Instruments datasheet of the LM358 that was wrongly copied from National Semiconductor. The maximum output voltage is shown to be about 1.15V less than the supply voltage when the output sourcing current is very low.

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cant find another op amp able to not use rail to rail
Then use a 6V supply voltage so the output of an LM358 can go up to almost 5V.

#### Attachments

• LM358 output voltage when sourcing current.png
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Sharagim

### Sharagim

Points: 2
Hi

@audioguru
Thanks, The bottom left graph is exactely what's figure 10 in my ti datasheet.

****

There are a lot of rail to rail Opamps. What's wrong with them?

The problem is that the OP did never specify any output voltage range.
So we can take a lot of time looking for what we think he needs..

Klaus

Could you please let me know some general purpose rail-to-rail (IN/OUT)!

We do not know which opamps are available in your country. Thousands are available here.
Did you know that a rail-to-rail opamp has a rail-to-rail output only when it has no load? What is your load?

Thanks for replies.
I want to use this opamp output as ADC input for shifting up and down voltage and gain for increase Accuracy and impedance issues, so you see there isn't much of load on output.

Hi

Why keeping the specifications as a secret but asking for help with choosing the right Opamp?

Klaus

Hi,
No secret Klaus!
Just lack of my knowledge.
I tried to make it more general for others also if some one else have the same problem maybe your answers them also. seems not good.Sorry

Hi,

Hi,
No secret Klaus!
Just lack of my knowledge.
I tried to make it more general for others also if some one else have the same problem maybe your answers them also. seems not good.Sorry

I can not believe this. I'm sure you you know what supply voltage(-es) there are available.
Also you talk about it should be used as driver for an ADC. Every ADC has a name and specifications like input voltage range. Or at least a voltage range of interest for your application.
*****
Circuit design is a process that begins with defining the specifications. All the professionals had to learn this.
*****
Baking a cake doesn't start with asking for the right temperature for the oven. First you need to choose what kind of cake you want to bake.

Klaus

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