LM358 Output saturation at 3.707V. - Electronic Circuits Projects Diagrams Free

debrajdeb · 14th February 2008, 12:40 PM

Hi,

I am working on a temperature controller based on LM35 (Temperature sensor) and interfaced to PIC microcontroller ADC, with 5V as reference for ADC.

To increase the resolution of my signal from LM35, I have added an amplifier -- LM358, with a gain of 11 and supply of +5V and 0V.

My problem is that the output of LM358 (op-amp) saturates at 3.707V. I know that LM358 is not a rail-to-rail Op-amp, but I never expected the output to saturate at 3.707V (when supply is +5V and 0V). I decided to reduce the gain of amplifier so that the output never reaches 3.707V for my application need.

My query --

Where do I get this figure 0f 3.707V output saturation in the data sheet of LM358 (that will help me to figure out such error during design of any other project)?

**Nigel Goodwin** · 14th February 2008, 01:02 PM

I suggest you consult my PIC tutorials, where you can see how I designed around opamp limitations, and improved performance at the same time.

Papabravo · 14th February 2008, 01:12 PM

There is a parameter called "Output Voltage Swing" It has two items in the LM358 datatsheet. Voh is the output high voltage which is always lower than the supply voltage and Vol which is a couple of tens of millivolts.

Note also the "Common Mode Input Range" It is usually specified as V+ minus 1.5 to 2 Volts. Inputs at the rail caus problems also.

picbits · 14th February 2008, 10:03 PM

Use a seperate 7805 with two diodes in the GND leg which will bump up the supply to the opamp by another 1.4 (ish) volts. Feeding the LM358 with 6.4 volts intead of 5v will get you to that magic 5 volts output (well 5.1 volts).

Roff · 15th February 2008, 12:47 AM

Here's the relevant spec.

audioguru · 15th February 2008, 04:10 AM

The graph in the datasheet for the LM358 shows that its output gets 1.2V from the positive supply when it sources no current. Its loss is worse when it sources current.

speakerguy79 · 15th February 2008, 04:23 AM

Just get a R-R op amp from TI. TLV2462 is a good part.

Hero999 · 15th February 2008, 06:14 PM

Just power the op-amp from a higher voltage supply.

The chances are, you're running the regulator from a >9V so it won't be a problem.

picbits · 15th February 2008, 06:57 PM

Quote:

Originally Posted by Hero999

Just power the op-amp from a higher voltage supply.

The chances are, you're running the regulator from a >9V so it won't be a problem.

Hence why I suggested running from a seperate 7805 with a couple of diodes in the gnd leg.

This would bump up the 5v output to approx 6.4 volts but more importantly should give the opamp a swing from around 300mv (assuming he doesn't have a negative power supply feeding it) to around 5.1 volts which is within the spec of the PIC input.

I've done a fair bit of research on the LM358 as I have around 4000 of them sitting on reels in the workshop

Hero999 · 15th February 2008, 07:13 PM

The LM358 can nearly go down to 0V and doesn't require a regulated power supply so another regulator would be pointless.

picbits · 15th February 2008, 07:24 PM

I was thinking more of limiting the maximum output to a 5.1 volts max.

That way you won't damage the microcontroller if the opamp was to output its maximum.

In simulations the LM358 was notoriously unreliable at lower voltage inputs unless used with a negative supply.

I found a workaround by using a 7805 with three diodes in the ground pin and a diode on the output pin before the feedback resistor. This would give me a reasonably reliable 0-5v swing.

Hero999 · 15th February 2008, 09:04 PM

That's a fair point but I thought microcontrollers had diodes to protect them from that sort of thing.

audioguru · 15th February 2008, 09:49 PM

The output of an opamp with its supply voltage too high might have enough current to destroy the input proctection diode of a PIC.

Hero999 · 15th February 2008, 10:35 PM

Not if you use a series resistor to limit it to a safe level.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Title	Starter	Forum	Replies	Latest
Blinking Output Circuit	bee-rad	Electronic Projects Design/Ideas/Reviews	9	16th November 2007 03:21 PM
Output = Input - Problems	adamthole	Micro Controllers	20	24th April 2007 05:09 AM
555 timer output help for a newbee...	WIZARDSV650S	Electronic Projects Design/Ideas/Reviews	6	6th April 2007 05:04 AM
Using PicBasic with Output compare for sounds	mduong77	Micro Controllers	7	29th March 2007 03:29 AM
How to design such output stage	eeglp	Electronic Projects Design/Ideas/Reviews	4	8th April 2003 07:01 PM

14th February 2008, 12:40 PM	(permalink)
debrajdeb	LM358 Output saturation at 3.707V. Hi, I am working on a temperature controller based on LM35 (Temperature sensor) and interfaced to PIC microcontroller ADC, with 5V as reference for ADC. To increase the resolution of my signal from LM35, I have added an amplifier -- LM358, with a gain of 11 and supply of +5V and 0V. My problem is that the output of LM358 (op-amp) saturates at 3.707V. I know that LM358 is not a rail-to-rail Op-amp, but I never expected the output to saturate at 3.707V (when supply is +5V and 0V). I decided to reduce the gain of amplifier so that the output never reaches 3.707V for my application need. My query -- Where do I get this figure 0f 3.707V output saturation in the data sheet of LM358 (that will help me to figure out such error during design of any other project)?

14th February 2008, 01:02 PM	(permalink)
Nigel Goodwin	I suggest you consult my PIC tutorials, where you can see how I designed around opamp limitations, and improved performance at the same time. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

14th February 2008, 01:12 PM	(permalink)
Papabravo	There is a parameter called "Output Voltage Swing" It has two items in the LM358 datatsheet. Voh is the output high voltage which is always lower than the supply voltage and Vol which is a couple of tens of millivolts. Note also the "Common Mode Input Range" It is usually specified as V+ minus 1.5 to 2 Volts. Inputs at the rail caus problems also. __________________ We never have time to do it right; but we always have time to do it over.

14th February 2008, 10:03 PM	(permalink)
picbits	Use a seperate 7805 with two diodes in the GND leg which will bump up the supply to the opamp by another 1.4 (ish) volts. Feeding the LM358 with 6.4 volts intead of 5v will get you to that magic 5 volts output (well 5.1 volts).

15th February 2008, 04:23 AM	(permalink)
speakerguy79	Just get a R-R op amp from TI. TLV2462 is a good part.

15th February 2008, 06:14 PM	(permalink)
Hero999	Just power the op-amp from a higher voltage supply. The chances are, you're running the regulator from a >9V so it won't be a problem. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez And http://www.silicontronics.com, same screen name as here.

15th February 2008, 07:13 PM	(permalink)
Hero999	The LM358 can nearly go down to 0V and doesn't require a regulated power supply so another regulator would be pointless. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez And http://www.silicontronics.com, same screen name as here.

15th February 2008, 07:24 PM	(permalink)
picbits	I was thinking more of limiting the maximum output to a 5.1 volts max. That way you won't damage the microcontroller if the opamp was to output its maximum. In simulations the LM358 was notoriously unreliable at lower voltage inputs unless used with a negative supply. I found a workaround by using a 7805 with three diodes in the ground pin and a diode on the output pin before the feedback resistor. This would give me a reasonably reliable 0-5v swing.

15th February 2008, 09:04 PM	(permalink)
Hero999	That's a fair point but I thought microcontrollers had diodes to protect them from that sort of thing. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez And http://www.silicontronics.com, same screen name as here.

15th February 2008, 09:49 PM	(permalink)
audioguru	The output of an opamp with its supply voltage too high might have enough current to destroy the input proctection diode of a PIC. __________________ Uncle $crooge

15th February 2008, 10:35 PM	(permalink)
Hero999	Not if you use a series resistor to limit it to a safe level. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez And http://www.silicontronics.com, same screen name as here.