Post by thread:[EE] rail-to-rail LM358N alternative

Do you need rail to rail inputs or outputs or both? Do you really need the output to go all the way to the 24V rail or is it more that you need it to go all the way to GND? The LM358 is a real "jelly bean" cheap op-amp - it seems strange to want to drop-in something with complete rail-to-rail inputs and outputs. Sean On Mon, Apr 2, 2012 at 7:06 AM, Dumitru Stama <spam_OUTspanacTakeThisOuTgmail.com> wrote: > Hello guys, > can anyone help me by suggesting a rail-to-rail alternative to LM358N ? > I need to use a +24V single power supply and LMC6482 can only go as > high as +15V. > > Thank you > Spanac >

At 08:12 AM 02/04/2012, you wrote: > > LT1678 might work for you, assuming you're using it as an op-amp and > > not as a comparator. > >Yes, as a differential op-amp actually. The only scarry thing about >your suggestion is the price. >Is there a cheaper alternative or almost all rail-to-rail op-amps are >this expensive ? Depends if you need rail-to-rail input AND output, what bandwidth, etc. For example, OPA2171 is only $1.85 in small quantities, is R-R out, and the input CM range includes the lower rail, but only within 2V of the upper rail. High voltage RRIO op-amps tend to be expensive, and because you essentially need more than one input stage to deal with different CM voltages, they often do some funny things as you change the CM voltage (like the offset voltage shifting around a bit at the transitions). BTW, the LTC part might be cheaper if you get it direct from Linear. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" .....speffKILLspam@spam@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com

> Depends if you need rail-to-rail input AND output, what bandwidth, > etc. > > For example, OPA2171 is only $1.85 in small quantities, is R-R out, > and the input CM range includes the lower rail, but only within 2V of the > upper rail. I need it to amplify the voltage drop over a sense resistor. If the sense resistor is 0.1 ohms and i have a 100mA current through it then the voltage across it will be something like 0.01V. I need the opamp to amplify it 15 times and output 150mV. I need its output to be as close as it can to GND, the +24V supply is just a requirement for this circuit, the voltages will never reach that value

{Quote hidden}> -----Original Message----- > From: piclist-bouncesKILLspammit.edu [.....piclist-bouncesKILLspam.....mit.edu] On Behalf > Of Dumitru Stama > Sent: 02 April 2012 15:06 > To: Microcontroller discussion list - Public. > Subject: Re: [EE] rail-to-rail LM358N alternative > > > Depends if you need rail-to-rail input AND output, what bandwidth, > > etc. > > > > For example, OPA2171 is only $1.85 in small quantities, is R-R out, > > and the input CM range includes the lower rail, but only within 2V of > the > > upper rail. > > I need it to amplify the voltage drop over a sense resistor. > If the sense resistor is 0.1 ohms and i have a 100mA current through > it then the voltage across it will be something like 0.01V. I need the > opamp to amplify it 15 times and output 150mV. > I need its output to be as close as it can to GND, the +24V supply is > just a requirement for this circuit, the voltages will never reach > that value. Have you tried loading the output the output of LM358 with a resistor to ground? (e.g. ~10k). They will typically get within ~5mv of ground which will be significantly exceeded by potential input offset voltage issues in this particular case. Cheers Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. =======================================================================

Microchip 602 or 6002 cannot cope with +24V supply voltage. TI's OPA look promising though, i will try to order one for testing > Have you tried loading the output the output of LM358 with a resistor to ground? (e.g. ~10k). They will typically get within ~5mv of ground which will be significantly exceeded by potential input offset voltage issues in this particular case. > Mike I am using it as a differential amplifier and the output is already tied to negative input using a 10k resistor. I don't think it will work

Look on the manufacturer's websites for "High Side Current Sense Amplifiers" or "Current Monitor" amplifiers. Almost all the analog suppliers make them, they are typically low cost, low power differential amplifiers with high CMV specifications, low input offset voltage and moderate to high gain. Many allow the gain to be set with a single resistor. A gain of 15 as you require is right in the middle of the range most of them offer. They're made to do exactly what you're looking for. Matt Pobursky Maximum Performance Systems On Mon, 2 Apr 2012 17:05:48 +0300, Dumitru Stama wrote: {Quote hidden}>> Depends if you need rail-to-rail input AND output, what bandwidth, etc. >> >> For example, OPA2171 is only $1.85 in small quantities, is R-R out, and >> the input CM range includes the lower rail, but only within 2V of the >> upper rail. >> > > I need it to amplify the voltage drop over a sense resistor. If the sense > resistor is 0.1 ohms and i have a 100mA current through it then the > voltage across it will be something like 0.01V. I need the opamp to > amplify it 15 times and output 150mV. > I need its output to be as close as it can to GND, the +24V supply is > just a requirement for this circuit, the voltages will never reach that > value.

> I need it to amplify the voltage drop over a sense resistor. > If the sense resistor is 0.1 ohms and i have a 100mA current through it then the > voltage across it will be something like 0.01V. I need the opamp to amplify it 15 > times and output 150mV. > I need its output to be as close as it can to GND, the +24V supply is just a > requirement for this circuit, the voltages will never reach that value. Then if you really are looking for an op-amp then you looking for the wrong thing. Check out the following devices Linear Technology LT6100 LT6106 (goes to 36V) Analog Devices AD8210 - this does bi-directional current sense, but they have others in the current sense family. Zetex ZDS100x Current Sense devices (they have trimmed this range considerably since being taken over by Diodes Inc). Supertex Inc HV78xx family Current Sense chips (some go to 450V for mains use!). If you really must use an op-amp then maybe Analog Devices AD8675 36V R-R Op-Amp might do, but I know nothing about the pricing. -- Scanned by iCritical.

> Then if you really are looking for an op-amp then you looking for the wrong thing. > Check out the following devices > Linear Technology > LT6100 > LT6106 (goes to 36V) > Analog Devices > AD8210 - this does bi-directional current sense, but they have others in the current sense family. > Zetex > ZDS100x Current Sense devices (they have trimmed this range considerably since being taken over by Diodes Inc). > Supertex Inc > HV78xx family Current Sense chips (some go to 450V for mains use!). > If you really must use an op-amp then maybe Analog Devices AD8675 36V R-R Op-Amp might do, but I know nothing about the pricing. Thank you for your suggestions. I will search for a high side current amplifier too

Hi, Which rail is the current sense resistor closer to? In other words, is one end of the sense resistor connected to gnd? Sean On Mon, Apr 2, 2012 at 10:05 AM, Dumitru Stama <EraseMEspanacspam_OUTTakeThisOuTgmail.com> wrote: {Quote hidden}>> Depends if you need rail-to-rail input AND output, what bandwidth, >> etc. >> >> For example, OPA2171 is only $1.85 in small quantities, is R-R out, >> and the input CM range includes the lower rail, but only within 2V of the >> upper rail. > > I need it to amplify the voltage drop over a sense resistor. > If the sense resistor is 0.1 ohms and i have a 100mA current through > it then the voltage across it will be something like 0.01V. I need the > opamp to amplify it 15 times and output 150mV. > I need its output to be as close as it can to GND, the +24V supply is > just a requirement for this circuit, the voltages will never reach > that value. >