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[Moved]: OPAMP with zero output voltage low

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kappa_am

kappa_am

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Hi all,
I am working on an instrumental device with a single supply. I need an OPAMP with zero output when it is in the low state. I mean VOUT-VSS=0V or at least below 5mv at 1mA. Is there any chip providing this? if not, is there any method to compensate deviation?

Thanks
 

Re: OPAMP with zero output voltage low

Hi,

Look for "rail-to-rail-output" opamps.
Either you find the "how close to rail" information in a chart or selection guide (maybe online) at opamp manufacturers or distributors.
Else you have to read the datasheets individually.

Klaus
 

Re: OPAMP with zero output voltage low

Thanks for your reply
I have been looking for rail-to-rail OPAMPs. but the minimum output low voltage that I found is about 30mV. I didn't find a reference showing this quantity online. Actually, I got exhausted looking to datasheets, So I decided to post a thread.
do you know any chip with this characteristic?
 

Re: OPAMP with zero output voltage low

Hi,

Please take care about I/O current at this low output voltage.

You say 1mA at 5mV. The 1mA is "sink current"?

Best is if you could provide a schematic with the connected load.
Why do you need this low output voltage?

Klaus
 

Re: OPAMP with zero output voltage low

There won' be any single supply device which can provide exactly 0.00 volts output. Specially if it has to sink some current.

If the requirement is very critical, you may require a small negative supply.
 

Actually the LM324 (LM358) can go to 5mV, esp if you add a 100 ohm pull down on the o/p - this is one of the little known features of these devices - not guaranteed on data sheet unfortunately - but for high volumes you could ask the manufacturer to screen.
 

From what I understand an op amp has a half-bridge for its output stage. Its cousin the comparator has an NPN transistor which simply pulls output low. (A pullup resistor is needed if you want positive output.) If that does not do what you want, suppose you were to try the opposite arrangement... PNP at high side, and resistor at low side? Turn off the PNP, and the resistor pulls to 0V ground.
 

I know of no single supply op amp that can go to within a few mV of ground when sinking 1mA of current, if that is what you need.
What frequency response range do you need?
 

Actually the LM324 (LM358) can go to 5mV, esp if you add a 100 ohm pull down on the o/p.
Click to expand...
It can, but hardly when sinking 1 mA as requested. And the pull-down limits the positive output voltage according to sourcing characteristic.

The technical problem with the requested output characteristic is that OPs are designed short circuit proof if ever possible. This doesn't allow to make the output transistors so large that they achieve 5 ohm or below Rdson in case of the most popular CMOS OPs. So it could be that there's no suitable IC solution.

Actually, I got exhausted looking to datasheets, So I decided to post a thread.
Click to expand...
I think it's O.K. in so far that an Edaboard member might have already solved the problem. But I presume you don't expect others to browse the datasheets for you.

People already started to design alternative circuits (discrete output buffers) for you. To deserve the assistance, you should at least post complete requirements.
 

Thank you all for your response.
People already started to design alternative circuits (discrete output buffers) for you. To deserve the assistance, you should at least post complete requirements.
Click to expand...
I want to use this chip in a precise rectifier. This circuit will rectify the sensed current. The low output is especially important for me because I need to take an important decision when current is very small. the downward circuits draw (peak detector, zero detector, etc) 1mA max.
I think it's O.K. in so far that an Edaboard member might have already solved the problem. But I presume you don't expect others to browse the datasheets for you.
Click to expand...
I do not expect some one do my work, not only it is not rational and out of morality, it shows how an individual can be irresponsible and unrespectful. I post the thread in hope that somebody has seen such a chip in his/her previous works or researches.

What frequency response range do you need?
Click to expand...
DC up to 1kHz


Thanks for your helps.
 

Can you use a small bias voltage -- something where you output a signal between 100mV and +Vdd-100mV? Then you could generate 100mV or other voltages that might be useful for threshold comparisons.
 

yes, the other common method is to make a small -3v psu and power the opamp off that - then 0.00 is easy ....
 

Other possibilities:
1) Drive a discrete transistor with the output of the opamp and change the circuit to suite that
2) Bias the circuit to a 'virtual ground' (though times I've considered this I've realized that a negative supply is just as easy)
 

Here's a precision rectifier that works down to the mV level.
But the output cannot sink any significant current.

Precision Rectifier2.PNG
 

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