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Topic: Single Supply Op Amp / Voltage Follower (Read 2419 times) previous topic - next topic

tocpcs

Jan 10, 2013, 12:30 pm Last Edit: Jan 10, 2013, 01:16 pm by tocpcs Reason: 1
I'm looking at the TLC2272 (or alternatives). What I need is a single supply Op Amp, capable of operating at 5V, and able to Follow a voltage from gnd to +5V.

Looking at the TLC2272 datasheet - it's noted the recommended operating condition is to VDD - 1.5V (so I can only expect an input of 3.5 at the output in voltage follower mode)?
I'm looking for it to have a high input impedance (no interference on circuit being monitored).

I think I can see the TLV2371 op amp as doing it - the datasheet shows that the common mode input voltage range is 0 to VDD.
I take it to mean that if I power it with a single 5V supply, and place 5V on the input, and got the DMM, I'd expect to read 5V on the output.
I think 1000Gohms means 1000 giga ohms of input resistance - so this would mean it would not load the circuit being monitored, therefore ensuring it is as if a DMM was monitoring it..

Am I right with that?

dhenry

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I take it to mean that if I power it with a single 5V supply, and place 5V on the input, and got the DMM, I'd expect to read 5V on the output.


No.

The same to your next question.

TI has a good app note on opamps, "OpAmp for everyone". Google it.

Gibson486

you are looking for a rail to rail op amp.... they do not go fully to Vdd, but it is pretty close.

tocpcs

It needs to be single supply too - I thought TLV2371 was a good pick?
Why not?

dhenry

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I thought TLV2371 was a good pick?


It is a great pick if you need a slow opamp with high input impedance, low idle current and reasonably good output (voltage) swings;

It is a terrible pick if you need to output lots of current, with extremely close-to-rail swings, or temperature stability, etc.

Whether it is a good pick depends on how you intend to use the opamp.

MarkT


It needs to be single supply too - I thought TLV2371 was a good pick?
Why not?


There's no such thing as a single-supply opamp, all opamps have a single supply!

Sometimes people call op-amps that operate down to the negative rail "single-supply" but that's really
sloppy terminology (invented by marketroids I bet).

Many opamps are genuinely rail-to-rail in the input circuit these days, but rail-to-rail outputs are
less common.  Even a genuine rail-to-rail output opamp won't drive to the rails at anything like full power
or bandwidth - there are a few with dual output stages (bipolar and MOSFET) that do better than
the pack though.  Most designs are a compromise between competing desires for low-noise, high
bandwidth, low power, high open-loop gain, low input-offset voltage and rail-to-rail output drive.
[ I won't respond to messages, use the forum please ]

oric_dan

Quote
Looking at the TLC2272 datasheet - it's noted the recommended operating condition is to VDD - 1.5V (so I can only expect an input of 3.5 at the output in voltage follower mode)?
I'm looking for it to have a high input impedance (no interference on circuit being monitored).

Stated more accurately, the d/s refers to that as the "Input voltage range", and yes, it will
only track voltages up to Vdd-1.5V. So, the outputs are "rail-to-rail" [or pretty close], but
the input range is only rail-to-(rail-1.5V). However, the TLV2371 is rail-to-rail on both inputs
and outputs, so that's what you want.

And Mark makes a good point. You will only get close to rail-to-rail output if the opAmp
output is not loaded down. CMOS opAmps have typical output impedances somewheres
around 100 ohms or so, so a 1K load would drag the output down by 10%.

CMOS opAmps are not the fastest in general, but with 3 Mhz gain-bandwidth product, the
TLV2371 is fast enough for most microcontroller apps. Especially as a follower amp with
unity-gain, the bandwidth BW = GBW/Gain = 3Mhz/1 = 3 Mhz.


SirNickity

There's no such thing as a single-supply opamp, all opamps have a single supply!


Ehh... while I see your point, I take exception with the wording.  I'm sure this is obvious to most, but it's extremely common to use the phrase "single supply" to refer to Gnd/Vcc supplies, and "dual supply" to refer to V+/V- supplies, where Gnd is presumably in the exact middle to support symmetric AC signals.

They both use a single pair of supply pins, sure, but one is made from a single PSU rail, the other from dual rails, hence the difference in terminology.  You'll confuse the heck out of a new guy for insisting differently.  :-)  IMHO, of course.

MarkT

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Ehh... while I see your point, I take exception with the wording.  I'm sure this is obvious to most, but it's extremely common to use the phrase "single supply" to refer to Gnd/Vcc supplies, and "dual supply" to refer to V+/V- supplies, where Gnd is presumably in the exact middle to support symmetric AC signals.


Yes, but that's nothing to do with the op-amp, that's the circuit.  Of course in a single supply circuit you either have to create
a virtual signal ground (or sometimes get away with using the negative rail as signal ground), but the same opamp can function
in either context.
[ I won't respond to messages, use the forum please ]

bot-thoughts

Hm, perhaps this particular thread is the best foothold from which to launch a worldwide campaign to change the terminology ;)
http://www.bot-thoughts.com/

tocpcs

I'm sure I can forgive an error of 0.1V if the input is 5, and output is 4.9.

bot-thoughts

if you can find out how much the motor draws from a datasheet or at full stall that would be a big help.
http://www.bot-thoughts.com/

dhenry

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to change the terminology


I don't know if the opamp cares about terminology.

retrolefty

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I'm sure this is obvious to most, but it's extremely common to use the phrase "single supply" to refer to Gnd/Vcc supplies, and "dual supply" to refer to V+/V- supplies, where Gnd is presumably in the exact middle to support symmetric AC signals.


Keep in mind that one can power a op-amp with non equal 'dual supply' values, such as running at +5vdc and -1v, the 'dual supplies' do not have to be of equal magnitude . This is often done where it's important that the output voltage can reach 0 vdc without having to use 'rail -to-rail' op-amps which can approach zero output when powered from say a single +5vdc voltage but can't actually reach true 0.

Lefty

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