Single Supply Op Amp / Voltage Follower

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?

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.

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

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

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.

tocpcs:
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.

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.

MarkT:
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. :slight_smile: IMHO, of course.

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.

Hm, perhaps this particular thread is the best foothold from which to launch a worldwide campaign to change the terminology :wink:

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

if you can find out how much the motor draws from a datasheet or at full stall that would be a big help.

to change the terminology

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

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