Temperature indicator circuit with LEDs

[dazz]

Cheers Hero. I really need to learn to read datasheets more carefully.

"Output Load Current... max 50uA"

doesn't get much clearer than that 

A unity gain buffer, made with the cheap and cheerful LM358 will be better.

Wouldn't that opamp need to be full rail to rail output?

[Zero999]

Cheers Hero. I really need to learn to read datasheets more carefully.

"Output Load Current... max 50uA"

doesn't get much clearer than that 

A unity gain buffer, made with the cheap and cheerful LM358 will be better.

Wouldn't that opamp need to be full rail to rail output?

Why do you think you'd need full rail to rail operation?

What range of voltages are you expecting from the temperature sensor? As long as the op-amp's inputs and outputs work over the range of voltages from the temperature sensor, then it will be fine.

[dazz]

Why do you think you'd need full rail to rail operation?

What range of voltages are you expecting from the temperature sensor? As long as the op-amp's inputs and outputs work over the range of voltages from the temperature sensor, then it will be fine.

The sensor output range is 100mV to 2V, but I'm just using the interval between 800mV - 2V really.
I'm confused now. The reason for my concern is that I simulated the buffer with a TL082 (that's the only opamp I have right now) and the output doesn't go all the way to zero. The TL082 datasheet states that the voltage swing for a +/-15V supply is -13.5 to +13.5V which seems consistent with the simulation showing the buffer kicks in when the input voltage gets above 1.5V

If I'm reading the datasheet correctly, the LM358's swing is pretty much the same:
"Input Common-Mode V+−1.5 V
Voltage Range

[Zero999]

Why do you think you'd need full rail to rail operation?

What range of voltages are you expecting from the temperature sensor? As long as the op-amp's inputs and outputs work over the range of voltages from the temperature sensor, then it will be fine.

The sensor output range is 100mV to 2V, but I'm just using the interval between 800mV - 2V really.
I'm confused now. The reason for my concern is that I simulated the buffer with a TL082 (that's the only opamp I have right now) and the output doesn't go all the way to zero. The TL082 datasheet states that the voltage swing for a +/-15V supply is -13.5 to +13.5V which seems consistent with the simulation showing the buffer kicks in when the input voltage gets above 1.5V

If I'm reading the datasheet correctly, the LM358's swing is pretty much the same:
"Input Common-Mode V+−1.5 V
Voltage Range

Yes, the TL082 is unsuitable for this because its inputs don't function close enough to the negative rail.

The LM358 should be fine for what you're doing. The part of the data sheet you've quoted is the maximum i.e. the highest end of the common mode range, which is V+-1.5V. The lower end is 0V and is specified with a single 5V supply. In other words, the LM358 will work with its inputs between the voltage on pin 4 and pin 8 minus 1.5V. In fact the LM358 is slightly better than the data sheet suggests because it will work when its inputs are slightly below 0V, down to about -0.3V but it's not guaranteed to, so be cautious about using it in designs where it inputs can go too negative.

In short you don't need a rail-to-rail op-amp, just a single supply op-amp, which the LM358 is.

[dazz]

Oh, I see.

thanks again, Hero 

[dazz]

I love LTSpice, I just love it

[Zero999]

I love LTSpice, I just love it

I love LTSpice too, but be careful, it can trip you up. It's easy to design a bad circuit which will work perfectly in LTSpice but perform really poorly or not work at all, in real life.

Here's an example.

.asc file
https://www.eevblog.com/forum/projects/unstable-dc-opamp-sinusoidal-signal-centering/msg1714391/#msg1714391

[dazz]

I love LTSpice, I just love it

I love LTSpice too, but be careful, it can trip you up. It's easy to design a bad circuit which will work perfectly in LTSpice but perform really poorly or not work at all, in real life.

Here's an example.

.asc file
https://www.eevblog.com/forum/projects/unstable-dc-opamp-sinusoidal-signal-centering/msg1714391/#msg1714391

I understand that there are limitations to what spice can simulate and I will definitely follow your advice. I'm wondering if swapping the universal opamp for a TL082 would make it fail the way it does in real life

[Zero999]

Yes, the circuit will not work with the TL082, because its inputs need to be a couple of volts from the TL082's negative rail for it to work. A single supply op-amp, such as the LM358 is required for it to work.

[dazz]

Yes, the circuit will not work with the TL082, because its inputs need to be a couple of volts from the TL082's negative rail for it to work. A single supply op-amp, such as the LM358 is required for it to work.

I don't understand why the circuit you posted wouldn't work in RL though. The inputs of the opamps are nowhere near -15V at any time, right?
Also, the simulation I did with the TL082 seems to suggest that it would work simply by adding a coupling cap at the OP output. What am I missing here please?

[Zero999]

Yes, the circuit will not work with the TL082, because its inputs need to be a couple of volts from the TL082's negative rail for it to work. A single supply op-amp, such as the LM358 is required for it to work.

I don't understand why the circuit you posted wouldn't work in RL though. The inputs of the opamps are nowhere near -15V at any time, right?
Also, the simulation I did with the TL082 seems to suggest that it would work simply by adding a coupling cap at the OP output. What am I missing here please?

Please disregard my previous post. It was about the temperature indicator circuit, not the circuit I posted.

The circuit I posted will not work because the two op-amps will not have exactly the same gain and offset voltages, as is the case with component models. U1 is operating as an inverting amplifier, with a gain of 10 and as you can see, is working perfectly in your circuit. The problem is U2 which has no negative feedback and is operating open-loop. In your circuit, the TL082 model must work slightly differently, when the inputs are connected the opposite way, presumably because of its offset voltage, which is taking the output more positive, but it still has the same gain, so the signal is the same amplitude. If you built this in real life, expect this to be worse. U2's output will most likely saturate at either supply rail. It might also output a square wave or a very tiny signal biased somewhere in between the power rails. The behaviour will be unpredictable, because the open loop gain and the offset voltage of U2 is unknown.

[dazz]

Gotcha! Thanks for the detailed explanation. So I guess that's why open loop configurations are used only for things like comparators & ADC's where they're supposed to operate with the output at the rails

[Zero999]

Gotcha! Thanks for the detailed explanation. So I guess that's why open loop configurations are used only for things like comparators & ADC's where they're supposed to operate with the output at the rails

Yes, op-amps are only used open loop in comparator applications. The negative feedback in a linear circuit stabilises the gain, so the op-amp's open loop gain makes little different to the total gain of the circuit.