STM32F303 op amps

modcan · ‎2013-07-04

Posted on July 04, 2013 at 11:52

I am a bit confused about the F303 op amps. If they can be used in inverting mode

won't a positive voltage into the minus input result in a negative output voltage?

Of course it won't happen because they are rail to rail opamps.

I need a summing input for two signals so can I use a negative offset like any other op amp to prevent it from trying to go below ground? It just seems wrong

to have a negative voltage connected to a uC

frankmeyer9 · ‎2013-07-04

Posted on July 04, 2013 at 13:36

I am a bit confused about the F303 op amps.

Perhaps you are a little bit confused by opamps in general. I recommend a good textbook, , for instance, especially the section a bout single-supply circuits.

(Edit: if the forum eats the link again, look there : www.ti.com/lit/an/slod006b/slod006b.pdf)

Of course it won't happen because they are rail to rail opamps.

This has nothing to do with rail-to-rail.

In these single-supply applications, the positive opamp supply is connected with Vdd, and the negative supply with GND. The opamp has no ''ground'', but usually you connect one of the inputs to a ''virtual ground'' of Vdd/2, formed by a resistive network.

When connecting (+) Vin to this virtual ground, and an input signal to (-) Vin for an inverting configuration, the output can become ''negative'' in reference to this virtual ground, but of course not below GND.

...rail to rail opamps.

Take the old 709 opamp, for instance, which was not a rail-to-rail design. Rated for a +-15V nominal supply voltage, it had a output swing of +-12V. That 3V towards the ''rails'' were lost because of the internal circuit design. Rail-to-rail designs reaching output swings just a few 100 milivolts away from the supply voltage.

And BTW, for an opamp supplied with 3.3V, this is an absolute necessity.

modcan · ‎2013-07-04

Posted on July 04, 2013 at 18:12

Don't be so rude with your presumptions about my knowledge of op amps.

I have a very good understanding of typical op amps. You have to admit having op amps

built into a uC is unusual and I was just wondering if there was any special considerations in this case. If you have tried using the op amps in the F303 and can say they behave as typical op amps in every way then that's great. This is the info I'm after.

frankmeyer9 · ‎2013-07-04

Posted on July 04, 2013 at 20:07

Don't be so rude ...

It might be a matter of language, but it wasn't ment to be rude. I did not say analog opamp design is a trivial matter, and I consult that recommended manual too if interfacing a MCU with analog signals.

You have to admit having op amps built into a uC is unusual and I was just wondering if there was any special considerations in this case.

Except ot the fact that these internal opamps are necessarily supplied asymetrically (i.e. single-supply, between Vdd and GND) and with relatively low voltages, no.

The integration of opamps is quite coomon these days.

However, you should note that this is a compromise. They are located on one silicon with ''noisy'' digital logic, and, as said, have to live with low, asymmetric supply voltages. The noise, offset and gain/bandwith performance is certainly affected by these facts, but they are definitely cheaper, and require much less design and layout efforts, compared to external circuitry. And they often have useful features like programmable gain.

modcan · ‎2013-07-04

Posted on July 04, 2013 at 23:45

Ok fair enough.

So configuring it in a typical inverting form I can input 0 to MINUS 3.3V with a pot through a 100k resistor at the minus input and tie the positive input to ground and then put a 100K between the minus and output pins I will get 0 to +3.3V swing from the output or to the ADC in this case? That would be cool if it can do that.

I am not after a vdd/2 configuration as this will be a summing node with several inputs

from pots and jacks each with their own summing resistor.

frankmeyer9 · ‎2013-07-05

Posted on July 05, 2013 at 09:54

As you know, the opamp amplifies the voltage difference between the (+) input and the (-) input. You can bind the reference input to any potential you wish, not only Vdd/2, as long as it's in the [GND .. Vdd] range. Input that would result in output voltages outside that range produce an output clipping.

Because internal components of the opamp are saturating close to the rails, the linearity is slightly reduced there. That increases the THD figures, which might not be of relevance in your case.