# How to design a buffer amplifier for STM32 ADC

So the purpose of this circuit is to buffer a 0-5V signal coming from a potentiometer sensor. The processor is 3.3V tolerate. the sampling switch resistance as 6K. the processor is a STM32F429

are there any issues with what i've done?

• R102 and R103 are pretty small, I'd go with 2k and 4k at least. So ADC1_S is where your potentiometer is coming from, right? But what exactly are you trying to accomplish? Are you trying to make 0-5V to 0-3.3V? -- Also, what's up with the capacitors and diodes, is this something you're making for NASA? Top notch ultra duper super mega safety to the maximum. Aug 11, 2017 at 4:19
• How long will the ADC input sampler be in Track mode, before changing to Hold mode? What # bits are possible with the ADC? What # bits do you require. What is the input current draw by the ADC, from the input-resistors? Math: Iin = Fsample * Csample * Vinmax. At 1MHz and 10pF and 5v, the Iin is 50 microAmps. Aug 11, 2017 at 4:34
• @analogsystemsrf I would say another very important thing is "Will you trimm/calibrate the offset introduced by the buffer circuit?" Aug 11, 2017 at 7:22
• @analogsystemsrf At 1MHz for the potentiometer - lets do 50 conversions per rotation - it is 20k rotations per second. If ones fingers do 3cm per rotation the average finger speed has to be about 600m/s - 2MACH. 20000 sonic booms per second - not bad. if fingers have to change direction in lets say 1/100 of the rotation time : change of the speed = 1200m/s 5*10^-7s acceleration 2.4*10^9 m/s2 - 244648318g - 1MSPS for the potentiometer? Ridiculous Aug 11, 2017 at 20:18

are there any issues with what i've done?

Maybe: -

• The op-amp input offset voltage can be as high as 3 mV so this will tend to add a small error to your potentiometer output but, it's probably OK.
• Input currents are quite small for this device so any volt drop across R101 will likely be negligible.
• The common mode input range IS going to be a problem - when powered from 5 volts, the maximum input voltage is 4 volts typically and your pot could produce 5 volts. I think this sounds like a showstopper.
• The high level voltage output is typically 4.65 with 1 mA load (and your load is going to be more like 6 or 7 mA so this is a showstopper.

So you have a couple of showstoppers but if you use very high values of resistors and form a potential divider on your potentiometer output you might be able to live with the lack of linearity due to loading the pot. Then use your amplifier circuit without the output pot-divider of R102 and R103.

• Okay if i power the op amp with 12V instead will this solve the common mode input range problem? And then youre saying the current through R102 and R103 will cause issues? Are you suggesting moving this divider to the left side of the op amp and increase the resistance to reduce its affect on the sensor impedance? Aug 11, 2017 at 13:41
• Yes it will solve it and it will also solve the output limiting at 4.65 volts under heavier loads. But, check the op-amp can run at 12 volts. Aug 11, 2017 at 13:42
• And then youre saying the current through R102 and R103 will cause issues? Are you suggesting moving this divider to the left side of the op amp and increase the resistance to reduce its affect on the sensors impedance? Aug 11, 2017 at 13:49
• If you can increase the power supply voltage then keep the potential divider where it is on the output but consider raising the resistor values so as not to load the output of the op-amp too much. Aug 11, 2017 at 13:55

I think that you complicate simply things.

For the potentiometer just do the resistor voltage divider. I don't think that someone will be able to rotate it so quick - and 20 -100 conversions per second will be more than enough. Buffer is worth when you need to do fast conversions from the high impedance source.