Author Topic: Interfacing an opamp output with the STM32 ADC  (Read 8530 times)

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Offline Yansi

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Re: Interfacing an opamp output with the STM32 ADC
« Reply #25 on: January 14, 2019, 03:55:33 pm »
I have already posted,  how an an ADC buffer might look like. The schematic just does not include the ADC input decoupling cap, which may be something around the few nF (1 to 3nF). 

The cap there is there to decrease the output impedance of the buffer at high frequency (the sampling current pulses), and the circuit is wired so that the DC output impedance is zero, yet it is fully stable with that capacitive load.
 

Offline mvs

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Re: Interfacing an opamp output with the STM32 ADC
« Reply #26 on: January 14, 2019, 04:42:38 pm »
What I have in the circuit is a CA3130 amplifier. the output goes through a resistor divider and then goes to the MCU pin (F103C8). I have not used a decoupling cap for the ADC input, but for sure for VDDA and other pins as datasheet says. as I mentioned now I tested that I can reduce the supply voltage of the opamp to 5V. therefore I can reduce the resistor values.
if you think this match will not work, I'll order an STM32F303 (or an external ADC) and a 3.3V opamp. otherwise, if you have a suggestion for the existing circuit, I really appreciate it.
You can try to reduce resistor values and add some small decoupling capacitor, like 100-220pF, but there is a limit, what your opamp can drive. You may go for some more suitable opamp, like THS4031.
 

Offline VanitarNordicTopic starter

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Re: Interfacing an opamp output with the STM32 ADC
« Reply #27 on: January 14, 2019, 05:11:10 pm »
Quote
You can try to reduce resistor values and add some small decoupling capacitor, like 100-220pF, but there is a limit, what your opamp can drive. You may go for some more suitable opamp, like THS4031.

I can not use dual supply opamps because it is a battery powered application. Therefore I selected a rail to rail opamp. it should have low bias current, therefore a Fet or CMOS opamps can work. the opamp input should be sensitive enough. The input frequency is below 1KHz, and the gain is around 1000. I was thinking to use TSV991 if this opamp could not work properly.
 

Offline Yansi

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Re: Interfacing an opamp output with the STM32 ADC
« Reply #28 on: January 14, 2019, 06:45:20 pm »
Why the hell do you need then a 1MSps or more for a 1kHz signal?  :o

With such insane gain, your first enemy is noise. Depending on the source impedance, a FET input or BJT input opamp might suit. Most of the RRIO single 5V opamps are CMOS/FET input, with rather high noise. But I may be a bit offtopic with that.

Driving the ADCs to insane speeds and pushing the limits also produces more noise (less ENOB) and creates more tough requirements for the buffering circuitry.



 

Offline VanitarNordicTopic starter

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Re: Interfacing an opamp output with the STM32 ADC
« Reply #29 on: January 15, 2019, 08:01:55 am »
Finally, I decided to go with STM32F303CC.

as few of you mentioned correctly, it seems the focus on F3 parts are for ADC and analog signals processing, which introduces internal opamps which can be configured as buffers or even amplifiers to fix the impedance issue. I think the best result could be achieved by the F3.
 


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