Current range to ADC in STM32F107

[ramadevi]

Posted on April 26, 2010 at 13:12

Current range to ADC in STM32F107

[raptorhal2]

Posted on May 17, 2011 at 13:48

I presume you mean the input current for each ADC channel. Table 5.3.16 in the Rev 2 STM32F105/7 Reference Manual says the channel input impedance is 1 K Ohm.

Check the capability of your interfacing operational amplifier. The literature may say rail-to-rail, but they are usually current limited near the rail. For example., a TI TLV2262 will not go above 2.8V into a 1K load, but a higher power TLV2472 will go above 3V. The bottom rail is no problem, since the current required goes to zero. Cheers, Hal.

[ramadevi]

Posted on May 17, 2011 at 13:48

I meant the input current for the ADC channel only. In the data sheet, I could see the external impedance as 50KOhms. If the input voltage range to the ADC is 0 to 3.3V, then is the max input current = 3.3/50K = 0.066microamps?

I am not using op-amp. I would like to use a voltage divider to reduce the voltage level. I might be wrong. Please help me to understand this.

[raptorhal2]

Posted on May 17, 2011 at 13:48

My apologies for confusing you. The information is in data sheet Table 50 of Section 5.3.16. RADC is 1 K Ohms, so the input current is 3.3 ma.

If you want to use a resistive network, then you need to refer to data sheet Table 51 for constraints on the input impedance. A more detailed description can be found in AN 2834 ....Best ADC Accuracy, section 1.2.5 and 2.2.5. When I tried to use voltage dividing resistors, I ran into ground loop problems between the PCB and my tester box. An op amp will provide very low ADC input impedance, isolate the signal from ground, and limit the signal at the amp's reference voltage preventing blowing an ADC channel. It is the preferred solution if you have the PCB space and power available. Cheers, Hal

[ramadevi]

Posted on May 17, 2011 at 13:48

Thank you very much for your inputs.

I used a potential divider with 5K and 1K and have the taken the output to feed the ADC. Here the current did not exceed 3.3mA which is the high current to ADC. I am planning to use a Zener diode in series to the ADC channel to avoid over voltage.