High Impedance Analog Input Specs

[scott239955_stm1_stmicro]

Posted on September 27, 2016 at 22:26

I've been looking in vain all day for specs on the analog input GPIO. Any help or referral to the proper docs would be much appreciated.  

It's referred to as 'high impedance', but 'high' is not quantified that I can find. I'm measuring approximately 0.65V on the GPIOs in Brown Out Reset, when VDD is 2.2V.Thanks! 

[Amel NASRI]

Posted on September 28, 2016 at 12:32

Hi deyo.scott,

I suggest you have a look to the datasheet and reference manual of the product you are selecting.Please let me know if you don't find the information you are looking for there.-Mayla-

[scott239955_stm1_stmicro]

Posted on September 28, 2016 at 16:09Thanks for the reply. I can't believe I left out the part number! STM32L476.I checked the latest ref manual, and the datasheet. I looked through errata and all app notes that I thought might apply, to no avail. The ADC inputs are spec'd at 50k ohm impedance, but I can't find anything that shows what the GPIO actually should be showing at reset. I see that they're supposed to be high impedance analog inputs, and I see the diagram for how that connection is made in the reference manual, but I can't quite tell if it's floating, held low, or showing ~0.7V because of the diode drop. And I don't see the actual impedance spec anywhere. 

[LMI2]

Posted on September 28, 2016 at 20:08

I googled STM32f4 adc. The third was a link to a PDF

https://www.google.fi/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwjHpbTi0rLPAhUI8ywKHZjiDUUQFggnMAI&url=http://www.st.com/resource/en/application_note/cd00211314.pdf&usg=AFQjCNG82XNcKfSPUJeoSoGox_iLZTTHZQ&sig2=YuF0SAKV54XXBMcv941XzQ&bvm=bv.134052249%2cd.bGg

What I remember is that you need a low impedance source because of the varying capacitive loading, not resistance.

[scott239955_stm1_stmicro]

Posted on September 28, 2016 at 20:35Sorry if I wasn't clear. There is plenty of spec information on the ADC inputs, but I'm looking for impedance, voltage level, etc., when a GPIO is set as an analog input, which it is set to while the processor is in brown out reset. 

[John F.]

Posted on September 29, 2016 at 09:13

The Reference Manual RM0351 Rev 4 says,

7.3.12 Analog configuration When the I/O port is programmed as analog configuration: • The output buffer is disabled • The Schmitt trigger input is deactivated, providing zero consumption for every analog value of the I/O pin. The output of the Schmitt trigger is forced to a constant value (0). • The weak pull-up and pull-down resistors are disabled by hardware • Read access to the input data register gets the value “0� Figure 22 shows the high-impedance, analog-input configuration of the I/O port bit. (please look at the manual for the diagram). So I think there should just be small leakage currents. Also ... 1) It may not be sensible to try to measure a voltage (without looking at the current in or out of the input for various test voltages). 2) You wrote, ''ADC inputs are spec'd at 50k ohm impedance'' but that's the maximum source impedance you can use to drive the ADC to get accurate results (R Ain External input impedance). I agree it's not clear what the individual pin leakage currents might be. It looks like ST specify a device current in various modes presumably with most pins not connected and then seperately specify I/O leakage currents which look to be up to several hundred nano amps. It will probably vary across batches of devices and with temperature.

[Amel NASRI]

Posted on September 29, 2016 at 10:50

Hi,

The pins leakage is specified in the table '' I/O static characteristics'' of the datasheet, depending on the I/O structure (FT, TT...) and VIN.So for pins used in analog mode, you have to check FT_a or TT_a characteristics.-Mayla--Mayla-