Where can I find the ADC input impedance of STM32H743 in differential and single-ended configurations?

[MasterT]

No.

pF means 10^-12.

kHz : 10^+3.

[Robmar]

Let me revise:-

R-in = 1 / (2Pi * 6pF * 192k) = 138k ohms

dropped the K!

[Robmar]

I have been testing the ADC input impedance of the STM32H743 and have observed the discrepancy between DC and AC impedance measurements.

DC Measurement:

Using a bias resistor method, I measured a DC input impedance of ~709kΩ, derived from a 22kΩ bias resistor dropping 0.49V off a 1.61V reference voltage, indicating a 2.2µA bias current.

AC Impedance Expectation:

For an ADC sampling rate of 192kHz and a sampling capacitance of 6pF, the expected AC impedance should follow:

Rin = 1 / (2π × fADC × CADC)

Substituting the values:

Rin = 1 / (2π × 192kHz × 6pF) ≈ 138kΩ

This suggests that while the AC impedance could be 138kΩ, the DC impedance is significantly higher at 709kΩ.

Clarifications Needed:

1. Is this discrepancy expected? Does the ADC input exhibit different impedance characteristics at DC versus AC due to internal switching behavior?

2. How does bias current influence ADC performance? Are there known effects of steady-state bias currents on charge transfer dynamics?

3. What is the recommended bias current for optimal ADC operation at 192/384 KHz sampling rates? Should biasing conditions be adjusted to ensure accurate sampling behavior?

4. Are there recommended methods to experimentally verify the true AC impedance seen by the ADC input?

I would greatly appreciate any insights or guidance on these points.

[AScha.3]

> Is this discrepancy expected? Does the ADC input exhibit different impedance characteristics at DC versus AC due to internal switching behavior?

Hi Rob ,

you didnt answer to my post, but ask again (what i cannot understand ),

just tell me, whats the problem:

- you dont understand, how this kind of ADC is working , and what side effects it has ?

- you want detailed precise data about the caps and switches in the chip design , to do...what ?

- you dont accept the recommended input impedance (from ds, rm or ANxxx ) for the ADC ?

[Robmar]

Thank you for your response. Let me clarify my concerns:

• I understand how this type of ADC works, but I am investigating whether the difference between DC and AC impedance is expected behavior due to internal ADC switching dynamics.

• I am not requesting chip design details, but rather an explanation of whether this impedance discrepancy aligns with typical ADC behavior.

• I acknowledge the recommended ADC input impedance from the datasheet, but my real-world measurements show a different behavior, which I would like to understand.

The ultimate aim of my inquiry is to determine whether my design could be improved to enhance sensitivity to weak signals or reduce noise. If internal ADC effects influence input impedance differently at DC versus AC, this could impact front-end signal conditioning.

[AScha.3]

Okay, but from your question it seems, you don't understand or just accept the way it's working.

So there's no DC input resistance, just the errors of the switches and mosfets at the pin, giving some uA in any direction, also probably changing with temperature.

That's nothing special for a digital circuit in "off" state.

And the AC input impedance is the result of the sampling cap (some 5pF or so) switched to the pin for the sampling time you defined and the unknown voltage level it's coming back from the SAR conversion.

Just do a simulation in swcad, if you want to see what comes out any conversion rate you set.

Otherwise...for best precision for conversion, try a low ESR ceramic cap at the input, just as big as the signal allows, you want to see. And low impedance driving the input, If high speed of ac signal is needed. And set the sampling time to...the best working range. Just try different settings, i found its not same for all ADC, it's more a best fit, you have to find out for your design/board and Input circuit. So this is just some work to do, don't expect someone else it's doing that for you. With oversampling and a cap at the input these ADC can give surprising good resolution, but don't expect wonders, much better than ds is giving.

[Robmar]

I was taught at college to "Never assume", and modeling in SWCAD is waste of time, as much as LTSpice, it won't model anything unless there is an accurate model which STM have not released for the H743 ADC input.

If you want to achieve mediocracy, accept things as they are, its not my motto!

[AScha.3]

ymmv.