# STM32H7 ADC characteristics

#### HKet

Joined May 9, 2024
2
Hi,

I'm working on a setup with a filter and some electronics to get an analog signal ready for a ST microcontroller (STM32H723VG). I want to simulate everything, even the part inside the microcontroller that handles the signal. But the problem is, I can't find the info I need in the microcontroller's manual. The manual does have a table that shows how long it takes to sample different signals, depending on how much resistance the source has. But I'm not sure how to use this info to figure out the resistance and capacitance inside the microcontroller.
So far, I've tried to guess the resistance and capacitance by looking at the sampling times in the table. I came up with a guess of 1K ohm for the resistance and 8.5 pF for the capacitance. But I'm not sure if 1 kilo-ohm is right. I want to sample at about 4MSPS, but 1 kilo-ohm might be too much resistance to do that quickly enough within 0.5LSB and with the required sampling time of 60-70 nanoseconds.

Here are some details about the setup:

Microcontroller: STM32H723VG
ADC input range: 0-3.3V
Signal Bandwidth: 1MHz (-3db)
Filter attenuation at 4MHz: -40db
ADC Resolution: 12 bits
ADC channel type: Fast Channel
Sampling time (2.5): 62ns
Sampling frequency in dual mode: 8MSPS (two ADC at 4MSPS)

Note: The images show the last stage for level shifting, charge bucket and ADC internal

Any help or advice would be great.

Thanks!

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#### MrChips

Joined Oct 2, 2009
31,057
Table 80 in the STM32H723 datasheet gives the R and C.

R (max) depends on the ADC resolution selected.

16-bit Rin max = 170 Ω
14-bit Rin max = 435 Ω
12-bit Rin max = 1150 Ω
18-bit Rin max = 5650 Ω
8-bit Rin max = 26500 Ω

C = 4 pF

#### HKet

Joined May 9, 2024
2
Table 80 in the STM32H723 datasheet gives the R and C.

R (max) depends on the ADC resolution selected.

16-bit Rin max = 170 Ω
14-bit Rin max = 435 Ω
12-bit Rin max = 1150 Ω
18-bit Rin max = 5650 Ω
8-bit Rin max = 26500 Ω

C = 4 pF
The table outlines the maximum external input impedance, for instance, it indicates that an 1150 ohm input source resistance results in a 2 LSB error for a 12-bit resolution. However, it doesn't detail the required input impedance for simulation nor the additional capacitance of the multiplexer.