Is ADC driver (op-amp based) is needed for measuring over cable with STM32 ADC?

[Davood_Kesha]

Hello ST team,

I am designing a hardware with STM32G4 family. I want to measure a differential signal by MCU's ADCs over a cable of 50 cm. Is it needed to use op-amp as ADC driver or I can connect it to the MCU pins directly?
 

[MasterT]

You may do direct connection in case coax cable is loaded with 50 Ohm matching resistor, means other end has high current driver.

[AScha.3]

Hi,

the cable doesn't change the requirement for the low impedance , the ADC needs for good conversion result.  

Just if you want good signal quality coming to the ADC, have screened cable and good ground , low impedance.

So just have same opamp or low impedance at the cable input, as you would use without the cable.

The needed input impedance depends on sampling time, see ds:

 

[Ozone]

That depends on the signal and your requirements.

For a low sampling frequency with relatively long sampling times and a “quiet” environment, you would surely get away without an opamp. I would still use shielded cable, though.

In the presence of EMI noise or for higher sampling rates, you will need some low-impedance amplifier.
 

[LCE]

Don’t forget to protect the inputs.

Alone due to this fact I would use an opamp + serial Rs + Cs with the same power rails as the STM32.

Plus protection for opamp inputs.

[rich.g.williams]

One way is to use an Instrumentation Amplifier that is truly differential input and unipolar output. The STM32G4 has unipolar input for signal in 0 to 3.6V so ideal for Instrumentation Amplifier feed. Most ADC’s need a low impedance feed. If you don’t use anything you may well have common mode problems.