OpAmp PGA mode - noisy output - STM32G431

[mechatron]

Hi,

Using the STM32G431 in PGA mode vs in standalone mode (external resistor network for Gain), I am getting different noise levels on the final output (measured using ADC).

The noise generated in PGA mode is twice as big as in standalone mode. What could be the reason?

I expect a change in gain due to the tolerance specified in the datasheet, but did not expect the noise level to increase.

 

Note - I tested with multiple combinations for external gain (100R and 1k5, 1k and 15k, 10k and 150k) to reach 16 gain. All the three resulted in similar noise level. Hence, thermal noise could not be the reason is what I assume, in case of PGA. Is some other effect taking presedence here? 

[MasterT]

RM says: “amplifier with a non-inverting gain ranging from 2 to 64, or with inverting gain ranging from
-1 to -63.”

The minimum gain 2 in non-inverting mode implies noise gain 2. Same for inverting G -1, noise of internal opa-amp getting factor G +2 amplification. You can use capacitor in parallel to feedback resistors to minimize BW and noise consequently.

 

[mechatron]

Hi ​@MasterT ,

 

Thanks for the response. Is this filtering always required while using OpAmp in PGA mode?

I am trying to understand why there is a difference in noise for external gain 16 vs PGA gain 16.

[MasterT]

One of many possible reasons may be where low leg of internal resistor is connected to ground on silicon crystal, since no “ground-plane” exists on chip.  Using external resistors you may get better option in this regard.

Filtering is just a remedy for pure noise performance of the internal op-amp, 500 nV/Hz is extremely high value, any lm358 has lower noise.

What is the application?