Unexpected OPAMP Output Voltage When Enabling OPAMP on EVSPIN32G4 DUAL

[pelino]

I am using EVSPIN32G4 DUAL and observed the following behavior.

When calling:

LL_OPAMP_Enable(OPAMPx);

 

 

the OPAMP output immediately shows about 208 mV.

I measured this voltage at TP25.

This behavior occurs even though DAC3 is completely disabled.
After calling LL_OPAMP_Enable(), I checked the DAC3 registers and confirmed that:

 

All related DAC_CR and DAC_DHR12R registers remain set to zero.

 

I checked these registers because DAC3 Channel 2 is configured as the OPAMP minus (−) input, as shown in the configuration below.

 

My questions are:

1. Why does the OPAMP output show approximately 208 mV when the OPAMP is enabled ,even though DAC3 has no value configured and is fully disabled?

Thank you.

Best regards,

Pelino

[TDK]

Op amps still do their thing even if their inputs are floating. What are you expecting to happen? It's best not to let input float or you end up with undefined behavior on the output.

If you measure TP18/TP19/TP25, it should be consistent with the op-amp behavior.

[pelino]

According to UM2896, the comparator negative input (Vth) can be connected to the DAC or an internal reference.

 

On the EVSPIN32G4-DUAL board, when DAC3 is enabled but no DAC value is written,

the OPAMP output at TP25 stays at 208 mV.

My question is simply:

What is the actual default Vth voltage on the EVSPIN32G4-DUAL board in this condition, and where is it defined (hardware or firmware)?

Best regards,

Pelino

[TDK]

> DAC3 is completely disabled.

> DAC3 is enabled but no DAC value is written,

Which is it? Your register values above suggest it's disabled.

If the input is floating, the state will depend on what is connected to m1_shunt, but it doesn't seem very useful to me.