Gain issue about ad7190

Hi, 

I guess the question here is if you are pertaining to system calibration or internal calibration? 

If it is internal calibration only then I don't think there's a huge difference in terms of error when buffers are enabled or disabled. However, if it is a system calibration then a large external input impedance impedance may have an impact when buffers are disabled. 

Thanks,

Jellenie

ok, thank you very much.

I have another problems.

1. In datasheet, the offset error is 0.5uVtyp(chop enabled). The datasheed also said as following:

With chop enabled, an internal zero-scale calibration is not needed.

So, i have to accept 0.5uV offset at least if the chop is enabled,  and i can't remove these offset.

It means i have 0.5uV error at least because i can't use offset calibrate to remove it.

Then the noise is about 8nV. Because the offset error is larger then the noise very much, if i could not remove the offset error,

how can the noise perofromance  reflect the value?

2. is the offset dynamic? 

2.1 if i change channels, will the offset change?

2.2 if i change gain, will the offset change?

2.3 will the offset change after working several days continuously?

best regaardings.

Hi, 

1. I guess the purpose of chopping is to remove any external offset errors. However, the internal chopper itself have an offset error which is typ. 0.5uV. So the question here is how this offset value will affect your target accuracy? Aside from offset another error contributor is the gain error so if you also performed internal gain calibrations the gain error is still larger than the noise value (0.007%FS at Gain >1). So practically speaking the noise is not the only error present when you performed internal calibrations. If you really wanted to remove both gain and offset errors I guess the option is to perform a system calibrations. 

2. I guess the offset itself is a static error. However, the error depends on the system level design and not only on the ADC error. So I guess the offset error of the system may possibly larger than the offset error of the ADC itself. 

2.1. I think same answer as above it depends on your system level design. But as the ADC alone we do not expect huge difference in terms each channel offset errors.

2.2 No, the offset is different with the gain error. So if you change the gain it is the gain error that will change. 

2.3 It depends whether the system level design is stable and if it will affect the temperature of the ADC or other part of the system that may drift over temp. As we know gain or offset error will also drift with respect to temperature. So if the temperature is stable then we do not expect shift in errors but if a huge change in operating temperature then there should be no significant shift in errors while operating for several days.  

Thanks,

Jellenie

Hi, 

1. I guess the purpose of chopping is to remove any external offset errors. However, the internal chopper itself have an offset error which is typ. 0.5uV. So the question here is how this offset value will affect your target accuracy? Aside from offset another error contributor is the gain error so if you also performed internal gain calibrations the gain error is still larger than the noise value (0.007%FS at Gain >1). So practically speaking the noise is not the only error present when you performed internal calibrations. If you really wanted to remove both gain and offset errors I guess the option is to perform a system calibrations. 

2. I guess the offset itself is a static error. However, the error depends on the system level design and not only on the ADC error. So I guess the offset error of the system may possibly larger than the offset error of the ADC itself. 

2.1. I think same answer as above it depends on your system level design. But as the ADC alone we do not expect huge difference in terms each channel offset errors.

2.2 No, the offset is different with the gain error. So if you change the gain it is the gain error that will change. 

2.3 It depends whether the system level design is stable and if it will affect the temperature of the ADC or other part of the system that may drift over temp. As we know gain or offset error will also drift with respect to temperature. So if the temperature is stable then we do not expect shift in errors but if a huge change in operating temperature then there should be no significant shift in errors while operating for several days.  

Thanks,

Jellenie