NorthGuy
Well-Known Member
This reference data sheet specifies 400ppm accuracy. Assume you can trim it with a very good benchtop volmeter such as Fluke 8846A to 30ppm and it's going to hold it (which is to be tested).
The combined error of the measurement is a square root((reference error)^2 + (input error)^2). If reference error is 30ppm, an input error of 20ppm will give you a combined error of 36ppm. Any further increase in input accuracy will not improve the combined error by much. Therefore, you have no reason to imrove input accuracy beyond 20ppm. This is 20uV for 1V measurement.
If you don't calibrate the reference and it stays at 400ppm, there's no reason to improve input accuracy beyond 250ppm, which is 0.25mV for 1V measurement.
24-bit ADC resolves about 0.06ppm. This does not mean that you get 0.06ppm accuracy. Your ADC data sheet specifies 14ppm max non-linearity error, 2mV (2000ppm!) offset error and 12ppm gain error. If you strip offset and gain errors (which will require to decrease already slow samplng rate 3 times), you still get stuck with 14ppm non-linearity, so your last 8 bits will never be accurate. This is another reason not to increase your input accuracy beyond 20ppm.
It is possible that you could get better accuracy by oversampling with a fast 16-bit ADC rather than by getting 24-bit from a slow ADC. Depends on the signal you measure I guess.
The combined error of the measurement is a square root((reference error)^2 + (input error)^2). If reference error is 30ppm, an input error of 20ppm will give you a combined error of 36ppm. Any further increase in input accuracy will not improve the combined error by much. Therefore, you have no reason to imrove input accuracy beyond 20ppm. This is 20uV for 1V measurement.
If you don't calibrate the reference and it stays at 400ppm, there's no reason to improve input accuracy beyond 250ppm, which is 0.25mV for 1V measurement.
24-bit ADC resolves about 0.06ppm. This does not mean that you get 0.06ppm accuracy. Your ADC data sheet specifies 14ppm max non-linearity error, 2mV (2000ppm!) offset error and 12ppm gain error. If you strip offset and gain errors (which will require to decrease already slow samplng rate 3 times), you still get stuck with 14ppm non-linearity, so your last 8 bits will never be accurate. This is another reason not to increase your input accuracy beyond 20ppm.
It is possible that you could get better accuracy by oversampling with a fast 16-bit ADC rather than by getting 24-bit from a slow ADC. Depends on the signal you measure I guess.