External or Internal Voltage Reference, that is the question?

3 different configurations are tested to see if the internal reference voltage is good enough or if an external one is needed for best accuracy.

For the test a LM4040-2.5 0.1% tolerance was used.

Reference	ADC INT REF	ADC EXT REF	DAC REF
Internal	2.4999V	0	2.4964V
External - 10 Ohm resistors	2.5033V	2.4991V	2.4991V
External - no resistors	2.5033V	2.4991V	2.4991V

Now I output a set of values through a DAC and read the voltage level back with an ADC and compare input and output values. The voltage range for ADC and DAC are set to 0v to +10V

Internal reference:

DAC	ADC	U [V]
0x0000	0x000	-0.0070
0x0400	0x0401	2.4920
0x0800	0x0801	4.9919
0x0C00	0x0C00	7.4886
0x0FFF	0x0FED	9.9282

External reference, input and output with series 10 Ohm resistor:

Circuit for the external reference:

DAC	ADC	U [V]
0x0000	0x000	-0.0041
0x0400	0x03FB	2.4920
0x0800	0x07FE	4.9983
0x0C00	0x0BFE	7.4976
0x0FFF	0x0FE6	9.9361

External reference, input and output shorted without series Ohm resistor:

DAC	ADC	U [V]
0x0000	0x000	-0.0042
0x0400	0x03FD	2.4970
0x0800	0x07FE	4.9992
0x0C00	0x0C00	7.4990
0x0FFF	0x0FE9	9.9381

The DAC does not seem to be capable of driving all the way up to the maximum value. More measurements are needed.

Returned ADC values seem to be best with the internal reference. The external reference without series resistors has the best voltage measurements, but adding cost to the BOM. Lower voltage measurements seem to be shifted 4-12mV (2-5 LSBs) to the negative side.

So many LSBs

Rev 03

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