Delta 35 Specifications download pdf (Page 15)

AN217

Rev. 0.2 15

Table 1 is a copy of the posted specification of the ADC0’s typical noise table from the C8051F35x family data

sheet. This table shows the expected, typical rms noise of the ADC at the output of the SINC3 filter for given

decimation ratios and gain settings.

If measuring a signal that requires a 10 Hz output word rate with unity gain, we set the decimation ratio for 1920

(remember: use the highest decimation ratio setting possible). This gives an expected V

NOISERMS

of 2.38 µV

(from Table 1). This is the noise number used to predict the expected effective resolution.

With a 2.5 V voltage reference, each LSB in a bipolar measurement is as follows:

REF

= 2.5 V/2

= 298 nV.

Therefore, the standard deviation (rms noise) in terms of codes is as follows:

2.38 µV/298 nV = ~8 LSBs or output codes of noise.

In practical terms, if the analog inputs are grounded for a noise test (both AIN+/AIN– are connected to analog

ground and a bipolar measurement is made), you could expect to observe a standard deviation of 8 codes. This

equates to an effective resolution of 23 bits – 3 bits = 20-bits effective resolution (it takes 3 bits to represent a

variation of eight binary output codes, so we subtract 3 bits from the signed 24-bit output word).

If concerned with noise-free resolution, we proceed with the same method but instead use the rms noise voltage

(one standard deviation) multiplied by 6.6. Multiplying the rms noise by 6.6 results in a noise level in which 99.9%

of all codes will be included. This leaves only the bits that will remain constant from sample-to-sample in our noise

test.

Again, using the 2.38 µV rms noise voltage (from Table 1), 99.9% of all sampled output codes should be contained

within 2.38 µV x 6.6 = 15.7 µV. An LSB = 298 nV, and so the total number of noise-varying bits is as follows:

15.7 µV/298 nV = ~53 codes.

53 bits requires 26 or 6-bits of a digital code to represent 53 different output codes, and so 23 bits – 6 bits = 17-bits

of noise-free resolution.

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Delta 35 Specifications Page 15