AN3322 datasheet(11/38 Pages) STMICROELECTRONICS

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AN3322

Watt-hour meter using the STM32

Figure 3. Typical thermal noise present on STM32 meter platform. Histogram shows

noise equivalent to white Gaussian noise

Measured thermal noise (overall on the board) degrades the ADC resolution and accuracy

by 2 to 5 LSB. This means that the number of effective bits drops from 12 to less than 10.

To recover from this situation it is necessary to use techniques for NOEB (number of

effective bits) enhancement, see Section 1.9. Using oversampling requires, in some cases,

the addition of the dithering signal. According to the noise level found in this system, it is not

necessary to add the dithering signal. It's addition does not bring any enhancement in the

performance.

1.7

Crosstalk between subsequent multiplexed channels

When reading more channels subsequently with maximum speed, values sampled from the

subsequent channel may be influenced by the reading of the previous channel. This is

called crosstalk between subsequent multiplexed channels.

It is caused by the balance of external impedance and capacitance of a signal source and

an internal impedance and capacitance of the sample and hold circuit in the given ADC. The

longer the sample and hold phase, the lower the correlation between subsequent samples.

The technique to overcome this phenomenon presented in this section is based on reading

every channel in the sequence twice and only the second value is taken as the valid one.

The error is reduced from 0.07% of ADC full range to less than 0.0001%.

The total conversion time is calculated as follows:

Equation 5

Tconv = sampling time + 12.5 cycles

Example 2

With an ADCCLK = 14 MHz and a sampling time of 1.5 cycles:

Tconv_14MHz = 1.5 + 12.5 = 14 cycles = 1 µs

AN3322 Datasheet(PDF) 11 Page - STMicroelectronics