STR720 datasheet(337/401 Pages) STMICROELECTRONICS

337 / 401 page

STR720 - S-D ANALOG/DIGITAL CONVERTER (ADC)

337/401

The word lengths needed in each integrator and differentiator are given by the formula

b=3log2(N) + 1. This gives a maximum word length of 21 bits for the internal registers. As the

output is only 16 bits, the LSBs are discarded.

Note

Only the 12 most significant bits out of this 16 bits are guaranteed to be accurate

according to the device specification, even if all 16 bits can be read.

Since the first zero of the decimation filter is located at

while the first replica of the spectrum of the modulator after decimation is located at:

care must be taken to ensure that the Nyquist criterion is respected to avoid alias, i.e.:

The description above refers to the operations in normal decimation mode. When low-rate

decimation mode is selected, the registers in the decimated section will be updated when the

count gets to 1700 instead of 170 and the world length of the registers will be 33 bits instead

of 23, the final value containing bits 33:18 of the final differentiator. The reset of the filter will

occur every 5120 cycles instead of every 512.

22.3.8

Bandgap Reference

An on-chip bandgap reference generates a 1.22 V reference. This is used to generate two

voltages used by the modulator — VCM & VREF. VCM is designed to be 1.25 V, the midpoint of

the converter’s voltage range and VREF is the feedback reference, 1.74 V. As the bandgap

reference is not trimmed, absolute values of VCM & VREF could be inaccurate by up to± 5%.

This will lead to gain and offset errors in the converter which can be calibrated out digitally if

necessary.

To calibrate the converter it is necessary to input the minimum and maximum inputs

supported — 0 V and 2.5 V. The digital output for a 0 V input is the offset and the gain of the

converter is given by:

The offset and gain correction factors can be stored digitally and applied to all outputs from

the converter, for all four input channels.

171

---------

2048

------------

Nyq

Max

≥

2.5Voutput

0Voutput

–

()

2.5

----------------------------------------------------------------

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