AN3137 datasheet(23/42 Pages) STMICROELECTRONICS

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AN3137

Methods for precision improvement

Doc ID 16983 Rev 3

23/42

4.2.3

Hardware design rules

Good hardware design can minimize many external influences on the measured signal and

measuring device. Therefore good design is also a method of improving precision, or rather

a method of minimizing additional errors caused by the external hardware. The following list

gives the main rules for minimizing external influences.

Grounding:

Care must be taken in the design of the PCB grounding. Ground paths on a PCB have

nonzero resistance (and impedance) therefore current flowing through these paths

generates a voltage drop. This voltage drop changes the voltages at points that are

important for the ADC. For example it changes the reference voltage on the microcontroller

pins slightly or influences the input signal on the microcontroller analog input pins. Good

design minimizes this voltage drop. Methods of doing this are: minimizing the ground path

impedance and minimizing the current through important ground paths (topology).

To minimize ground impedance: use massive ground paths. To minimize current through

sensitive connections: use star topology, use independent PCB lines from input signal pins

to microcontroller analog inputs, use independent PCB lines from the reference voltage

source to microcontroller reference voltage inputs. Simply: supply current should not flow

through signal ground - use signal ground and power ground in design.

Voltage filtering:

To minimize any external influences on the precise voltage levels used by the ADC, it is

good to use filters on these voltage sources. Good filters must be applied on the reference

voltage and on the supply voltage. Filter design depends on the noise amplitude present on

the voltage sources, the external noise disturbance and the required precision.

Preamplifier usage:

If the measured signal is too small (in comparison to the ADC range) then it is good to use

external preamplifier. This amplifier can fit the input signal range to the ADC range and can

also insert offsets between input signal and ADC input. Care must be taken in the

preamplifier design in order not to generate additional errors (for example additional offset,

amplifier gain stability, linearity, frequency response...).

Frequency independence:

In cases where an AC signal is measured, frequency is an important characteristic of the

input signal path. When measuring fast changes in the input signal then the frequency

response of the signal path must be flat, even at high frequencies. The parasitic resistance

and parasitic capacitance on PCB signal paths must be minimized. Otherwise, when

measuring DC signal levels, adding an RC filter to the input signal data path improves

robustness against external noise.

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