AS8501 Datasheet, PDF (35/40 Pages) ams AG – High precision voltage and current measurement sensor interface

AS8501 - Preliminary Data Sheet
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level measurements and high resolution.
9.3 Noise considerations
for every low level measuring system it is essential to know the origin of noise and to accept the limitations given by it. Three major sources of noise
have to be considered. The input voltage noise and the input current noise of the amplifier and the thermal noise (Johnson noise) of resistors in the
external circuitry around the amplifier. Due to the fact that these three sources are not correlated they can be added in the well known square root
equation.
In most applications the input resistor or input divider is low ohmic (i.e. below 10 kOhms) which mean that the noise voltage produced by the input
current noise is negligible compared to the input voltage noise. The input noise density (En) of the AS8501 is with only 35 nV/sqr(Hz) extremely low.
This could be achieved with a special internal analog and digital chopper circuitry which eliminates the CMOS typical 1/f-noise completely. Even though
the overall noise will be dominated by the input amplifier as long as the external resistors are below 10 kOhm.
The total noise voltage generated at a given frequency resp. in a given frequency band (BW) is given by:
Un= En*sqr(BW)
This square root dependence can be seen very nicely in fig. 9.10. The typical square-root shaped dependence is found for both the peak to peak noise
as well as for the equivalent RMS noise.
The bandwidth resp. the sampling frequency of the AS8501 can be adapted to the requirements of the application by programming the internal digital
filter via the SDI bus. For a sampling frequency of 16kHz the input voltage RMS noise is less than 5µV, whereas at 500 Hz already 1µV (or 1LSB) is
reached.
If the customer needs even higher resolution at a lower measuring speed the internal integration time can be further increased but due to the limitation
of the digital noise ( 1LSB) it is better to perform an external averaging in the attached µC. In this way the resolution of the system can be considerably
increased to less than 0.1 µV for sampling rates of 5 Hz and below which corresponds to an effective AD-converter width of more than 20 bits. (see fig.
9.10)
9.4 Shielding, guarding
In many applications it is difficult to gain full benefit from the AS8501 performance since a number of external error sources can disturb the
measurement. To achieve the maximum performance the design engineer has to take care specially of the layout of the PC-board and the sense
connections to the external components. To avoid noise pick-up from external magnetic fields all tracks on the PC-board should be parallel strip lines
and they should be traced as close as possible to each other. External sensing cables should be twisted and kept away from current carrying cables as
far as possible. For longer cables a shielding is sometimes helpful but care should be taken that the shield is not connected to one of the sense leads.
For an optimum performance it should be open on one side, the other side should be connected to the central (star like) analog common point.
In very sensitive applications it may be wise to use a guard ring around both inputs and it should be connected again to the analog common point. This
procedure minimises leakage currents and parasitic capacitances between different terminals and components on the PC-board.
EMV interferences can be affectively avoided in most cases by using standard SMD-type high frequency filters in the analog input lines
as well as in the digital output lines.
Revision 1.1, 04-April-06
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