4935337418894055612263500285AN611_0| Application Note

4935337418894055612263500285AN611_0| Application Note

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AN-611 APPLICATION NOTE

50 Hz/60 Hz Rejection on - ADCs
By Adrian Sherry

INTRODUCTION Rejection of 50 Hz and 60 Hz interference is a requirement in many industrial settings. This Application Note outlines how the AD7708/AD7718, AD7709, AD7719, AD7782/AD7783 - ADCs are used for optimal rejection of these frequencies. POWER LINE FREQUENCIES Electrical ac power is distributed worldwide at one of two frequencies, 50 Hz or 60 Hz. This frequency can be picked up as undesired interference on electrical signals, either via the power supply transformers or via radiation from unshielded power cables or electrical equipment. There can also be interference present at harmonics of these frequencies, i.e., 100 Hz/150 Hz or 120 Hz/180 Hz, and so on, although these components are generally lower than the fundamental. The exact frequency of the mains signal can typically vary by up to 1 Hz over time. Mains interference presents significant problems when trying to measure low level signals with a high resolution ADC.
UNSHIELDED POWER CABLE

One solution for attenuating this interference is to use a low-pass analog filter. To obtain good rejection of 50 Hz and 60 Hz, this filter needs a low cutoff frequency and/or high order. A low cutoff frequency will limit the bandwidth of signals that can be measured, while high order analog filters are costly in terms of component count and board space. Also, the cutoff frequency may be prone to drift. An alternative is to use a digital filter. This filter can be optimized for rejection at one power line frequency or can attenuate both frequencies simultaneously, allowing the equipment to be used worldwide without reconfiguration. The features of the filter to consider are rejection at 50 Hz 1 Hz and/or 60 Hz 1 Hz, rejection at harmonics of these frequencies, the settling time of the filter, and the complexity of the filter (affects power consumption, and so on.). The rejection required depends on the magnitude of the interference and the noise level of the system, e.g., 60 dB of rejection is sufficient t
o attenuate a 1 mV interference level down to 1 uV. - ADCs - ADCs include a digital filter as one of the components in their architecture. If correctly configured, this filter can provide impressive rejection of power line frequencies while still having sufficient bandwidth for measuring the input signal. The filter used on this family of ADCs from Analog Devices offers good mains frequency rejection while providing reasonably high update frequencies. SINC3 FILTER (UNCHOPPED) The digital filter used in these ADCs is a sinc3 filter. The response of this filter depends on the filter sampling rate, fS (32.768 kHz), and a register value SF This sets the update . rate of the ADC, the position of the notches in the frequency response, and the noise of the ADC conversions. The frequency response of the sinc3 filter is given by:

50Hz/60Hz INTERFERENCE

SENSOR

ADC

Figure 1. Source of 50 Hz/60 Hz Interference REJECTING INTERFERENCE Using differential signals allows rejection of any common-mode 50 Hz/60 Hz interference, provided the system has good common-mode rejection. However, this does not provide any rejection of normal mode interference.

1 sin(8SF f / fS ) 8SF sin( f / fS )
and the ADC update rate is:

fS 8SF REV. 0