10 M:

-

+

10 M:

10 M:

10 M:

10 M:

270 pF

270 pF

270 pF

CR2032 Coin Cell

225 mAh = 5 circuits @ 9.5 yrs.

60 Hz Twin T Notch Filter

10 M:

270 pF

Remote Sensor

Signal

+

60 Hz

To ADC

Signal × 2

(No 60 Hz)

200 2s/DIV

LPV521

www.ti.com

SNOSB14D – AUGUST 2009 – REVISED DECEMBER 2014

Figure 63. Step Response

8.1.2 EMI Suppression

The near-ubiquity of cellular, Bluetooth, and Wi-Fi signals and the rapid rise of sensing systems incorporating

wireless radios make electromagnetic interference (EMI) an evermore important design consideration for

precision signal paths. Though RF signals lie outside the op amp band, RF carrier switching can modulate the

DC offset of the op amp. Also some common RF modulation schemes can induce down-converted components.

The added DC offset and the induced signals are amplified with the signal of interest and thus corrupt the

measurement. The LPV521 uses on chip filters to reject these unwanted RF signals at the inputs and power

supply pins; thereby preserving the integrity of the precision signal path.

Twisted pair cabling and the active front-end’s common-mode rejection provide immunity against low-frequency

noise (i.e. 60-Hz or 50-Hz mains) but are ineffective against RF interference. Even a few centimeters of PCB

trace and wiring for sensors located close to the amplifier can pick up significant 1 GHz RF. The integrated EMI

filters of the LPV521 reduce or eliminate external shielding and filtering requirements, thereby increasing system

robustness. A larger EMIRR means more rejection of the RF interference. For more information on EMIRR,

please refer to AN-1698.

8.2 Typical Applications

8.2.1 60-Hz Twin T-Notch Filter

Figure 64. 60-Hz Notch Filter

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