9.3.2 Detailed Design Procedure

The notch frequency is set by:

To achieve a 60-Hz notch use R = 10 MΩ and C = 270 pF. If eliminating 50-Hz noise, use R = 11.8 MΩ and C = 270 pF.

The twin T notch filter works by having two separate paths from V_IN to the input of the amplifier. A low-frequency path through the series input resistors and another separate high-frequency path through the series input capacitors. However, at frequencies around the notch frequency, the two paths have opposing phase angles, and the two signals tend to cancel at the input of the amplifier.

To ensure that the target center frequency is achieved and to maximize the notch depth (Q factor) the filter must be as balanced as possible. To obtain circuit balance, while overcoming limitations of available standard resistor and capacitor values, use passives in parallel to achieve the 2C and R/2 circuit requirements for the filter components that connect to ground.

To make sure passive component values stay as expected, clean the board with alcohol, rinse with deionized water, and air dry. Make sure board remains in a relatively low humidity environment to minimize moisture which may increase the conductivity of board components. Also large resistors come with considerable parasitic stray capacitance which effects can be reduced by cutting out the ground plane below components of concern.

Large resistors are used in the feedback network to minimize battery drain. When designing with large resistors, resistor thermal noise, op amp current noise, as well as op-amp voltage noise, must be considered in the noise analysis of the circuit. The noise analysis for the circuit in Figure 36 can be done over a bandwidth of 2 kHz, which takes the conservative approach of overestimating the bandwidth (TLV8544 typical GBW/A_V is lower, where A_V is the gain of the system). The total noise at the output is approximately 800 µVpp, which is excellent considering the total consumption of the circuit is only 500 nA per channel. The dominant noise terms are op-amp voltage noise, current noise through the feedback network (430 µVp-p), and current noise through the notch filter network (280 µVp-p). Thus the total noise of the circuit is below 1/2 LSB of a 10-bit system with a 2-V reference, which is 1 mV.