High precision analog front end (signal conditioner) presented here can be used for portable ECG equipment that requires single-supply operation. There are two kind of amplifiers on the circuit diagram, the instrumentation amplifier INA326 and the dual precision amplifier OPA2335. Both operate from a single 5V supply and apply autozeroing techniques. Autozeroing techniques will keep the initial offset and offset drift over temperature and time near zero.
The INA326 input gain is set to 5. This gain is obtained since G1= 2R2/Rg = 2(200/80). To cancel resistors noise, we can put capacitor 750-pF paralleled to R2. While the output stage around A3 provides a gain of G2=1 Mohm/5Kohm = 200, the 3-dB frequency of the integrator A4 is set to 0.05 Hz. Low-noise biasing of the 2.5-V bias voltage to the amplifiers and the 16-bit, 100-kSPS, SAR-ADC ADS8321 is provided by REF3125(the precision voltage).
The input common-mode voltage is fed back via the amplifiers A1 and A2 to the right leg of the patient for further 50/60 Hz noise rejection. This approach requires only a few microamps of current. This current is used to significanly improve the common-mode rejection and to ensure compliance with the UL544 standard. [Source: Texas Instruments Application Note]