AD8221 bridge preamplifier circuit

AD8221 bridge preamplifier circuit

Figure 1-1 shows a bridge preamplifier circuit composed of AD8221 , a typical instrumentation amplifier application. When the signal is detected, the resistance of the bridge resistor changes, causing the bridge to lose balance and generating a differential voltage change across the bridge. The signal output of the bridge is this differential voltage, which is directly connected to the input of the instrumentation amplifier. In addition, a constant DC voltage is also applied to the two inputs. This DC voltage is usually equal or common mode at the two inputs. Regarding the main function of the instrumentation amplifier, it usually suppresses the common mode DC voltage or any other voltage common to the two inputs, while amplifying the differential signal voltage, that is, the voltage difference between the two inputs.

In contrast, if a standard op amp is used in this application, it will only amplify the signal voltage across it and any DC signals, noise, or other common-mode voltages. Therefore, the signal will be buried in the DC offset voltage and noise. For this reason, even the best op amp cannot effectively extract weak signals.