In this design, a ±10-V, 10-kHz bandwidth, bipolar signal is attenuated and converted to a single-ended signal and filtered by a 3rd-order Butterworth filter to drive a single-ended analog-to-digital converter (ADC). By using the OPA2206, the input of the signal chain is protected from overvoltages up to 40 V beyond either supply. This signal-chain design is common for programmable logic controllers (PLCs), low-power data acquisition systems (DAQs) and field instruments where high precision, low power and signal fault protection are needed.

The OPA2206 was selected for this application because of the high supply range, high dc precision (4-µV offset and 0.08-µV/°C offset drift), and low power consumption (220-µA quiescent current) that minimizes thermal dissipation requirements. Because of the internal OVP topology, the device provides better dc and ac accuracy under normal operating conditions compared to passive external protection and results in a smaller system solution. Be sure to connect a zener diode between each supply to ground to provide a return path for the current that is generated during a fault condition.

The first stage of the signal chain is an attenuator and level-shifter. The input signal to this stage is bipolar ±10 V that is attenuated to ±2.5 V, and then level-shifted so that the output is a single-ended, 0-V to 5-V signal. The feedback and gain resistors were selected as 20 kΩ and 80 kΩ, respectively. Thus, the combined impedance is 100 kΩ, which lowers the input current to the signal chain, and minimizes errors resulting from higher output impedance sensors.

The second stage of the signal chain uses the second channel of the OPA2206 to create a 3rd-order Butterworth filter with a –3-dB response of 20 kHz. For more information on filter design, refer to Texas Instrument's filter design tool.

The output of this signal chain is shown in Figure 8-5 and the filter response is shown in Figure 8-3.