4 Proposed Solution

A simple solution to this problem is to connect a resistor from each IA input to a system ground or other bias voltage. This solution provides a path for the input bias current to properly bias the IA inputs. In a dual-supply configuration (a sample schematic is shown in Figure 4-1A), each resistor is connected to ground. In a single-supply configuration, in order to maximize the input voltage range, the input bias resistors are usually connected to a voltage (Vbias) at mid-supply, as shown in Figure 4-1B. This mid-supply connection serves a secondary purpose in that a typical IA input generally cannot swing all the way to the power-supply rails. Thus, connecting the resistors to mid-supply maximizes the IA input dynamic range. Similarly, because the output of an IA cannot swing to the rails, connecting a reference voltage (Vref) to mid-supply maximizes the output voltage dynamic swing. Be aware that both Vref and Vbias must be able to sink and source current; therefore, using a low drop-out regulator (LDO) here may be unacceptable because an LDO can only source current. A buffer or a voltage reference is usually needed to drive the reference pin on an IA.