Instrumentation amplifiers are a two-stage circuit used to extract and amplify differential input signals in the presence of common-mode voltages, as shown in Figure 1-1. The first stage forms a high-input-impedance circuit that amplifies the differential signal but passes the common mode signal without amplification. The second stage of the design is typically a difference amplifier that largely removes the common-mode signal while referencing the output to a specific reference voltage.

In many IA applications, accurate, low-level ac signal processing is desired, while dc signals are rejected at the output. The easiest way to implement these functions is to ac-couple the IA. Designers ac-couple by adding capacitors in series with the IA inputs to block the dc input voltages, which effectively forms a high-pass filter. This method eliminates the need to accommodate the dc input signal before the IA gain stage pushes the dc input signal to saturation, a nonlinear operating condition. Therefore, this method of passing only the ac signal allows for higher gain and wider dynamic range.

For example, assume an IA has a 100-Hz sine wave with an amplitude of 100 mV in the presence of a 5-V common-mode voltage and a 3-V dc voltage. The desired output is a ±1-V signal. Using these operating conditions, the instrumentation amplifier must be configured with a gain of 10V/V. Applying the INA818, one of TI’s high-precision, low-power, low-noise IAs, the circuit schematic and transient analysis is shown in Figure 2-1.

While the 5-V common-mode voltage is rejected by the IA, the 3-V dc voltage sums with the input differential voltage shown by the VDiff curve. In a gain of 10V/V, the output signal is saturated against the positive power supply rail. Even though the desired signal to amplify was the 100 mV / 100 Hz sine wave, the 3-V dc voltage prevented the instrumentation amplifier output from representing only the amplified ac signal.

Eliminating the dc nonlinearity at the output due to the presence of a dc input voltage is important. Engineers often mistakenly configure an ac-coupled IA circuit by adding a capacitor in series with each input terminal, but without providing a path for the input bias current. This mistake is illustrated in Figure 3-1. If a capacitor is connected in series with an instrumentation amplifier input without a dc path for current to flow, then over time, the Ib of the IA charges the capacitor until the output is driven to one of the rails, as shown by the IN+ trace in Figure 3-1.

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.