8.2.2.8 IA Structure, Voltage Monitor

The instrumentation amplifier has high-impedance NPN transistor inputs that do not load the output signal, which is especially important in current output mode. The output signal is a controlled current that is multiplexed either to the SET pin (to close the voltage output loop) or to IA_OUT (for external access).

The principal circuit is shown in Figure 44. The two input buffer amplifiers reproduce the input difference voltage across R_GAIN. The resulting current through this resistor is bidirectionally mirrored to the output. That mirroring results in the transfer function of Equation 6:

Equation 6.

The accuracy and drift of R_GAIN defines the accuracy of the voltage to current conversion. The high accuracy and stability of the current mirrors result from a cycling chopper technique.

Figure 44. IA Block Diagram

The output current, IA_OUT, of the instrumentation amplifier is limited to protect the internal circuitry. This current limit has two settings controlled by the state of M2 (see Electrical Characteristics: Instrumentation Amplifier (IA), Short-Circuit Current specification).

With M2 = low, the nominal R_GAIN of 10 kΩ allows an input voltage of 20 V_PP, which produces an output current of 4 mA_PP. When using lower resistors for R_GAIN that can allow higher currents, the IA output current limitation must be taken into account.