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Amplifier-Guide-Slide5

The simplicity of the ICF architecture gives rise to two immediate advantages. First, it is lower power than traditional designs. This is because it does not require two or three op-amps to make the instrumentation amplifier, and therefore simply requires less transistors to build. The second advantage is that it doesn’t require internal trimmed thin-film resistor networks to achieve high CMRR and gain accuracy. This allows these in-amps to be manufactured and sold at lower cost. Another major advantage of the ICF architecture is that the internal signals are current rather than voltage. As a result, internal nodes cannot saturate and there is no hexagon plot. However, users must be careful of the differential input limit for ICF in-amps, which may impose an additional limit to output swing at low gains. Further benefit comes from the fact that the gain is set by two resistors and the gain error and drift is determined by the relative match of these two resistors. ICF in-amp users can design to system gain accuracy and drift requirements using anything from low cost resistors to tightly matched resistor networks, such as the Susumu RM series. The AD8237, for example, has 0.005 % maximum gain error and 0.5 ppm/°C maximum gain drift, enabling very high precision performance. Additionally, some applications benefit from an in-amp with a high impedance reference pin. For example, the REF pin is often used to level-shift the output. If a resistor divider is used to generate the level-shifting voltage, most in-amps would require a buffer between the divider and the REF pin to maintain a high CMRR. This buffer is not necessary with the ICF architecture, enabling power, space, and cost savings. Given the advantages highlighted here, as well as a few others, a number of application circuits can be developed that improve upon what can be done with traditional instrumentation amplifiers. The following slides will list just a few of these useful circuits.

PTM Published on: 2012-12-10