5 Component Selection and Trade-Offs

Adding resistors and capacitors comes with trade-offs, including increased noise and limited board space. Typical resistor values used for this purpose, seen as R1 and R2 in Figure 5-1, range from 100 kΩ to 1 MΩ. The values are usually chosen to be large compared to the differential source resistance. However, large resistor values introduce higher thermal noise and dc offset due to the instrumentation amplifier input bias current flowing through the resistors and developing a voltage drop across the resistors. Additionally, the input bias current flows into the input impedances and produces voltages that add to the overall system error, as shown in Figure 5-1 and Equation 1:

Using higher-value capacitors allows for smaller resistors; however, large capacitors consume more board space. In addition to the value of the capacitor, the type of capacitor plays a large role in component selection. Depending on the application, a designer may need to opt for a specific capacitor grade because the grade may have an impact on linearity and distortion performance. C0G-grade capacitors offer the most stable capacitance of any ceramic capacitor over temperature, voltage, and frequency, and thus produce the lowest distortion. Ceramic X7R- and X5R-grade capacitors are not as stable over temperature, voltage, and frequency, and may result in unacceptable distortion over frequency.

In addition to the trade-offs of noise, offset error, and board space, these added components may affect overall precision. The resistors and capacitors added to the circuit in both input paths must be precisely matched to each other. That means R1 must match R2, and C1 must match C2. Any mismatch of these components degrades the ac common-mode rejection ratio (CMRR), which in turn converts this common-mode signal into a differential signal, and introduces error at the output of the circuit. One way to preserve CMRR is by lowering the cut-off frequency of the RC combination. However, this method requires larger-value resistors and capacitors, which again contributes to noise and board space. Another way of increasing precision is to add a third resistor that is typically one-tenth value of the other two resistors. Connect this third resistor between the instrumentation amplifier inputs.

Figure 5-2 shows the output error improvement by adding the third resistor. These circuits showcase the INA849, which has an input bias current of 50 nA (maximum). As shown in Equation 1, the input bias current flows into the input impedances to produce a voltage that then appears at the output of the instrumentation amplifier as error. Again, if the IA is configured with gain, the error is also amplified, adding even more error at the output. Adding a third resistor between the inputs of the IA, as shown in the following circuits, reduces the overall input impedance, which results in less system error at the output. In the left circuit, two perfectly matched 10-MΩ resistors are used. Adding the third 1-MΩ resistor reduces the output error significantly.

Unfortunately, resistors cannot be perfectly matched in production; instead, resistors are labeled according to tolerance. A 1% 1-MΩ resistor can be up to 1% off and still be within tolerance. Figure 5-3 shows two circuits, where R1 and R2 varied 1% in opposite directions; the worst possible scenario. However, adding the third resistor significantly reduced the output error. Depending on the system requirements, this third resistor can allow a designer to use lower tolerance resistors and still achieve a high-precision output.

Be aware that this third resistor has a unique set of drawbacks. While adding the third resistor allows a designer to counteract the impact of large resistor mismatch between R1 and R2, adding the third resistor does reduce the overall impedance. Reducing the input impedance may affect the sensor circuit driving the IA. The designer must make sure the impedance formed by R1 through R3 is still large compared to the source output impedance. Reducing the input impedance also changes the corner frequency of the dc-blocking high-pass filter. Depending on the application, this corner frequency may need to adjusted by increasing the size of the coupling capacitors.