9.1 Use Case
- Couple a high impedance signal source to a low impedance signal sink
- Take an input signal and amplify it by a certain amount, G (gain). The gain is directly proportional to the R2/R1 resistor value ratio present in the feedback loop (see Figure 9-1).
- The input impedance to an ADC has a low value hence the non-inverting amplifier acts as a buffer
- Increases the dynamic range of the input signal improving the sampling resolution
Consider the case when the input signal to an ADC is 25 mV referred to the system’s ground. The ADC has a 10-bit resolution, and the voltage reference has been selected as 1.024V. It means, as per the data sheet, that the quantization error, of one least significant bit (LSb), translates to 1 mV, which represents 4% of the input signal. If an accurate measurement is desired, this can represent a high source of error. However, it can be improved by amplifying the input signal before sampling it via the ADC. With a gain of 16, the input signal will be amplified to 400 mV before sampling. It follows that the quantization error of 1 mV, represents only 0.25% of the input signal, which is a considerable improvement.
It is common for applications requiring a non-inverting amplifier to have a fixed gain, set via the external resistors, as part of the negative feedback loop. These applications assume that the input signal will fit within a fixed, predefined range. However, such a setup is limiting if the input signal has a high dynamic range, or the transducers (sensors) exhibit a change in their output signal over time, due to external (e.g., environmental) or internal (e.g., aging) factors. For such cases, it is beneficial to be able to modify the gain of the amplifier without a change in components. A change in gain either upwards or downwards will bring the signal of interest within its specified range once more. This can be done through a programmable gain amplifier (PGA) where the feedback resistor ratio can be adjusted between several values. It is likely that these applications involve a control loop where the algorithm is constantly monitoring the control and feedback signals and is able to make a decision when a different gain is required. PGAs have a wide area of applications such as audio and voice, data acquisition, industrial and medical instrumentation, lighting, motor control, power control, and test equipment.