Signal AmplificationSeptember 1, 2006 By: Ed Ramsden Sensors
With the recent introduction of cheap ΔΣ analog-to-digital converters (ADCs) offering resolutions of 24 bits or more, you'd think that the digital revolution is complete, and that the need for analog design has passed. Twenty-four bits gives you a resolution of better than 1 μV on a 10 V span, so these high-resolution converters will make it easy to solve many interfacing problems with a minimum of additional circuitry.
The transducer side of sensing, however, hasn't exactly been stagnant. New applications are placing increasingly stringent demands on recovering signals from existing transducers, and new transducer techniques are often dependent on subtle and sophisticated signal processing and recovery techniques. In many cases, micro sensors such as MEMS devices make micro signals (Figure 1), which eventually have to be boosted to levels where they can be handled by a system's ADC.
Figure 1. Micro sensors make micro signals that have to be raised to levels manageable by the system's ADC
Amplification is the set of techniques used to boost a signal's strength. Figure 2 shows a combination of an idealized transducer and an idealized amplifier. The key features of the transducer model are an open-circuit voltage (VOCT) and an output impedance (rOT). The amplifier has an input impedance rIN, an output impedance rOA, and an open-circuit output voltage defined as VOCA = AVVIN, where AV is the amplifier's gain.
Figure 2. An idealized transducer and idealized amplifier are combined here
Maintaining Accurate Gain
While the overall goal is to increase the amplitude of the transducer's output signal, there are a number of secondary goals that must be considered when selecting or designing an amplifier. One of the most important of these in many sensor systems is to maintain accurate gain. In the system of Figure 2, there are two fundamental ways you can achieve this.
The first is to simply make the amplifier's input impedance much higher than the transducer's output impedance. The signal seen at the amplifier's input will be VOCT × [rIN/(rIN+rOT)], which is about equal to VOCT when rIN >>rOT. For example, with transducer output impedances less than a few megohms, a simple op amp amplifier circuit such as the one shown in Figure 3 can often be used. When implemented with a suitable FET-input op amp, this circuit can provide in excess of 1010 Ω of input impedance at DC. Using a very high input impedance amplifier is often an adequate and simple solution to many interface problems.
Figure 3. In some cases, a simple op amp circuit can help you maintain accurate gain
In some cases, especially those involving high-frequency signals or very small signals, the high-input impedance solution may not be adequate. At high frequencies, an amplifier's input impedance may be dominated by a reactive component. For example, a FET-input amplifier that provides 1012 Ω input impedance at DC may have a 1 pF input capacitance, which appears as roughly 160 kΩ at 1 MHz. So much for the benefit of high DC impedance.
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