Sensors & transducers
A VARIATION OF THE CLASSIC INSTRUMENTATION AMPLIFIER (PGIA) OFFERS MORE DESIGN FLEXIBILITY
By Hooman Hashemi, product applications engineer, Analog Devices
As useful and versatile as instrumentation amplifiers (IAs) are when it comes to interfacing to a transducer, there are constraints that hamper the design of variable gain IAs or programmable gain instrumentation amplifiers (PGIAs), also referred to as software programmable gain amplifiers (SPGAs) in some literature. The need for such PGIAs arises because of the often-encountered case of adapting the circuit to a wide range of sensors or environmental conditions. With a fixed gain, the system designer may have to contend with suboptimal SNR, which could compromise precision. In this article, I will present another tool and methodology to facilitate such work and I will go through the design steps that allow one to quickly home-in on the external component values needed to create a precise PGIA using a newly released instrumentation amplifier.
A NEW INSTRUMENTATION AMPLIFIER ARCHITECTURE
One common instrumentation amplifier architecture is shown in Figure 1.
The gain is set by the value of the external resistor RG. To create a PGIA with such a device, one merely needs to switch the value of RG. This is commonly done using an analogue switch or a mux. However, some of the non-ideal behaviours of an analogue switch complicate this task - examples being the switch on-resistance, its channel capacitance, and the variation of channel resistance with applied voltage.
One variation on the standard instrumentation amplifier architecture is shown in Figure 2. Notice how the RG pins are broken up as ±RG,S and ±RG,F and individually pinned out and accessible from the outside of the device package. A useful and important feature of the architecture shown in Figure 2 is the ability to configure the instrumentation amplifier so that it can switch between several distinct gain values while minimising any gain error because of the finite switch resistances. This feature can be used to create a PGIA. As mentioned, any resistor programmable in-amp can be enabled to vary its gain by switching the value of the gain-setting resistor accordingly. However, there are significant drawbacks to this such as:
Large gain error due to switch on-resistance (RON) nominal value and its variation.
High gain values may be impossible to achieve due to the low switch RON values required.
Signal distortion due to switch non-linearity. That is because signal current flows directly through RON and thus any variation in its value as a function of voltage causes distortion.
Figure 1. Classic instrumentation amplifier.
resistor. Configured this way, RON is only a small fraction of the total internal 12.1kΩ feedback resistance and thus has little impact on gain error and drift. By the same token, distortion due to the switch non-linearity is minimised because of the RON value being a small fraction of the overall feedback resistance causing little or no effect due to variation of its value with voltage. Furthermore, the input stage of this device is comprised of a current feedback amplifier (CFA) architecture, which by its nature allows less variation in bandwidth or speed as gain is varied when compared to a traditional voltage feedback amplifier. All this culminates in the ability to create a precision PGIA with accurate gain steps using low cost external analogue switches. Figure 4 shows a simplified diagram of the PGIA to demonstrate how different taps of the resistor ladder, implemented by analogue switches (eight total) shorted two at a time to set the gain, configure the circuit. In this diagram, the two switch banks are depicted in one of the four possible gain values; that being with the –RG,S and +RG,S pins shorted to the RF3/RF4 junction.
Figure 2. LT6372-1 architecture allows access to some of the IA internal nodes.
The LT6372-1 can alleviate these concerns when configured as a PGIA, shown in Figure 3, because of how the RG,F and RG,S pins are separately pinned out. In this schematic, the signal from a Wheatstone bridge, consisting of R5 to R8, is amplified with four possible gain values selectable by the user depending on which SW1 switch position is selected. The LT6372 family pinout allows one to create a PGIA that takes advantage of varying the ratio of RF/RG to get the desired gain values.
Furthermore, the U1, U2 analogue switch RON is minimised as a source of gain error because it can be placed in series with the input stage inverting terminal and its feedback
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DESIGN STEPS TO COMPUTE EXTERNAL RESISTORS FOR ANY GAIN
Figure 3 shows the complete PGIA configuration, including the switches required, which can accommodate an arbitrarily large gain range. Four possible gain values are included here, but it is possible to increase that number by adding more switches to the design. As noted earlier, the feature of allowing access to the RG,F and RG,S pins enables one to increase RF for large gains, and to reduce RG for low gains to create a versatile PGIA. For purposes of gain computation, one can consider the feedback resistor to be the internal 12.1kΩ trimmed resistance plus other resistances in series with RG,F on its way to reach the RG,S terminal. Conversely, the gain setting resistor is the
February 2026 Instrumentation Monthly
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