Data acquisition systems with wide dynamic range often require some way to adjust the input signal level to the analog-to-digital converter (ADC). For optimal performance of an ADC, the maximum input signal should match its full-scale voltage. This can be accomplished with a programmable gain amplifier circuit.

This circuit utilizes a quad SPST switch ( ADG1611 ) and a resistor programmable instrumentation amplifier ( AD620 ) to provide programmable gain functionality.

Four single-pole single-throw switches are connected to four precision resistors. These switches can be used to control the external gain setting resistor value R _G to set the gain value.

In this application, low switch on-resistance is critical, and the ADG1611 has the industry's lowest on-resistance R _ON (1 Ω typical) and is available in the smallest 16-pin, 4 mm × 4 mm LFCSP package.

This circuit uses the industry standard low-cost AD620 and quad switch ADG1611 combination to achieve unparalleled performance and provide programmable gain characteristics with all the advantages of a precision instrumentation amplifier.

Figure 1 shows the programmable gain instrumentation amplifier circuit, which consists of a ±5 V four-channel SPDT switch with ultra-low on-resistance R _{ON , the industry standard AD620 instrumentation amplifier, and four 0.1% standard resistors.}

The ADG1611's ultra-low on-resistance makes it an ideal solution for gain switching applications where low on-resistance and low distortion performance are critical. The AD620 is a low-cost, high-precision instrumentation amplifier that requires only an external resistor _RG on pins 1 and 8 to set the gain from 1 to 10,000.

Using the ADG1611 and AD620 combination, designers can control the gain of the AD620 by switching different gain setting resistors in R _G. This circuit provides a low-power, low-cost programmable gain instrumentation amplifier solution.

The gain can be changed by selecting different combinations of switches S1, S2, S3 and S4 to change R _G . Sixteen possible gain settings can be controlled through the ADG1611 parallel interface. The gain of the AD620 is programmed with a resistor between Pin 1 and Pin 8. The AD620 is designed to provide accurate gain using 0.1% to 1% tolerance resistors.

Its gain is easily calculated using the following equation:

Figure 1 shows a circuit setup requiring gains of 1, 50, 100, 500, and 1000. Table 1 shows the control pins IN1 through IN4 of the ADG1611, which control the resistance between Pin 1 and Pin 8 of the ADG620. This circuit utilizes standard 0.1% resistors to achieve the gain settings below. The table also shows the gain obtained by adding the on-resistance of the ADG1611 to the signal chain and how temperature affects the gain. The ADG1611 switch has ultra-low on-resistance, which is ideal because R _ON is much smaller than R _ON and R _ON changes very little with temperature. Figure 2 shows the gain error due to temperature changes in the ADG1611 R _{ON .}

The test circuit in the lab includes an automatic switching mode that automatically switches the gain from 1 to 50, 100, 500, 1000 and back to 1.

Figure 3 shows the circuit waveforms when the gain is sequentially switched from 50 to 1000.

The combination of the ADG1611 and AD620 provides a low-cost, high-precision programmable gain instrumentation amplifier solution that can achieve 16 levels of programmable gain.

In order to achieve the desired performance of the circuits discussed in this article, excellent layout, grounding, and decoupling techniques must be used (see Tutorial MT-031 and Tutorial MT-101 ). At least a four-layer PCB should be used: one is the ground layer, one is the power layer, and the other two are signal layers.