Build a variable-gain inverting amplifier using a digital rheostat and an operational amplifier

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Compact, Low Cost, 5 V, Variable Gain Inverting Amplifier Using the AD5270/AD5272 Digital Rheostat and the AD8615 Op Amp

Circuit Description

The circuit shown in Figure 1 uses the AD5270/AD5272 digital rheostat and the AD8615 operational amplifier to provide a compact, low cost, low voltage, variable gain inverting amplifier. The small size and low cost of the AD5270/AD5272 (10-lead 3 mm × 3 mm × 0.8 mm LFCSP) and AD8615 (5-lead TSOT-23) packages provide an industry-leading solution for analog signal processing circuits.

The circuit offers 1024 different gains that can be controlled through an SPI (AD5270) or I2C (AD5272) compatible serial digital interface. The AD5270/AD5272 features ± 1% resistor tolerance performance, providing low gain error over the entire resistor range, as shown in Figure 2.

This circuit supports rail-to-rail input and output, can be operated with either a +5 V single supply or a ± 2.5 V dual supply, and can provide up to ± 150 mA of output current.

In addition, the AD5270/AD5272 contain a 50-time programmable memory that allows custom gain settings at power-up.

The high accuracy, low noise, and low total harmonic distortion (THD) characteristics of this circuit make it ideal for signal conditioning applications in instrumentation.

This circuit uses the AD5270/AD5272 digital rheostat and the AD8615 CMOS operational amplifier to provide a low cost, variable gain inverting amplifier.

The AD8615 connected in reverse mode amplifies the input signal VIN. This operational amplifier features low noise, high slew rate, and rail-to-rail input and output.

The maximum circuit gain is determined by Equation 1.

The maximum current flowing through the AD5270/AD5272 (RAW = 20 kΩ version) is ±3 mA, which limits the maximum input voltage, VIN, according to the circuit gain, as shown in Equation 2.

When the input signal connected to VIN is higher than the theoretical maximum value determined by Equation 2, R2 should be increased and the new gain recalculated using Equation 1.

Additionally, the minimum gain should be calculated to reduce the error due to leakage current of the AD5270/AD5272. To make the leakage current error negligible, the current flowing through R2 should be at least 100 times the worst-case leakage current of 50 nA. Therefore, the minimum current through R2 should be 5 μA, which also determines the minimum value of R2 in Equation 3.

Figure 2 shows the possible range of R2 values ​​based on the op amp's input voltage under these assumptions.

The ±1% internal resistor tolerance of the AD5270/AD5272 ensures low gain error, as shown in Figure 3.

The circuit gain calculation formula is:

Where D is the code loaded into the digital potentiometer.