Variable Gain Noninverting Amplifier Using the AD5292 Digital Potentiometer and the OP184 Op Amp
The circuit offers 1024 different gains, controllable through an SPI-compatible serial digital interface. The ±1% resistor tolerance performance of the AD5292 provides low gain error over the full resistor range, as shown in Figure 2.
The circuit supports rail-to-rail inputs and outputs for both single-supply operation at +30 V and dual-supply operation at ±15 V, and is capable of delivering up to ±6.5 mA output current.
In addition, the AD5292 has an internal 20-times programmable memory that allows a customized gain setting at power-up.
The circuit provides accuracy, low noise, and low THD and is well suited for signal instrumentation conditioning.
Figure 1. Variable Gain NonInverting Amplifier Simplified Schematic (Decoupling and All Connections Not Shown)
The input signal, VIN, is amplified by the OP184. The op amp offers low noise, high slew rate, and rail-to-rail inputs and outputs.
The maximum circuit gain is defined in Equation 1.
The maximum current through the AD5292 is ±3 mA, which limits the maximum input voltage, VIN, based on the circuit gain as described in Equation 2.
When the input signal connected to VIN is higher than the theoretical maximum value from Equation 2, R2 should be increased, and the new gain can be recalculated using Equation 1.
The ±1% internal resistor tolerance of the AD5292 ensures a low gain error, as shown in Figure 2.
The circuit gain equation is
where D is the code loaded in the digital potentiometer.
Figure 2. Gain and Gain Error vs. Decimal Code
Figure 3. Gain and Phase vs. Frequency for AC Input Signal
When the circuit input is an ac signal, the parasitic capacitances of the digital potentiometer can cause undesirable oscillation in the output. This can be avoided, however, by connecting a small capacitor, C1, between the inverter input and its output. A value of 10 pF was used for the gain and phase plots shown in Figure 3.
A simple modification of the circuit provides a logarithmic gain function, as shown in Figure 4. In this case, the digital potentiometer is configured in the ratiometric mode.
Figure 4. Logarithmic Gain Circuit
The circuit gain is defined in Equation 4:
where D is the code loaded in the digital potentiometer. A gain plot vs. code is shown in Figure 5.
The AD5292 has a 20-times programmable memory, which allows presetting the output voltage in a specific value at power-up.
Excellent layout, grounding, and decoupling techniques must be utilized in order to achieve the desired performance from the circuits discussed in this note (see Tutorial MT-031, Grounding Data Converters and Solving the Mystery of AGND and DGND and Tutorial MT-101, Decoupling Techniques). As a minimum, a 4-layer PCB should be used with one ground plane layer, one power plane layer, and two signal layers.
|OP184||Single-Supply Rail-to-Rail Input/Output Operational Amplifier|
|AD5291||Single Channel, 256-Position, 1% R-Tol, Digital Potentiometer with 20-Times Programmable Memory||
|AD5292||Single Channel, 1024-Position, 1% R-Tol, Digital Potentiometer with 20-Times Programmable Memory||
|AD5293||Single Channel, 1024-Position, 1% R-Tol, Digital Potentiometer||