Overview
Design Resources
Device Drivers
Software such as C code and/or FPGA code, used to communicate with component's digital interface.
Features & Benefits
 Digital Pot and InAmp Create Programmable Gain InAmp
 Low Cost
 High Voltage
 Optimized for Low Noise
Circuit Function & Benefits
The circuit offers 1,024 different gain settings, controllable through an SPI 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 provides a high performance instrumentation amplifier that delivers the industry’s highest CMRR over frequency in its class and dynamic programmable gain for both single supply operation at +30 V and dual supply operation at ±15 V. In addition, the AD5292 has an internal 20times programmable memory that allows the user to customize the instrumentation amplifier gain at powerup.
The circuit provides accurate, low noise, high gain and is well suited for signal instrumentation conditioning, precision data acquisition, biomedical analysis, and aerospace instrumentation.
Circuit Description
The differential input signal, +IN and –IN, is amplified by the AD8221. The instrumentation amplifier offers accuracy, low noise, high CMRR, and high slew rate.
The maximum circuit gain is defined in Equation 1, where R_{AW_MIN} is the wiper resistance of the AD5292 in the rheostat mode and represents the minimum value of the gainsetting resistance (100 Ω).
The circuit gain formula for any particular AD5292 resistance is
This equation is plotted in Figure 3 as a function of D, the decimal code.
The maximum current allowed through the AD5292 is ±3 mA, which limits the allowable circuit gain as a function of differential input voltage.
Equation 3 shows the maximum gain limit as a function of the differential input voltage, V_{IN}. This equation is derived by substituting R_{AB} = V_{IN} /3 mA into Equation 1. The equation is plotted in Figure 5.
Equation 1 limits the maximum circuit gain to 500. Equation 2 can be solved for D, yielding Equation 4, which calculates the minimum allowable resistance (in terms of the digital code) in the AD5292 without exceeding the current limit.
where D is the code loaded in the digital potentiometer, and G is the maximum gain calculated from Equation 3.
When the input to the circuit is an ac signal, the parasitic capacitances in the digital potentiometer can cause a reduction in the maximum AD8221 bandwidth. A gain and phase plot is shown in Figure 5.
The AD5292 has a 20times programmable memory, which allows presetting the output voltage in a specific value at powerup.
Excellent layout, grounding, and decoupling techniques must be used to achieve the desired performance from the circuits discussed in this note (see MT031 Tutorial and MT101 Tutorial). As a minimum, a 4layer PCB should be used with one ground plane layer, one power plane layer, and two signal layers.
Discussions
Sample Products
Samples
Product 
Description 
Available Product

AD8220  JFET Input Instrumentation Amplifier with RailtoRail Output 
AD8220ARMZ AD8220BRMZ AD8220TRMZEP AD8220WARMZ 
AD5291  Single Channel, 256Position, 1% RTol, Digital Potentiometer with 20Times Programmable Memory 
AD5291BRUZ100 AD5291BRUZ20 AD5291BRUZ50 
AD5292  Single Channel, 1024Position, 1% RTol, Digital Potentiometer with 20Times Programmable Memory 
AD5292BRUZ100 AD5292BRUZ20 AD5292BRUZ50 AD5292SRUZ20EP 
AD5293  Single Channel, 1024Position, 1% RTolerance Digital Potentiometer 
AD5293BRUZ100 AD5293BRUZ20 AD5293BRUZ50 
AD8227  Wide Supply Range, RailtoRail Output Instrumentation Amplifier 
AD8227ARMZ AD8227ARZ 
AD8221  Precision Instrumentation Amplifier 
AD8221ARMZ AD8221ARZ AD8221BRZ AD8221TRMZEP 