» Circuits 11032016 Digitally programmable instrumentation amplifier offers autozeroingAnalog Devices » AD8231Marian Stofka EDN The current trend in advanced instrumentation amps is to use no external resistors. In these amplifiers, a gaincontrol word, comprising a binarycoded one, sets the voltage gain. Several integer gains within one to 1000 are currently available; however, this range does not yet include a gain of three. Although externalresistorfree amplifiers with a gain of three are available, they are neither instrumentation amps nor autozeroed devices (Reference 1). These features are essential in applications requiring accurate processing of lowlevel voltages. You can use the circuit in Figure 1 for applications requiring instrumentation amps having voltage gains of three or 10 and the ability to process voltages as low as 1 mV.
This design achieves a voltage gain of three by using the “algorithm” of 3 = 2 + 1. The circuit comprises two units of the Analog Devices digitally gainprogrammable, autozeroed AD8231 instrumentation amp. These ICs have voltage gains that are programmable as powers of two – that is, one, two, four … 128 (Reference 2). Amplifier A_{1} in IC_{1} is preset to provide a gain of two, and auxiliary amp A_{2} in IC_{2 }is preset to a gain of one. The noninverting and inverting inputs of A_{1} and A_{2} connect together. The output of A_{2} connects with reference input REF_{1}, and reference input REF_{2} serves as a freely usable reference. You can thus calculate the output voltage as V_{OUT} = V_{OUT1} + V_{REF1} = V_{OUT1} + V_{OUT2} = 2ΔV_{IN} + ΔV_{IN} = 3ΔV_{IN}, where ΔV_{IN} is the inputdifference voltage. Similarly, you can achieve a voltage gain of 10 according to a symbolic formula of 10 = 8 + 2. This time, A_{1} has a voltage gain of eight, and A_{2} has a gain of two. Using Reference 2, you can derive that, for gains of both three and 10, A_{1}A_{1} = A_{0}A_{2}. Therefore, the gaincontrol pins connect and remain low for a gain of three, and the high at these pins sets the gain to 10. Note that three approaches the square root of 10, or approximately 3.16. You can therefore consider it as roughly the geometric center of a decade. ReferencesMaterials on the topic 

