Autoranging circuit simplifies hardware and software

Dana Romero, Phonex, Midvale, UT -- 7/22/1999

A natural approach to autoranging is to use an inverting op amp and switch in one of N feedback resistors at a time for a gain of A=-R_F/R_O (Figure 1). However, the inverting configuration suffers from low input impedance of Z_IN=R_O because the negative input is at virtual ground. Also, another inverter is necessary to provide positive voltage ranges because of positive signals at the input.

An alternative approach is to use a noninverting op amp that has a high input impedance and a gain equation of A=1+R_F/R_O (Figure 2). This circuit augments the typical 0 to 5V input range of a µC with ranges of 0 to 1V and 0 to 10V. The circuit in Figure 2 uses a 68HC16 µC, but the results generally apply to any µC with an ADC. The unusual arrangement of circuit components provides three input ranges that use only a single op amp, two spdt relays, and resistors with standard values. Also, the circuit simplifies the autoranging software because after K₂ switches, the status of K₁ is irrelevant.

With the relays in the positions that Figure 2 indicates, R_F equals R₁, which in turn equals R_O. Thus, the amplifier gain is 2. Combined with the voltage divider of R₄/R₅, the overall circuit gain is 1/2, which is the gain necessary for a 0 to 10V input at V_IN to provide 0 to 5V at V_AD.

When K₁ switches, R_F becomes R₂+R₁=2R_O+R_O=3R_O for a gain of 1+3R_O/R_O=4. Dividing by 4 gives an overall gain of 1, which is suitable for a 0 to 5V range at V_IN. At this point, it becomes obvious why the voltage divider sits before the amplifier: With V_IN near 5V, multiplying by 4 would saturate the op amp.

When K₂ switches, the position of K₁ is irrelevant, and R_F equals R₃+R₁=18R_O+R_O=19R_O. Thus, A=1+19R_O/R_O=20. Now, the overall gain is 20/4=5, or exactly the value necessary for a V_IN range of 0 to 1V to produce an output of 0 to 5V.

Although the gain calculation works out exactly with standard 5% resistor values, for greater accuracy, you can switch the resistors to the nearest 1% values. Or, even better, sort through a batch of 5% resistors and use a good multimeter to find resistors that are within 1% of nominal. (DI #2381)

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