Real-world data-acquisition systems require amplifying weak signals to match the full-scale input range of an A/D converter. Unfortunately, when you configure them as gain blocks, most common amplifiers have both gain errors and offset drift. The typical two-resistor gain-setting arrangement found in many op-amp circuits has serious accuracy and drift limitations. With standard 1% resistors, the circuit gain can be off by as much as 2%. Also, the gain can vary with temperature, because each resistor drifts differently. You can use monolithic resistor networks for precise gain setting, but these components are expensive and consume valuable pc-board space. The circuits of Figure 1 and Figure 2 offer improved performance and lower cost; they are also smaller. The single-µSOIC approach is the smallest available for this function, and the circuits require no external components. Figure 1 shows an AD628 precision gain block connected to provide a voltage gain of 10. The gain block itself comprises two internal amplifiers: a gain-of-0.1 difference amplifier, A₁, followed by an uncommitted buffer amplifier, A₂. You can configure it to provide different gains by strapping or grounding the appropriate pins.

For a gain of 10, the input signal connects between the V_REF pin (Pin 3) and ground, instead of to the op amp's inputs. With the input tied to the V_REF pin, the voltage at the noninverting input of A₁ equals V_IN(100 kΩ/110 kΩ), or V_IN(10/11). The inverting input of A₂ (Pin 6) is grounded; therefore, feedback from the output of A₂ forces the noninverting input of A₂ to be 0V. The output of A₁ must then also be at 0V. The voltage on the inverting input of A₁ must be equal to the voltage on the noninverting input of A₁, so both equal V_IN(10/11). Thus, the output voltage of A₂, V_OUT, equals

providing a precise gain of 10 with no external components.

The companion circuit of Figure 2 provides a gain of –10. This time, the input connects between the inverting input of A₂ (Pin 6) and ground. Operation is similar to that of Figure 1, but A₂ now inverts the input signal by 180°. With the V_REF pin grounded, the noninverting input of A₁ is at 0V, so feedback forces the inverting input of A₁ to 0V as well. Because A₁ operates at a gain of 0.1, the output of A₂ necessary to force the inverting input of A1 to 0V is –10V_IN. The two connections exhibit different input impedances. When you drive the V_REF input (Pin 3) for a gain of 10, the input impedance to ground is 110 kΩ; it is approximately 50 GΩ when you drive the noninverting input of A₂ (Pin 6) for a gain of –10. All resistors are internal to the gain block, so both accuracy and drift are excellent. These circuits have gain accuracy better than 0.1%, with a gain temperature coefficient lower than 5 ppm/°C. The –3-dB bandwidth is approximately 110 kHz with a 10-mV input and 95 kHz with a 100-mV input. Although ±15V supplies are appropriate, you may operate these circuits with dual supplies from ±2.25V to ±18V.