Single IC provides gains of 10 and –10
Edited by Bill Travis
Moshe Gerstenhaber and Charles Kitchin, Analog Devices, Wilmington, MA -- EDN, August 7, 2003
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 VREF pin (Pin 3) and ground, instead of to the op amp's inputs. With the input tied to the VREF pin, the voltage at the noninverting input of A1 equals VIN(100 kΩ/110 kΩ), or VIN(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 VIN(10/11). Thus, the output voltage of A2, VOUT, 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 VREF 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 –10VIN. The two connections exhibit different input impedances. When you drive the VREF 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.
