Advantage's of Microchip's Cascaded Op Amps

TB077 Advantages of Microchip's Cascaded Op Amps Author: Kumen Blake Microchip Technology Inc. V IN V OUT INTRODUCTION Microchip has introduced cascaded dual op amps: R R 2 4 R R 1 3 MCP627X (2 MHz), MCP628X (5 MHz) and MCP629X (10 MHz). Features of this new topology include: ) pin for both amplifiers* Chip Select (CS FIGURE 2: Cascaded Amplifiers. Small 8-pin packages (PDIP-8, SOIC-8, MSOP-8) There are several interesting variations of the circuit in Pinout similar to the industry standard for duals Figure 2 that are supported by these parts. For The two op amps are connected so they will support instance, the input op amp (op amp A) can be many application circuits. configured as an inverting amplifier, as an inverting (Miller) integrator or as a difference amplifier. The output amplifier can be set up as a unity-gain buffer to FUNCTIONALITY isolate the load from op amp A. This results in an The output of op amp A is connected internally to the overall performance improvement when driving heavy non-inverting input of op amp B (see Figure 1). Com- loads (e.g., 1 k?). /V = pin # 1)bining these into one pin (V OUTA INB+ It is also possible to make a low-pass filter using just S input (pin # 5) to an 8-pinmakes it possible to add a C resistors (no capacitors) (see Figure 3). This is a high package. Both op amps function as normal op amps. and Rbandwidth, second-order filter. The resistors R 1 2 set the DC gain, while R maintains stability and 3 V CS controls the filter Q. The filter cutoff frequency isDD (8)(5) typically 10% to 50% of the op amp gain bandwidth product (GBWP). (3)V INA+ V (7) OUTB V (2) INA- V REF (1) (6) (4) V /V + V - V OUTA INB INB SS V REF R 3 FIGURE 1: Pinout Configuration. V V OUT IN The CS input helps conserve power in many popular RR 12 dual op amp applications. This pinout makes it rela- tively easy to modify many existing designs for these FIGURE 3: Low-pass Filter with no parts. Capacitors. The MCP627X (DS21810), MCP628X (DS21811) and APPLICATIONS MCP629X (DS21812) data sheets provide more application circuits (e.g., an inverting integrator with The most common application circuit supported by improved performance). These data sheets also shed these parts is cascaded amplifiers (see Figure 2). more light on the circuits shown in this Technical Brief. These parts allow for easy layout in this common application. Usually, most of the gain is produced by the first stage. ? 2004 Microchip Technology Inc. DS91077A-page 1