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1 THAT Corporation Design Note 137 Substituting THAT 2180- and 2181-Series VCAs for THAT 2150-Series VCAs in Existing Designs Abstract THAT Corporation's 2180- and 2181-series VCAs are pin-for-pin compatible, improved performance replacements for the 2150-series VCAs. Designers may convert existing 2150-series designs to use the 2180/2181 VCAs without making any changes to existing PCB layouts. This application note details the few component value changes and component deletions required to obtain optimum performance with the 2180- and 2181-series VCAs in existing 2150-series designs. Choosing the correct VCA Table 1 shows the correct VCA to use when switching from a THAT 2150-series part to one of the THAT 218x-series. When retrofitting the 218x VCAs, designers may replace the 215x-series VCA with a trimmable VCA or a trimless VCA. For high performance applications, the choice is normally to choose a trimmable 2181Lx or 2181Sx. However, where distortion performance is less critical and trim-free production is a goal, the 2180Lx is the right choice. Old VCA Precision Trimless THAT 2155 THAT 2181LC or THAT2181SC THAT 2180LC THAT 2150A THAT 2181LB or THAT2181SB THAT 2180LB THAT 2151 THAT 2181LA or THAT2181SA THAT 2180LA Table 1 2180 Series The 2180-series VCAs are pre-trimmed at wafer stage for low THD and control-voltage feedthrough without further adjustment. The parts are available in three grades (2180LA, 2180LB, and 2180LC) selected for factory-trimmed distortion performance, allowing the user to optimize cost vs. performance. Figure 1 shows a typical 2150-series application circuit in which the negative control port (pin 3) is used for gain control. Components R2, R3, and VR1 provide for external adjustment of VCA symmetry. Figure 2 shows the same circuit adapted for use with a 2180-series VCA. Note that external symmetry trim components R2, R3, and VR1 have been deleted. In particular, R2 must not be used with the 2180 Series--it will upset the internal symmetry trim. Capacitor C3 may be reduced in value to as low as 22 pF in order to increase the circuit bandwidth if desired. Figures 3 and 5 show similar 2150-series application circuits wherein the positive control port (pins 2 and 4) or both control ports are used, respectively. Figures 4 and 6 show the 2180-series counterparts to these circuits. Copyright