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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.

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