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where REXT is the externally selected resistor value needed to achieve the desired gain setting, Rsi is the fixed parallel resistorin Figure 4, and RFIX is the effective fixed value of the remaining internal resistors: RS2, RS3, or (RS2 11 RS3) depend- ing on the pin connections. Note that the best process and temperature stability will be achieved by using the pre-programmed fixed gain options of Table l, since the gain is then set entirely by internal resistor ratios, which are typically accurate to +0.5%, and track quite well over process and temperature. When combining exter- nal resistors with the internal values to create an effective Rs value, note that the internal resistors have a typical tempera- ture coefficient of +700ppm/IC and an absolute value toler- ance of approximately +5%, yielding somewhat less predict- able and stable gain settings. With or without external resis- tors, the board layout should use short Gain Strap connec- tions to minimize parasitic resistance and inductance effects. The overall noise performance of the VCA2613 will vary as a function of gain. Table lI shows the typical input- and output-referred noise densities of the entire VCA2613 for maximum VCA and PGA gain; i.e., VCACNTL set t0 3.OV and all MGS bits set t0 7. Note that the input-referred noise values include the contribution of a 50 I fixed source imped- ance, and are therefore somewhat larger than the intrinsic input noise. As the LNP gain is reduced, the noise contribu- tion from the VCNPGA portion becomes more significant, resulting in higher input-referred noise. However, the output- referred noise, which is indicative of the overall SNR at that gain setting, is reduced.
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