MOSFETs reduce crosstalk effects on analog switches
( 01 Sep 2003 )
Stanley Chen, Global Mixed-Mode Technology, Taipei, Taiwan
Some cost-effective analog multiplexer/demultiplexer ICs, such as the CD4053 and CD4066, find frequent use as signal distributors. These digitally controlled analog switches have low on-resistance. However, with all channels in the same package, crosstalk can be annoying and unavoidable. Figure 1 provides a cost-effective and viable method of solving this problem. By simply adding some n-channel MOSFETs, such as the 2N7002 or 2N7000, the crosstalk effect becomes negligible. When the Channel Select signal is high, the CD4053's input pins A, B, and C assume a level of nearly 0V. This operation selects Channel 1. Therefore, the Y output connects to Y0 and Z to Z0. Meanwhile, the Channel Select signal turns on Q4 and Q5, thereby drastically attenuating the Channel 2 signal at Y1 and Z1. The crosstalk effect simultaneously decreases. When the Channel Select signal is set low, Y=Y1 and Z=Z1, and Q2 and Q3 turn on to attenuate the crosstalk effect. For the 2N7002, the RDS(ON) resistance is several ohms; therefore, the crosstalk decreases by more than 40dB. Add to that the analog switch's internal rejection ratio, and the total crosstalk rejection could be as high as 80dB. By using a few low-cost MOSFETs, you can drastically reduce crosstalk effects in an analog multiplexer.
