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Regarding the CD4051 analog multiplexer,

Data Sheet

If I connect VDD to +5 V and VSS, VEE to GND. The analog signals are coming from modules with the analog signal having a DC offset that can be as low as -300 mv.

Does this analog signal pass through the multiplexer as is, or will it be clipped at zero volts (VEE)?

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    \$\begingroup\$ The language in section 8.1 seems to suggest clipping at Vee \$\endgroup\$ Jul 23 at 0:37
  • \$\begingroup\$ what if I need to pass around -300 mV, with no negative supply \$\endgroup\$
    – Bavly
    Jul 23 at 0:41
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    \$\begingroup\$ I'd find a way to provide a negative supply. \$\endgroup\$ Jul 23 at 0:44
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    \$\begingroup\$ Probably a DC to DC converter. \$\endgroup\$ Jul 23 at 0:45
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    \$\begingroup\$ The operation of the 4051 is not guaranteed for inputs<VEE. So it may, or may not pass the signal through. Once the ESD protection diodes start to conduct, the input voltage will be clamped at whatever their forward drop is. \$\endgroup\$
    – SteveSh
    Jul 23 at 1:02
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It will work okay, in fact series resistance will be near minimum, but leakage will be relatively high (perhaps in the few uA range) increase at high junction temperature (perhaps >100uA at Tj=100°C) due to forward biasing the isolation diffusion junctions and input protection diodes.

The transmission gate resistance will typically be more than 200\$\Omega\$ (more at higher temperatures) so error could typically exceed 10% at Tj=100°C even if the source is low impedance.

If that's unacceptable (and keep in mind that higher negative voltage can damage the chip should excess current be allowed to flow) you might want to consider a charge pump such as the TC7660 or ICL7660. An 8 pin package plus two capacitors are all that is required. enter image description here

Since the CD4051 (unlike the 4016/4066) includes input level shifters, you can simply connect the 0/5V control inputs directly. That is my recommendation if you need to accurately pass signals of -300mV over a range of temperature.

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The Schottky ESD diodes will be conducting but the amount of clipping depends on the impedance ratio of source to diode.

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  • \$\begingroup\$ Thanks a lot. How can I estimate the minimum voltage that can pass without clipping?. I am using HB100 radar sensor IF pin as source \$\endgroup\$
    – Bavly
    Jul 23 at 0:23
  • \$\begingroup\$ when the ESD diodes are conducting, Is it safe or we might damage the sensor? \$\endgroup\$
    – Bavly
    Jul 23 at 0:57
  • \$\begingroup\$ by sinking current \$\endgroup\$
    – Bavly
    Jul 23 at 0:58
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    \$\begingroup\$ Sensor is unspecified. I doubt it \$\endgroup\$ Jul 23 at 1:26
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A very "Heath Robinson" negative supply can be used by placing a series diode between circuit ground (diode anode) and connecting VEE to supply negative = diode cathode. Bypass the diode with a suitably large capacitor. This gives you an approximately -0.6 V negative supply with a silicon diode. Less for a Schottky diode. For more, use two diodes.

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