SN74HC4851-Q1: how to tell the high impedance of HC4051

Hale Tan

Part Number: SN74HC4851-Q1
Other Parts Discussed in Thread: TMUX4051-Q1, SN74HC4851

Hi team,

My customer wants to use HC4051 as below. But they want to make sure how the microcontroller recognize high level voltage, low level voltage and high impedance(ChannelX suspension) of HC4051. It seems that high level voltage and low level voltage is easy to tell but I am not sure how to tell the high impedance of HC4051. Could you provide your suggestion?

over 1 year ago

0 Clemens Ladisch over 1 year ago

Guru 244020 points

Configure a pull-down and then a pull-up resistor at the microcontroller input. If the voltage changes, the signal was high impedance.

0 Hale Tan over 1 year ago in reply to Clemens Ladisch

TI__Intellectual 2490 points

What signal does the the A/D input get? For example, the high level voltage for MCU is about 5V and the low level voltage for MCU is about 0V. If there is ChannelX suspension, what signal can the A/D input get?

0 Kameron Hill over 1 year ago in reply to Hale Tan

TI__Genius 10480 points

Hello Hale,

The switch being in high impendence or off(Channel X suspension) is determined by the voltage put on the enable pin( pin6). It is one of the control pins on the device which can be used to select which input is connected to the output.

On this table you can see that if the enable pin(INH) receives a high signal the channel will be in high impedance or off( Channel X suspension).

If the device is in channel x suspension. then the A/D input will see approximately 0v.

Please let me know if you require any further assistance.

Regards,

Kameron

0 Hale Tan over 1 year ago in reply to Kameron Hill

TI__Intellectual 2490 points

Hi Kameron,

Is there a mosfet between the COM pin and Channel PIN? According to the customer demands, if the voltage of channel X changed, such as 24V to 10V, will the output signal COM change? In this application, the customer need to tell 3 voltage level, low level voltage(0V), high level voltage(24V) and the voltage between 0V and 24V. So we want to know about the principle of the signal transmit from Y0 to COM.

0 Kameron Hill over 1 year ago in reply to Hale Tan

TI__Genius 10480 points

Hi Hale,

Yes there is a mosfet between the com pin and channel pin. All TI multiplexers are passive analog switches. Which mean they can pass signals from 0v to vcc from input( Channel x) to output ( Com Pin).

Hale Tan said:
According to the customer demands, if the voltage of channel X changed, such as 24V to 10V, will the output signal COM change?

Yes for example if the input voltage changes from 3v to 4v we will see 4v on the output, but but if you want to pass signals from 24V to 0V I suggest using the Tmux4051-Q1 instead. Here is a good comparison between these products

https://www.ti.com/compare-products/?id=520&type=GPT&partList=TMUX4051-Q1,SN74HC4851-Q1&columnList=3318,1143,480,3306,3307,1389,3143typ,3127typ,3193max,192typ,1611,1192,2954,2955,1130,261011,2192,-4

Finally just to reiterate this is an analog mux which can send any signal from 24V to 0V and anything in between.

Thanks and Regards,

Kameron