Output Voltage Level Drops when 50 ohm Load Applied

[sulemanzp]

Hi Experts,
Need your help for a circuit.
I made a circuit with CD4520 IC, the required output voltage is -8 volts but when load is attached to the circuit the voltage drops to approximately -3 volts (-2.286 Volts). So I found that ON resistance is 125 ohm so it is casing the voltage level drop.
So I replaced the IC with ADG1408 lower ON resistance 4.7 Ohm. Now the voltage drop is about 1 volts, but still the IC output current is 155 mA and 160 mA is required.
In below picture I gave the overview.
Experts kindly tell me a way to stable the voltage level with mux/switch or any other circuit enhancement.

Remember IN/OUT is a single line in which -8 volt signal sent and then received with -8 to -10 volt level. There is other circuit in parallel which handles the incoming -8 volt signal by enabling or disable the mux.

Thank you

 

[KlausST]

Hi,

with input voltage of 8V it is impossible to get 8V output, too. Especially with a 50 Ohm load.

(At least I don´t know a superconductive switch)

* either you have to rise the input voltage
* or you have to lower the (specified) output voltage.

****

Review your specifications.

Maybe you can specify an allowed voltage drop. Let´s say 500mV. Then you may calculate the allowed ON resistance of the switch.
Maybe a photomos relay can do the job.

Klaus

 

[skroz]

You have never heard about overloading. CMOS logic family can have up to 18V of supply and can deliver up to few mA output current. From this data you can quickly found that you are loading this circuit too much. First consequence is that voltage swing is too small not reaching logic levels needed for operation.

 

[schmitt trigger]

160 mA!
You won't get that much current out of a multiplexer.
Even if you did, the life of the poor IC would be very short,

I think you may have to use an external Mosfet booster.

 

[Audioguru]

Simple arithmetic shows the voltage divider you have with the CD4052 125 ohms switch (its maximum is 240 ohms) and your 50 ohms load.
The maximum allowed continuous output from a ADG1408 is 30mA or 100mA peak. The datasheet says it does not meet spec's at these maximum stresses and higher currents may cause permanent damage.

 

[KlausST]

Hi,

You won't get that much current out of a multiplexer.

I use an TS5A23160.
It is not to compare with an CD4052...
But the switch current capability is huge: 200mA continous, 400mA peak.

************

BUT I agree. Those CMOS Muxes (CD40xx, ADGxx) are more meant to switch signals than that high current.

Therefore the recommendation for a photomos.

***
But there is an ADG1604 in my stock.
datasheet says:
Continuous current per channel
LFCSP package: 504 mA
TSSOP package: 315 mA
Supply voltage up to +/-8V

Maybe worth a try.


Klaus

 

[sulemanzp]

ok guys I got your answer. MUX will not do my job, so whats the alternative to my circuit or what I have to do on output side to stable my voltage level as well as current. I can allow voltage drop of 500mV.
As I said that ADG shows -7 volts but the current shows limit. I want to add that I already replaced 1 damaged ADG on my board.
Please circuit suggestions are welcome.

 

[KlausST]

Hi,

Did you read the last lines of my post#6?

--> maybe worth a try.

Klaus

 

[sulemanzp]

Hi,

Did you read the last lines of my post#6?

--> maybe worth a try.

Klaus

Yes I read and I will try it in my circuit by replacing ADG1408. Yet that IC is not available in my office store room. I have to order it.

I am placing order in DIGIKEY = ADG1604BCPZ-REEL7CT-ND. Will update the post and mark the answer after testing it if nothing goes wrong.