Making opamp output open circuit

[hkBattousai]

Is there any way to enable/disable an opamp by means of controlling its supply voltage. I want to make opamp run in normal mode when both FETs conduct, and make opamp output high impedance when both FETs are cut-off.

My idea comes from the fact that, most opamps have output driving circuits same as the image below, or can be characterized like that:

My purpose is to desing an "analog multiplexer", after seeing how expensive analog multiplexer ICs are, I decided to desine my own circuit.
There will be several opamp ICs on the board, and a microcontroller is going to select an analog source by enabling one of those opamps. Though there is a problem in my desing: I won't be able to use a quad opamp package.

So, any ideas, could it work?

 

[Chippie]

hmmm... wrong answer

 

[Sceadwian]

I don't think you could switch an opamp to a high imepdance state like that, you'd have to remove it from the circuit entirely. You could use FET's on the output of the opamp, but the opamp is still going to be connected and active to the input, the FET would just be switched off to keep the output from going anywhere.

 

[hkBattousai]

I don't think you could switch an opamp to a high imepdance state like that, you'd have to remove it from the circuit entirely. You could use FET's on the output of the opamp, but the opamp is still going to be connected and active to the input, the FET would just be switched off to keep the output from going anywhere.

If I could manage to make the output of a disabled opamp 0V, it satisfies my needs. I can input all opamp outputs to a suming circuit. So the disabled opamps would not effect the output of the summing circuit.

 

[dougy83]

Looks weird... You can get opamps that have an output enable pin which should do what you speak of. Or just use a cheap analogue switch/MUX IC: e.g. (74HC)4066, (74HC)4051

 

[Sceadwian]

So they use a mosfet on the opamp output.

 

[hkBattousai]

Looks weird... You can get opamps that have an output enable pin which should do what you speak of. Or just use a cheap analogue switch/MUX IC: e.g. (74HC)4066, (74HC)4051

Cheap? Can you give me some numbers?
I search it in Farnell, the typical prices were above 25-30€.

 

[dougy83]

Cheap? Can you give me some numbers?
I search it in Farnell, the typical prices were above 25-30€.

Those numeric characters in my previous post were the part numbers.

**broken link removed**

 

[Roff]

Cheap? Can you give me some numbers?
I search it in Farnell, the typical prices were above 25-30€.

Are you sure there was not a decimal point in front of those prices?

 

[AllVol]

the 74HC4066 sells for 35 cents USD at Jameco.

 

[hkBattousai]

Are you sure there was not a decimal point in front of those prices?

It doesn't make sense to me either.

Please refer to this link:
Your Search Results | Farnell Turkiye

Are those chips really that expensive, or are those prises not for single IC packages, for instance for a tube of ICs instead?

 

[ericgibbs]

hi,
In the UK they are 27p each

**broken link removed**

 

[hkBattousai]

Ok, now I understand that analog multiplexers are not that much expensive. Maybe those I found at Farnell were some special function multiplexers.

I decided to consider this analog mux idea. Can you please tell me the performance of analog multiplexers? I know that it is impossible to transfer a signal keeping its exact from after passing it through a semiconducter circuit element. How much my signals will be modified after being switched by an analog multiplexer? What is the level or percentage of sensitivity?

I read datasheet of 405x family (x=1,2,3), There is no clear indication about this matter. I am new to this analog multiplexer thing, I don't even know their working principles.

 

[Hayato]

You didn't found any Multiplexer. You found a lot of ADCs, R/D convertors... But any Multiplexers.

https://tr.farnell.com/jsp/search/b...rch_001&Ntt=74HC4066&Ntx=mode+matchallpartial
-> 74HC4066D = €0.586

Switches, Multiplexers & Demultiplexers | Farnell Turkiye
-> 74HC4051M = €0.564

You should have checked your search. It was with wrong part numbers.

 

[dougy83]

Can you please tell me the performance of analog multiplexers?

Maybe check the datasheet for that.

They're supposed to exhibit a pretty good transfer characteristic of a resistor. I do know that most have a varying resistance depending on the voltage passed - so as long as they're driving a high impedance device, there should not be any issues.

 

[crutschow]

Three important characteristics of analog multiplexers to note are: on-resistance, signal amplitude for normal operation, and switching speed. These are noted in the multiplexer data sheet.

 

[Roff]

You also need to at least consider parasitic capacitances.

 

[Mikebits]

Here is an opamp with output disable pin. https://www.electro-tech-online.com/custompdfs/2010/01/LMH6732.pdf

You might be able to get a sample from national.

 

[Roff]

Intersil makes a 4 channel selectable op amp, the **broken link removed**. They are expensive (US $15.76 from Intersil).

 

[dougy83]

I get the feeling the OP is happy to use an analogue MUX now that he knows they are dirt cheap.