smple Analog Isolation circuit needed

[leonardo0]

Hi to forum

I have 5 boards equipped with 8 channel adc and display .The board inputs i need to connect in parallel with each other
so signals can show wherever a board is located .In front of each input there are shunt resistors 10K working as selectable
pull up or pull down . The problem i have is that if a board power supply lines are not connected with the rest of boards leakage through input
shunt resistors destroy all indications .I dont want to use jumpers with diodes. Is there a very small simple circuit to isolate the inputs
when a board is not powered ?.Voltage range is 0..30 VDC

 

[kam1787]

perhaps you could explain WHY you need to have the board inputs in parallel, first.

 

[jiripolivka]

Without a schematic your explanation is confusing.

To isolate ANALOG channels, there are special opamps used e.g. in medical applications. But they are complex and expensive.

 

[betwixt]

The simplest and possibly cheapest solution is to wire relays with their contacts in series with the analog inputs and their coils across the respective power rails of the card. It is lossless as far as the ADC measurement is concerned and relatively power efficient if sensitive relays are used.

Brian.

 

[leonardo0]

Hi

betwixt thanks your solution is fine .It needs only needs 8xrelay cost + board space that i don't have .
Possibly isolated solid state relays.Still expensive though
My application requires 5 displays separated together by 5 meters . Along with signal lines are vcc and gnd lines that power boards with 24 volts
No special accuracy is needed.
Possibly a p-fet is the solution .My question is if input goes negative will fet be destroyed ?.Is there a trick to prevent that ?.

 

[BradtheRad]

Is there a very small simple circuit to isolate the inputs
when a board is not powered ?

Have you looked at the 4066 IC? It's a bilateral (analog) switch. Four gates in one chip. Pull the control input high, the internal resistance goes down.
Pull the input low, internal resistance goes very high.

It works very much like a relay. Signals can be positive or negative, or AC.

 

[betwixt]

The 4066 (and 4016) cannot pass voltages below VSS or above VDD so to use them you would need a positive and negative supply. It could be a viable solution though, especially if all the switches were controlled simulataneously because the control pins could be linked and connected to the board supply.

Brian.