Need help with auto range switching circuit

[Beowolf]

I need a working range switching circuit.

I know that it can be done wit analog switching IC (4066 and similiar).

But all I found are unipolar circuits (analg swithces cannot pass negative voltage).

So it comes to finding a circuit that will pass the negative voltage.

I found a version where the switching IC was powered with 0 - 5V and analog ground was set to 2.5V.

I am interesed if there are other solutions, or if that solution can be applied in a circuit that uses an SPI interfaced ADC.

Will the logic levels of the digital part of the ADC be altered if I pull upt the analog ground to 2.5V

 

[IanP]

Re: Auto range circuit

Try to supply CMOS switches, such as the CD40660, of symmetrical power supply (+/-5V) ..
Below is one example ..

Regards,
IanP

 

[lambtron]

Re: Auto range circuit

Many analog switches and multiplexers CAN pass negative voltages. For example, when a DG508 is operated with dual supplies (±12V to ±15V) it will easily pass ±10V signals, perfect for range selection (i.e., gain or attenuation control). And the 4066 you mentioned can be operated from ±5V power, thus enabling it to pass analog voltages in the ±5V range.

Important note: analog switches and multiplexers are not like mechanical relays in that they have significant resistance when operating in the "on" state. When you use an analog switch to control gain, try to arrange the circuit so that no current flows through the switch. If your design does not do this, you will have voltage drops across the switch that are influenced by a variety of factors and you will not be able to easily calibrate out the resulting errors.

 

[Beowolf]

Auto range circuit

I know that I can supply the 4066 with +-5V, but what will be the controll logic levels??

A I see it, 1st channel will be on when I connect pin 13 to VDD (+5V). But what will be the state of the switch when I put pin 13 to GND? Will it be on or off?

 

[IanP]

Re: Auto range circuit

To control logic for 4066 switch which is fed of symmetrical supply (+/-5V) must also operate between -5V (for 0) and +5V (for 1) ..
This, however, shouldn be difficult to realize ..
For example, if you take the LM324 opamp (it contains 4 opamps in one enclosur - one opam per one ¼ 4066 switch) and use it as comparators - means, you will connect Vref to all (-) inputs and treat its (+) inuts as 0-5V logic input, the output of each opamp-comparator will operate in -5V to +5V logic ..

Regards,
IanP

 

[Beowolf]

Auto range circuit

IanP, that's a nice point.
I shall use threshold of 2.5V on - pin for the comparator...

 

[lambtron]

Re: Auto range circuit

I know that I can supply the 4066 with +-5V, but what will be the controll logic levels?? A I see it, 1st channel will be on when I connect pin 13 to VDD (+5V). But what will be the state of the switch when I put pin 13 to GND? Will it be on or off?

Why not use a 4053? You can tie Vee to -5V, Vdd to +5V, and then use standard 0 to 5V logic control!