cd4051 problem

dear all
i am designing a simpel temperature based circuit in which i am using 8051 , 12 bit adc,opamp stage, and cd4051 for analog muxltiplexing of the gains.

i am trying to design a programmable gain amplifier.

now the problem is when i connect cd4051 with opamp the resistances (8) get added in parallel to each other and i am not able to select only one resistance.my opamp is running on 5 volts supply.and cd 4051 alos is running on 5 volts.

wht might be the error.

If the CD4051 has pin 6, pin 7 and pin 8 grounded, then it is wired for a single supply and should work.
But the OP07 has a very limited input voltage range, near half the supply voltage. It would work much better if it has a negative 5V supply in addition to its positive 5V supply.

Some things to keep in mind:
1. The resistance of the CD4051 can be as high as 1000 ohms.
2. The input signal, when the OP07 is powered by a single 5 volt supply, must be between +2 and +3 volts.
3. The OP07 output does not have any range at all in the worst case.
You might want to try the OPA340PA from Texas Instruments (DigiKey stocks it).

sorry i was wrong the power supply to opamp is +15volts to _15 volts. i ahve tried running cd4051 qith all possible supply voltages ,ie. +15 volts and 5 volts ..still not able to get the answer

What is the value of the resistor to ground? 47K would give a gain of 2, max.

Russlk said:

What is the value of the resistor to ground? 47K would give a gain of 2, max.

Click to expand...

No, R8 is 220k. So if the resistor to ground is 47k then the max gain is 220/47 + 1= 5.68.