An n-channel MOSFET is an inexpensive way to connect and disconnect circuits to 0V. But if I have a fixed voltage (in this case, 2.5V) and want to connect fixed resistances of higher or lower voltage, the diode in the MOSFET will interfere?
There are analog switches, but they are expensive by comparison. I am looking for the equivalent of an inexpensive MOSFET that is 1Mohm when the gate is 0V, 0.5ohm or less when the gate is 5V (or 10V) above the reference voltage, but I want to handle voltages both higher and lower than the reference voltage.
The power in the circuit is tiny, in the milliwatt range, so dissipation is not an issue.
Is there any such device?
Here is an op amp circuit. It will allow variable gain depending on the whether the gates of M1 and M2 are high or low. This works when the resistors are connected to ground, and it should work when the resistors are connected to 2.5V as shown, as long as Vin is higher than 2.5V. I would like to know what to do if Vin is less than 2.5V.