Unity Gain Op Amp Switch

[whitecollar]

Hi guys,

I'm trying to use an OP-AMP as a non-inverting amp with a gain of 350, and ALSO as a unity gain buffer (but not at the same time). It's feeding the ADC input of a PIC micro controller.

Is it acceptable to use the MOSFET to switch the voltage divider to ground as in the circuit attached?

So:

1) FET OFF = OP-AMP acts as unity gain buffer.
2) FET ON = OP-AMP acts as non-inverting amplifier with gain of 350.

Are there any problems with this?

Thank you!

 

[keith1200rs]

R34 and R35 values are the wrong way round. In principle what you want to do will work. The usual problem is the effect of the MOSFET capacitance. Make sure you consider its effect and choose the MOSFET wisely (i.e. low capacitance).

Keith

 

[whitecollar]

Thanks Keith,

Ok, I've added a 1 MOhm resister to offset the gate capacitance of the FET. Also I've swapped the feedback resisters. I've added a 0.1 uF cap as a filter as I am just amplifying steady DC voltages, there nothing with a frequency component.

Updated circuit attached.

Thank you.

 

[keith1200rs]

I would suggest you add the option of putting a capacitor across R34. If there is going to be any instability due to the MOSFET capacitance then some capacitance there should fix it.

Keith

 

[alexan_e]

The is still a small problem i think, the input offset of your opamp is typically -4.5mv to 4.5mv,
and with an amplification factor of 350 times you will also amplify this voltage 350 times and you will have up to 1.5v dc offset in the output,
this offset will depend on the chip used and will also change in different temperatures.
You should either change to a low offset opamp or one that provides offset adjustment.

Alex

 

[keith1200rs]

Alex,

That is a very good point. In fact, with such a low level signal it may be worth considering an instrumentation amplifier depending on where the low level signal comes from.

An alternative strategy is to use two separate amplifiers and an analogue multiplexer. For the unity gain you could simply bypass the amplifier with a multiplexer unless the signal is high impedance.

Keith