Does anyone have any ideas for an amplifier circuit that meets the following constraints:
Input Range: +/-15V
Gain: 1V/V, output clamped at +/-5V
Bias Current: <1nA.
Amp Noise: <2uVpp 0.1 to 10 Hz, <50nV/rthz at 1k
Basically i'm trying to design a low noise, low power unity gain amplifier that clamps the output at +/-5V. I don't care about anything above +/-5V, but I don't want to amplifier inputs to draw any current when the input is below +/-15V.
I have two options for you. The simpler one is to use the AD8479 and run it at a supply voltage of +/-5V. It is a difference amplifier which has a fixed gain of 1. Because of its architecture and the large resistors that it has in its internal circuitry, an input of +/-15V would not damage the part. The tradeoff would be that with the higher resistors, you would expect higher noise in your circuit.
The second option is to use an in amp with a supply of +/-15V and configure it in a gain of 3. You could then use a resistor divider at the output, so that you divide the output by 3. Effectively you get the output in a gain of 1, while clamping your output to ~5V. A sample circuit used with one of our lowest noise in amps, the AD8421, is shown below.
Take note that since you have a resistor divider at the output, you would likely need to add a buffer after.
I hope this helps!
As per Kris's suggestion, a buffer amplifier running off 15V rails would be how I would tackle this. ADI don't seem to offer a lot of low power opamps that run off 30V total supply, but I would look at the ADA4062-2. You can use the 'gain of 3 trick' or follow it by some other clamping circuit, which should then be much easier as you don't have to worry about your 1nA max load any more.