2nd-order Sallen-key low pass filter with unity gain outputs rail voltage when input is 0V

Question

I was designing an active low pass filter for simple sigma-delta modulation signals. I found that the op-amp will output 3V when the input is 0V given that the power supply is 0~3.3V.

I simulate the circuit and find the same result. The circuit and the result are shown below. The blue line is the output of the op-amp.

According to my analysis, it is essentially a unity gain buffer with 10k2 input resistance in this scenario and the output should be 0V.

I noticed that the voltage will follow when V2 is slightly greater than 0V.

Why does that happen?

Answer 1

You are exceeding the input common mode voltage range of the opamp. The AD712 is designed to operate with a minimum of 10 Volt (±5 V). power supply.

You also have an input biasing problem. To get it to work with a single power supply, you need to bias the +input at half the supply. You can do this by adding a DC voltage source in series with V2.

For your experiments, use a ± power supply perhaps at ±5 V.

Answer 2

The first problem is you don't have an input voltage, it's zero volts. There are two ways to solve this. One is use a PWL source and use a step input (remember to use something like 10msec and 10.0000001msec for timestamps with the voltage of the first being 0 and the next being high (3.3V)).

The second problem is the simulation does a DC operating point, and so it runs the circuit and figures out the starting voltage. Sometimes it can get confused

"Ctrl-RightClick on the capacitor. In the SpiceLine cell, type "IC=0", for example, to set its initial voltage to 0V"

Changed 2V to 0V where applicable Source: How to define the initial charge of a capacitor in SPICE

You may also want to set the IC of the cap to 3.3V and leave the source at zero.

Answer 3

There are two things that can be taken from this graph in the datasheet: -

• The minimum supply voltage is +/- 5 volts (a 10 volt span)
• The valid input voltage range is about 3.5 volts for a +/- 5 volt supply

Additionally, the data sheets informs that the input voltage range is from +3.5 volts above the negative rail to 0.5 volts below the positive rail.

No-way will it work on a single 3.3 volt power rail.