Dear Sir
Hello, I used the OPA1654AIDR operational amplifier to design Sallen-Key Low Pass Filter (unity Gain). The test result found that when the input voltage of the operational amplifier is about 0V~0.2V, the output of the operational amplifier will have an error value (offset) . Is this result reasonable? The details are provided in the attached file, please refer to it again!
Hello,
The problem is that both the input common mode range and output range are too close to the power rail to accurately maintain accurate regulation. For input common mode range, the input voltage must be 0.5 V above the negative supply voltage.
Regarding the input range, the specification shows the input voltage must be 0.5 V above the negative supply.
In your circuit, you have both a buffer and a high-pass Sallen and Key. Both of these set the input common mode range of the amplifier to the same voltage as the input voltage. If the voltage at the input is < 0.5 V from the negative supply voltage (pin V-), the input is forced into a state that gives low open-loop gain and high error.
For output range, the output cannot force a voltage that is equivalent to the voltage on the negative supply. You can see in the data sheet that with a 2kOhm load, the output minimum/maximum specification for voltage delta from the supply voltage is 0.8 V:
In this case, a 2kOhm load resistor is connected to a voltage that is mid-supply (for +/-15 V supplies, connected to 0 V). In your configuration, I don't see any load, but, even with no load, the output cannot force a zero volt differential from the negative supply to the output.
Note as the voltage increases above the negative rail, the error is resolved. You will either need to add a negative supply to keep the same input range, or, shift the input range you are using above at least 0.5 V from the negative rail (V-).
Best Regards,
Mike
