LPF before or after the opamp?

[nickagian]

The general concept is the following: I have the HTG3533 humidity sensor and want to interface it with an ADC that has an internal voltage reference of 1.2V

What I plan to do is to use a voltage divider, consisting of two 1MΩ resistors. Thus, the output voltage of the sensor will be divided by 2 and be always in the input range of the ADC. After the voltage divider, I'll put a buffer constructed around an LPV511 OpAmp in order to provide small output resistance for the connection to the ADC.

Moreover, I would like to put an RC low-pass filter (something like a 5k with a 1 or 10u) in order to filter out any noise at the output line of the sensor, since the sensor will be located at a distance from the interface PCB.

My question is : Where to put this filter? Before the resistive network, just at the entrance of the signal at the PCB or after the buffer and before the ADC? I think that the best is to put it before the voltage divider. Is this correct?

I also have a (quite silly) question with this design: the data-sheet of the sensor says that its output impedance is 50Ω. Is there any problem that this small resistance is going to be connected to a huge input resistance of the analogue front-end? (As I said, I have a voltage divider consisting of two 1MΩ resistors. Their value can of course be reduced)

Thanks
Nikos

 

[crutschow]

I would just put a capacitor at the input to the op amp to ground and use the resistance of the 1MΩ divider (500kΩ) as the resistive part of the RC LP filter. A 0.1µF cap will give you the same rolloff as would a 5KΩ resistor and a 10µF cap.

The low output impedance of the op amp will minimize any noise pickup after that.

I would put the 1MΩ divider resistors close to the input to the op amp, since the low output impedance of the sensor will also minimize noise pickup between the sensor and the divider.

Your question about the 50Ω output impedance is not silly. In this case it will reduce the signal by a very small amount (by a ratio of 1,000,000/1,000,050 or 99.995%).

For added noise reduction use shielded coax wire (such as audio cable) for all signal connections.

 

[nickagian]

Thank you very much for your suggestion and your help! I haven't thought of this solution before.

This ratio by which the output signal will be reduced comes out from a voltage divisor actually, right? We have the 50Ω in series with the one 1Μ (1,000,050 Ω) and then the other 1M. Ok, I understand now. Yes, the output resistance should be much lower than the input resistance. I am always confused at this point, because I erroneously keep in my mind that maximum power transfer theorem where the input and output resistances should be equal.

Regarding the cable, I will indeed use a shielded one. And of course the shield should be connected to ground. Thank you.

By the way... do you think that this cut-off frequency of ~3Hz is too low? Should I go higher, perhaps with a capacitance of 0.01u and a cut-off of ~30Hz? I don't have any special specification for the filtering.

 

[crutschow]

--- Yes, the output resistance should be much lower than the input resistance. I am always confused at this point, because I erroneously keep in my mind that maximum power transfer theorem where the input and output resistances should be equal.

By the way... do you think that this cut-off frequency of ~3Hz is too low? Should I go higher, perhaps with a capacitance of 0.01u and a cut-off of ~30Hz? I don't have any special specification for the filtering.

Many people have some confusion about the maximum power transfer theorem. It's normally only used for things like RF circuits where you are interested in the maximum power to the transmitting antenna. Otherwise it's seldom used since it means half the power will be dissipated in the source and that's usually not desired. Typically you want most of the voltage (not necessarily power) to go to the load, thus you want a very low source output impedance.

The frequency of the LP filter is determined mainly by how fast the signal level is moving. You don't need the filter frequency response to be significantly higher than that.