Hello,
am using op amps to amplify the LM35s for the logomatic. Have lots of noise in a long cable. I thought op amps would help reducing it? Because they are able to drive against low impedance, should be even better against high impedance? But actually the LM35s at present do better without the amplifier…
Regards,
Steffen
How long is the cable? Is it screened?
Leon
its about 10 m, no screen. But it does well with just the LM35, should do way better with amplification?
Tricky to guess without seeing your actual setup. (10m of unshielded cable - I think you’re asking for trouble.) But it sounds as if perhaps the cable capacitance combined with the output impedance of the LM35 are doing some filtering for you. With the low impedance of the op amp, you get less filtering.
If that is the case, you could look closely at your grounding and supply at the sensor end. I presume you provide the supply via the same cable? The LM35 supply rejection / line regulation spec is pretty good. But if you’re picking up a lot of interference on the supply line, you might want to heavily filter or even post-regulate the supply.
Always best to eliminate or suppress the source of the noise. Still, a filter between the LM35 and the op-amp might help a lot.
Not knowing your setup, I’ll mention that it is usually a no-no to have the grounds connected to earth ground at both the sending and receiving end of the cable. You get significant current in your ground line that way. If for some reason you’re forced to ground at both ends, you’d probably be better off receiving differentially.
I don’t know what your temperature range is. But it does seem like you should be able to get better performance with some gain from the op-amp, even without a shield. For example, 50mV of cable noise would be less significant when added to a 3V signal than when added to a .5V signal.
-Hugh
Op amps don’t like capacitive loads, check the spec for the one you are using.
Leon
its not grounded. Got a 9V plug in power supply, no earth connection. Sensors hang on a 3wire cable, 0V, 9V, signal. Put a few capacitors and an LC filter in before the voltage goes into the cables.
However, was playing around. The LM35s run well on their own if you put 75ohm and 1µF from signal to ground just next to the sensor, before you feed it into the cable. Works reasonable well until 3-4 m cable. Hence I came up with the opamps. But if you put the same RC combination in either at the input or the output of the opamp, it works quite bad. But if I put a 2k resistor onto the opamp output and then into a 8-9m cable its just ±1 digit, perfect.
Yes, was one of the reasons to amplify by 3, so when dividing by 3 the noise is reduced by 3 as well. Plus, the logomatic has some offset every now and then, so I miss between 0 and 5°C, now I’d only miss 0-2°C…
I think the resistor is dealing with the capacitive load problem I mentioned.
Leon
yes. The 2k I took from the LM35 datasheet. Its getting worse if you put 10k in. And the capacitor adds capacitive load it seems, its better without.