I am trying to drive around 9 strain gauges with a 5V battery, after connecting the strain gauses to the battery, I found that the gauge outputs are very noisy. (since the strain gauges are used on moible platform, I can only use batteries rather than other power sourse with low noise)
To reduce the noise, I added a LM358N op amp as a voltage follower between the battery and the strain gagues. When the output pin of the op amp connects to no load, the output is 5V. However, when I connect that 5V output to the strain gauges, the output voltage of the op amp drops significantly as the number of the strain gauges increases. (even drops to less than 1V)
I think that the voltage drop may be caused by the low impedance of the strain gauges, which the LM358N can not handle. So my question is:
1. Are there any recommendations on op amps that can be used as voltage followers for low impedance outputs like strain gauges?
2. If proper op amps doesn't exist, what other components can I add in my circuit to eliminate or at least to reduce that voltage drop?
Looking forward to your suggestions and thank you in advance!
Usually, considerable noise is not generated by a battery but by circuits driven by the battery. The current through your 9 strain gauges is about 375 mA; please have a look at this table which shows op amps delivering at least 400 mA:
https://www.mouser.de/c/semiconductors/amplifier-ics/operational-amplifiers-op-amps/?number%20of%20channels=1%20Channel&output%20current%20per%20channel=400%20mA~~-&package%20%2F%20case=DFN-8~~DIP-8%7C~PDIP-8%7C~SOIC-8&supply%20voltage%20-%20max=12%20V~~400%20V&rp=semiconductors%2Famplifier-ics%2Foperational-amplifiers-op-amps%7C~Output%20Current%20per%20Channel%7C~Supply%20Voltage%20-%20Max%7C~Package%20%2F%20Case
But by just putting a voltage follower between voltage source and load the noise is not cancelled because the voltage follower follows the noise, too (as far as its bandwidth permits). At least, you have to insert a low pass filter between the 5V battery and the non-inverting input of the op amp.
Actually, you don't need the 5V battery anymore because the power is delivered by the 12V source. It would be enough to have a voltage divider plus a low pass filter which is driven by the 12V source and delivers 5V. Due to the op amp, you can use a voltage divider and a low pass filter with high impedance (relative to the impedance of your strain gauges).
Hope this helps.
Hello Junjie Luo ,
I agree with Joerg Fricke that the electrical noise you are seeing does not come from the battery. Can you tell us something more about how your strain gauges are used? For example, you said the strain gauges are used in a mobile application. Does this mobile application have an internal combustion engine associated with it, or electric motors? Also does this mobile application have an onboard computer, which could also be a noise source? It would seem to me that the source of the measurement noise is associated with something in your mobile application. Perhaps the easiest way to determine the source of the measurement noise is to simply turn off the power to the mobile application without turning off the power to the strain gauge array. If the measurement noise goes away, you have found the source. If the mobile application is indeed the source of the measurement noise, then the next thing would be to determine how this noise is being coupled to your array of strain gauges, i.e., is the coupling capacitive, inductive, or through some lack of Galvanic isolation?
Also, you have not explained how the signal from each strain gauge is measured and sent to you. I mean, if it is a mobile application, does it communicate the strain gauge signals wirelessly, or via an umbilical cord?
Without detailed information about your mobile application, it is hard to know how to help you.
Regards,
Tom Cuff
Thanks all of your guys for your kind comments. After considering all your helpful suggestions, my final solution is to change the LM358 to an AD812 amplifier as the voltage follower, together with the addition of a filter. The voltage over the strain gagues rises to around 2V under these changes, which is not as high as 5V, but is high enough for my measurements.
Any electronic component you add to your circuit (except the battery ) will add more noise to your circuit.
You should use only a 5VDC battery, with enough capacity (current capability);
the conductors (wires) should NOT be thin (to decrease the voltage drop along the wires),
and you should use cables with a screen , connected to earth in one point , to decrease the electrical noise from the environment
If you are using any o.a., like already mentioned , you are adding to the circuit the noise of the o.a.
If the supply of your strain gauges is smaller, you are losing precision of your measurements (Supply = 1/2 of initial supply means
Precision = 1/2 of initial precision
Hi Junjie Luo,
the noises mainly come from the circuits, environments, and the sensors themselves, instead of the batteries. You can use the oscilloscope to trace the noise source.
The reason for the big voltage drop is the used amplifier chip does not have a large source current capability, the maximum output current can be found in its datasheet.
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