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We have observed some unexpected data measurement fluctuations in our meter output. We would like to find out where exactly the fluctuations come from. There is a pressure sensor connects to our ADC (HX711 PDF). We have fixed ADC input with a fixed voltage and see no measurement fluctuations at the output. So we try to see what the voltage looks like when we connect the real pressure sensor to the ADC. I post the schematic here.

enter image description here

The sensor parameters are:

Tech Parameters unit value
sensitivity mV/V 2 ± 0.2
output at zero %R.C. ≤ 10
input impedance 400 +/- 10
output impedance 350 +/- 4
insulation resistance MΩ >= 5000 (50V DC)
suggest excite voltage V(DC/AC) 5~15
max excite voltage V(DC/AC) 20

We connected a scope probe at INA+ & INA-. However, from the scope we see waveform like this:

enter image description here

which does not make any sense. So my question here is what's the right way to monitor the INA+ & INA-? We really want to know whether the output from the sensor is ok or not?

To add our actual schematic:

The actual schematics

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  • \$\begingroup\$ Hi. What about the V_AVDD? Are you sure that is not noisy? If you check the bridge supply voltage and it's clean, then the noise is coming from the bridge itself. If you are using a load cell, they have long coil-like tracks inside. Maybe it picks magnetic noise from something close to it. \$\endgroup\$
    – Saadat
    Commented Apr 5 at 9:47
  • \$\begingroup\$ How is wired your pressure sensor to HX711? How do you measure INA+ INA- voltage? How is Vpp (=53.6 V ?) measured ? \$\endgroup\$
    – Antonio51
    Commented Apr 5 at 10:36
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    \$\begingroup\$ We have observed some unexpected data measurement fluctuations in our meter output <-- do they at all correspond with the waveform measured by your oscilloscope? Please also show your actual circuit used and the supply voltage supplied. Please also provide a link to the data sheet of the HX711. \$\endgroup\$
    – Andy aka
    Commented Apr 5 at 11:23
  • \$\begingroup\$ @Saadat We have tried to replace V_AVDD by a lab voltage source from the sensor. That did not help. \$\endgroup\$
    – neuron mac
    Commented Apr 5 at 14:25
  • \$\begingroup\$ @Antonio51 we simply hook a scope to INA+ & INA- And after we hooking the scope to INA the measurement reading fluctuates even worse. I guess the scope measurement is intrusive. \$\endgroup\$
    – neuron mac
    Commented Apr 5 at 14:28

1 Answer 1

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Your oscilloscope GND is probably grounded to earth. If you are using a single scope channel that can affect the signal in various ways depending on exactly how things are powered and wired.

You can try using two scope channels with GND connected to the circuit ground (and display the voltage difference), but the signal is quite small and even with probes set to 1:1 you won't see much. It should make that 300kHz noise that appears to be a switching power supply artifact mostly go away.

To see if the signal is 'okay' I would first check the AVDD is stable and low noise, using the oscilloscope (one channel, GND to circuit ground and input from AVDD- if it looks good on 10:1 probe and full bandwith then switch to AC-coupled and crank up the gain, and check again with probe set to 1:1 and -necessarily- with less bandwidth). Then I would use a decent 6-1/2 digit DVM with ~100nV resolution to measure the difference between INA+ and INA-.

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