For the HX711 (data sheet here) I am confused by what it expects as the input voltages. I think its this bit that is relevant:

"Channel A can be programmed with a gain of 128 or 64, corresponding to a full-scale differential input voltage of ±20mV or ±40mV respectively, when a 5V supply is connected to AVDD analog power supply pin. Channel B has a fixed gain of 32."

Does that mean that if I want a gain of 128 the +ve must be +20mv and the -ve must be -20mv? What If I am using strain gauges in a Wheatstone bridge that will change? I will be using 3V. When they say gain do they mean that my output will be (+ve)-(-ve)*128?

Thank you very much for your help

 

hi m,
I use the HX711.
The +/-mV inputs specification stated, give the maximum ADC count for that Gain setting. [ it is a 24Bit ADC]
It will work with a supply voltage of +2.7v thru +5.5v

The number of clock bits sets the Gain

Use INA inputs, leave the INB inputs unconnected.

NOTE: the HX711 expects a differential input, so INA+ and INA- should be at close to Vsupply/2

Do you follow.?
E

 

Thanks a lot

So the +/- 20mV and gain of 128 come from the full scale differential input range equation given in the data sheet?

Do you strictly have to stick to the values given in this table? For example can you use a wide range of gain values other that 128 or 64 in the equation. Or is the chip limited to just those two values? Is 128 the maximum possible gain?

What about the fact that the voltages change due to the strain gauges on the Wheatstone bridge? Have I understood correctly that the difference will always be the same because one side goes up by the same amount that the other side goes down?

What is it that tells you that (INA+) - (INA-) = Vsupply/2 ?

Thanks for your help with this, really appreciated!

 

hi m,
The Gains are fixed and the max gain is 128 on INA.
The HX datasheet states the Common mode voltage limits, normally with a bridge the common mode voltage input is approx Vsup/2.
You could get this on your project by placing a resistor in series of the same value as your strain gauge.
If your SG is 2.8 ohms, you would add a 2.8 ohm resistor in series.
That would set the junction voltage at approx Vsup/2., connected to INA+
The other half of the bridge could be two 1k resistors in series, connected to INA-

Do you want a diagram.
E

 

Thanks, that would be really helpful. Think I'm starting to get it now, thanks so much for your help. Was totally stuck. The gain is set by the PD_SCK Pulses right?

By common mode voltage do they mean that the difference between the -ve of the supply voltage and INA+ and also INA- must both be between AGND+1.2 & AVDD-1.3 V?

Can I just hook straight up to the input pins of the chip or do I need all the resistors and capacitors that are included on the board. Do you know where I can find out what they are doing?

 

hi,
For the Common mode voltage, consider a full Wheatstone bridge where the bridge is balanced, ie: no loading.
If the bridge excitation was say 3v, then both the +Sig and -Sig outputs to the amplifier would be at 3/2 = 1.5v, thats the common mode voltage.
This is in range permitted by the datasheet AGND+1.2 & AVDD-1.3 V.

Consider a bridge like yours with a 2.8ohm resistor connected to say a high side 100ohm resistor, that would make the junction of the resistors ~0.048v above ground,which is outside the CMV range.

So in order to meet the CMV limits of +1.2v thru 1.7v, your 2.8ohm strain gauge, will need a 2.8ohm in series, giving +1.5v
For the other half of the bridge I would use two 1K's, to give +1.5v.

I guess you realise that you require a micro-controller and program to drive the HX711 and also some way of displaying the result.?

If you intend power switching the HX, check the datasheet set up times and the output data rates.

Can I just hook straight up to the input pins of the chip or do I need all the resistors and capacitors that are included on the board

The HX771 I use are already mounted on a small PCB with all the supporting components.

E

EDIT:
If you buy the complete HX711 PCB assembly, the onboard transistor Q2, will not be able to supply the bridge excitation current required for your very low resistance bridge.