HX711 query (Amp/ADC for load cells)

[volican]

For the HX711 (data sheet here) I am a bit 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? FYI for my project I plan to use a 3V button cell. What impact will that have? Just to confirm when they say gain do they mean that my output will be (+ve)-(-ve)*128?

Thank you very much for your help :)

 

[lewando]

Does that mean that if I want a gain of 128 the +ve must be +20mv and the -ve must be -20mv?

The first of the "Key Electrical Characteristics", the full scale differential input range, needs to be followed. If exceeded, clipping of the input signal or physical damage could occur.

According to this line of the datasheet, for AVDD of 5 V and gain of 128, this results in +/- 20 mV. Take the +20 mV case for example, this is described as being equal to "V(inp) - V(inn)". This puts V(inp) at 10 mV and V(inn) at -10 mV.

You can experiment with this device by applying this differential voltage (with AVDD = 5 V and gain = 128) and verify that you indeed get a full scale ADC result.

 

[volican]

Thanks. Do the gain values have to stay as 128 or 64 or can they be a wide range of values. Could I just rearrange that formula to find what the gain will be? Does the HX711 work like that or is it saying that the gain can only be 128 or 64 and this is a limitation that the rest of the circuit will have to accommodate?
Many thanks for your help, that really helped me out. Was staring at it for hours.

 

[lewando]

Looks like the gain on channel A can only be 128 or 64. Page 5 of the referenced datasheet shows how to do this.

 

[volican]

Thanks a lot. Really appreciated.