HX711 query (Amp/ADC for load cells)

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Discussion Overview

The discussion revolves around the HX711, a device used for amplifying signals from load cells, particularly in the context of understanding its input voltage requirements and gain settings. Participants explore the implications of using different gain settings, the impact of supply voltage, and the behavior of the device in relation to strain gauges in a Wheatstone bridge configuration.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether a gain of 128 means the positive input must be +20mV and the negative input -20mV, expressing concern about varying inputs from strain gauges.
  • Another participant clarifies that the full-scale differential input range must be adhered to, warning that exceeding this could lead to clipping or damage, and provides an example of how to interpret the input voltages based on the datasheet.
  • A participant inquires about the flexibility of the gain values, asking if they can be varied beyond the specified 128 or 64, and whether the HX711 operates under a fixed gain limitation.
  • It is noted that the gain on channel A is indeed limited to 128 or 64, as referenced from the datasheet.

Areas of Agreement / Disagreement

Participants generally agree on the fixed nature of the gain settings for the HX711, but there remains uncertainty regarding the implications of input voltage variations and the specific configurations of strain gauges.

Contextual Notes

Participants express uncertainty about how the gain settings interact with varying input conditions and the implications of using a supply voltage different from the specified 5V, particularly with a 3V button cell.

Who May Find This Useful

This discussion may be useful for individuals working with load cells, strain gauges, or those interested in the technical specifications and operational limits of the HX711 ADC.

volican
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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 :)
 
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volican said:
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.
 
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.
 
Looks like the gain on channel A can only be 128 or 64. Page 5 of the referenced datasheet shows how to do this.
 
Thanks a lot. Really appreciated.
 

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