Strain gauge measurement using ADS1234

Canfer,

You can make any type of measurement with the ADS1234 as long as the input to the ADS1234 is within the analog input voltage range of the device.  This is shown in the Electrical Characteristics table on page 3 of the datasheet under the Parameter Common-Mode Input Range.  If you use a gain of 1 or 2 your input voltage can range from AVSS-0.1V up to AVDD+0.1V.  For higher gains the input voltage must be greater than AVSS+1.5 and less than AVDD-1.5V.  Of course the difference between AIN+ and AIN- must be within the measurement range relative to VREF and gain used.

For example, if AVDD is connected to +5V and AVSS is connected to GND, then when using gain of 128, the inputs must be within 1.5V and 3.5V.

Best regards,

Bob B

Hi Bob, 

Thanks for the reply. You said "For example, if AVDD is connected to +5V and AVSS is connected to GND, then when using gain of 128, the inputs must be within 1.5V and 3.5V." When I look at the weigh scale example at page 25 of the datasheet, the analog input range is very small because AINP1 and AINN1 are directly connected from wheatstone bridge. I guess the voltage difference between those pins is a few uVolts, right? How can ADS1234 read that differential voltage if analog input ranges are in 1.5 V and 3.5 V? 

Please forgive me if my question is basic but I confused. I purchased ten ADS1234's to use in a project but I can't go any further. 

Thanks in advance,

Best regards,

Canfer

Hi Bob, 

Firstly, I want to thank you for your detailed explanation. 

How can I find the full scale range? When I use a loadcell, the sensitivity of the loadcell is told in the datasheet like 2 mV/V. Let's say the excitation voltage value is 5V, then the full scale output value will be 10mV. But If use a single strain gage, there isn't any information about sensitivity such as ** mV/V. In this condition, I can't find a way to find the full scale value. 

Also I know the importance of the common mode voltage. If the amplifier is not rail to rail, the output can't swing close to Vs+ and Vs-. 

The thing that I'm having so much trouble to understand is that even if the gain is 128, differential voltage value (the potential difference between AINP1 and AINN1) will not be in the common mode voltage range which is 1.5 V and 3.5 V. I guess I don't know something which is very basic. 

Thanks in advance,

Best regards,

Hi Canfer,

Can you give me the manufacturer's information and model of the strain gauge you are using?  Is the exact structure of your setup the one given previously as in the 1/4 wave bridge and 120 ohm resistors?  If it is configured differently can you tell me how you are connecting it together?  Have you created a schematic?  If so, can you send it to me?

Thanks,

Bob B

Hi Bob, 

I'm sending three bridge configuration that I've made. I haven't been able to create a PCB or check again the circuit yet but I'm planning to go with this way. I thought that the voltage difference is always small with any of these bridge configuration. The only thing that changes is the formula as you know. I will be using the formulas in the "Bridge_Equation" file which is also attached. 

There is also a photo of the strain gage that I have. 

Thanks in advance, 

Best regards,

Canfer

5226.Strain_Measurement_Files.zip

Hi Canfer,

Thanks for the information.  You are correct that the bridge output voltage difference is small, and if all elements are about the same value for the bridge, then the balanced state of the bridge should be about 1/2 of the excitation voltage.  If the excitation voltage is the same as AVDD, then the common mode for the ADS1234 input will be 1/2 of AVDD which is ideal.

Best regards,

Bob B

Hi Bob, 

So you mean there is no problem with analog input range of these small voltages for ADS1234's AINP and AINP If I use the supply for the bridge and ADS1234, right? 

Best regards,

Canfer

Hi Bob,

Thanks for the information. I will follow the way I think due to your instructions. 

Best regards,

Canfer

Dear Bob,

I am interested in the same subject and I would like to thank you for the amazing support you provide to the designers. I am currently in the component identification stage for a similar strain gage logger, trying to find the best 24-bit ADC part for one-channel, low-SPS measurements.

In this topic, I would like to ask why  the ADS1232 part is not preferable to of the (more expensive) ADS1234 part, which has two instead of four available inputs.

Dear Canfer,

in your design, I just saw that bridge input is wrongly connected to AINP3 (pin 13) on the ADC, instead of AINP1 (pin 11).

Hi Vassilis,

The initial question was related specifically to the ADS1234, but you could also use the ADS1232.  Another option would be to use the fixed gain, single channel device which is the ADS1231.  The ADS1231 also has the benefit of a low-side switch that can be used to break the excitation to the bridge to save power when not converting.

You can take a look at the ADS1232REF or the ADS1231REF which are weigh reference designs for further information.  You may also want to take a look at the weigh scale design information found at this link as well:

http://www.ti.com/ww/en/industrial/sensors/weigh/design.html

Best regards,

Bob B

Dear Bob,

thank you very much for your support.

My concept is to design a battery-powered strain gauge/load cell logger, with wireless connectivity. As a result, I am looking for low-power consumption, yet with adequate measurement accuracy. I came across many options from various manufacturers (TI, Analog Devices etc), concluding either to ADS1232 or the newest ADS1220 part (similar price).

It's true that the ADS1232 has lower noise than ADS1220, however the latter is a winner concerning power consumption, lower minimum AVDD (e.g. suitable for a ratiometric 2.5V excitation), more gain levels and higher speed (up to 2kSPS, suitable for dynamic measurements also). I would very much appreciate your comment on the above dilemma.

Hi Vassilis,

There are always trade-offs in any design.  Some are less critical than others.  What I do is to create a couple of columns on paper and right the pros and cons of each device, and then weight them as to importance and decide based on those considerations.

Best regards,

Bob B

Dear Bob, thank you for your answer. I assume that you found my interpretation on the pros and cons of the above two devices to be correct. Just ordered a couple of ADS1220's to start with.

Hi Vassilis,

Yes, your interpretation is correct.

Best regards,

Bob B