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ADC conversion using Instrumentation Amplifier

Hi,

I have a problem deciding which component to choose. In my application I need to measure a loadcell. Sometimes 2 loadcells parallel or 3 loadcells parallel.

Currently what I am doing is to use AD620 in order to amplify signal to +/-10V and filter it, and use a separate A/D module to make measurement. The module is expensive and I need to do it by myself.

The tricky part in my application is that there is a lot of dead weight on my loadcell. I have tried to use complete analog front end IC like ad7730 and ad7190. But with both of them results were bad.

My loadcell is 10kg generally. 2mV/V. And my maximum product weight is 1000 gr. and I need to measure 0,05gr minimum. With a complete front end IC I need to obtain 200000 noise free bits. But there is also vibration in my system and I need to make measurement in 0.3 seconds. So it is impossible for an engineer like me to design such system.

So I am thinking to use an instrumentation amplifier instead of complete front end solution. My plan is as follows;

Supply my bridge with 10V. Also use that voltage to energize AD620. There will be also -10V source. So my output can swing around +/- 9V. With 10V excitation for 0,05gr weight output will be 0,1uV. If I adjust AD620 for 4000 gain. My output change will be 0,4mV. But there is a dead weight on my loadcell so I must adjust reference pin of AD620 so that when there is no weight output will be around zero volts. In order to adjust it, I am planning to use a DAC. But most DAC work with 5V, so there must be an other voltage regulator for it. Also the output of DAC must be fed to an op-amp because I need to get voltage swing around +/-10V. Later I am planning to use an active filter to filter out vibration and than to ADC. Which has capable of accepting bipolar input voltages.

With this design The things that can go wrong for me is drift of DAC voltage. Since output of AD620 is directly related to reference pin. Noise and drift can cause problem. Also since there will be seperate 5V source, can this source cause extra noise in my system? Can there be an other way to make reference voltage for AD620. What do you think of this solution? Any suggested part numbers?

Regards,

DC

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  • Hello DC,

    I think you bring a good point about complexity vs. dynamic range.

    In the case of single supplies: keep in mind that going from +5V to +/-10V you've only increased your dynamic range by a factor of 4 or 12dB (assuming all the error sources remain constant). This is not a huge advantage, the same design techniques for the signal chain would apply regardless of your power supply choice. Also, typical linear regulators such as LM337 (I'm assuming you meant 337 not 317 that for the negative regulator) require at least 1.5V beyond the power rail to work. Do you have bipolar supplies available for this? Would you need to add a switching supply to boost the voltage rails? Depending on what supplies you have available, you can decide how much complexity, extra power dissipation, and cost is added by going this way. Some of the inamps that Matt proposed are quite inexpensive vs. the additional regulators and components, which are not free.

    In the case of dual supplies: If you're concerned about the drift of the positive or negative rails, for a regular load cell this is a DC common-mode voltage variation, which is typically rejected by the instrumentation amplifier. Even if you get 100ppm/C if you have as low as 80dB CMRR (divide CMV by 10^4 = 10,000) the variation becomes 0.01ppm/C.

    Also keep in mind that +/-10V input ADCs are harder to find. The good news is that there are alternatives to this. For the sensitivity you mentioned, you are going to need at least an 18-bit ADC, unless you consider Harry's approach. If you feel like the single supply is not the best way to go, I recommend you take a look at the following circuit note (you can use the AD7690 or any other differential input ADC if you don't need the speed):

    http://www.analog.com/en/circuits-from-the-lab/CN0180/vc.html

    If you need to reject a vibration frequency (and in the best case is periodic) you should consider a sigma-delta ADC such as AD7791 or similar. You can also do the same with a SAR, but you may need to average on the digital domain. Depending on whether or not you have this processing power, you can decide what to do. But most of the time, you'll get better performance with a sigma-delta since it will look at the signal for the longest period of time (of course, you'll pay the price in total system throughput). Since you need to look at a few load cells, you're going to need an analog multiplexer such as ADG409. One more advantage of the SAR ADC in this case will be that it would allow you to interleave samples even if you average at the end, thus increasing your system throughput. The circuit note above should be capable of switching channels with high-slew rates so it won't be an issue.

    I hope this helps,

    Gustavo

Reply
  • Hello DC,

    I think you bring a good point about complexity vs. dynamic range.

    In the case of single supplies: keep in mind that going from +5V to +/-10V you've only increased your dynamic range by a factor of 4 or 12dB (assuming all the error sources remain constant). This is not a huge advantage, the same design techniques for the signal chain would apply regardless of your power supply choice. Also, typical linear regulators such as LM337 (I'm assuming you meant 337 not 317 that for the negative regulator) require at least 1.5V beyond the power rail to work. Do you have bipolar supplies available for this? Would you need to add a switching supply to boost the voltage rails? Depending on what supplies you have available, you can decide how much complexity, extra power dissipation, and cost is added by going this way. Some of the inamps that Matt proposed are quite inexpensive vs. the additional regulators and components, which are not free.

    In the case of dual supplies: If you're concerned about the drift of the positive or negative rails, for a regular load cell this is a DC common-mode voltage variation, which is typically rejected by the instrumentation amplifier. Even if you get 100ppm/C if you have as low as 80dB CMRR (divide CMV by 10^4 = 10,000) the variation becomes 0.01ppm/C.

    Also keep in mind that +/-10V input ADCs are harder to find. The good news is that there are alternatives to this. For the sensitivity you mentioned, you are going to need at least an 18-bit ADC, unless you consider Harry's approach. If you feel like the single supply is not the best way to go, I recommend you take a look at the following circuit note (you can use the AD7690 or any other differential input ADC if you don't need the speed):

    http://www.analog.com/en/circuits-from-the-lab/CN0180/vc.html

    If you need to reject a vibration frequency (and in the best case is periodic) you should consider a sigma-delta ADC such as AD7791 or similar. You can also do the same with a SAR, but you may need to average on the digital domain. Depending on whether or not you have this processing power, you can decide what to do. But most of the time, you'll get better performance with a sigma-delta since it will look at the signal for the longest period of time (of course, you'll pay the price in total system throughput). Since you need to look at a few load cells, you're going to need an analog multiplexer such as ADG409. One more advantage of the SAR ADC in this case will be that it would allow you to interleave samples even if you average at the end, thus increasing your system throughput. The circuit note above should be capable of switching channels with high-slew rates so it won't be an issue.

    I hope this helps,

    Gustavo

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