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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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  • Hi DC,

    I doubt the drift of the DAC will be the limiting factor for your system.  The effect of the DAC drift will be divided by the gain of the instrumentation amplifier.  I pulled up an inexpensive DAC from the ADI website:  the AD5601, which has 5 uV/C typical drift.  If you divide this drift by your gain of 4000 it is a tiny tiny 1.3 nV/C.  The drift of your load cell and instrumentation amplifier will far exceed this.

    If you want better performance, I would recommend concentrating on your load cell and instrumentation amplifier.  If you are able to get a load cell with greater sensitivity (perhaps by using a load cell with a lower max weight), this would likely give you the best improvement.

    You may want to consider the following instrumentation amplifier for better performance:

    AD8221:  similar to AD620 but better drift and noise

    AD8228:  Outstanding gain drift.  Max gain of 100.

    AD8231:  Excellent offset drift and good gain drift.  Single supply.  Max gain of 128, but comes with extra op that you can use for more gain.

    AD8230: Outstanding offset drift.  10V supply.  Higher noise than other in amps on this list, so you may need a lower bandwidth filter.

    AD8293G160:  Excellent offset drift and good gain drift.  Single supply.  Fixed gain of 160.  Inexpensive.

    Matt

Reply
  • Hi DC,

    I doubt the drift of the DAC will be the limiting factor for your system.  The effect of the DAC drift will be divided by the gain of the instrumentation amplifier.  I pulled up an inexpensive DAC from the ADI website:  the AD5601, which has 5 uV/C typical drift.  If you divide this drift by your gain of 4000 it is a tiny tiny 1.3 nV/C.  The drift of your load cell and instrumentation amplifier will far exceed this.

    If you want better performance, I would recommend concentrating on your load cell and instrumentation amplifier.  If you are able to get a load cell with greater sensitivity (perhaps by using a load cell with a lower max weight), this would likely give you the best improvement.

    You may want to consider the following instrumentation amplifier for better performance:

    AD8221:  similar to AD620 but better drift and noise

    AD8228:  Outstanding gain drift.  Max gain of 100.

    AD8231:  Excellent offset drift and good gain drift.  Single supply.  Max gain of 128, but comes with extra op that you can use for more gain.

    AD8230: Outstanding offset drift.  10V supply.  Higher noise than other in amps on this list, so you may need a lower bandwidth filter.

    AD8293G160:  Excellent offset drift and good gain drift.  Single supply.  Fixed gain of 160.  Inexpensive.

    Matt

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