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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,

    As I check single supply op-amps, their spec are not good as dual supply op-amps right? Or they are more expensive compared to dual supplies. But with this design only 1 voltage source is necessary.

    Speaking very broadly, you can divide our in amp portfolio into two types of technologies:

    - BJT based inamps:  these parts can operate on wide supplies (+/-15).  Depending on the design chosen, they may or may not work well on single supply.    BJT based in amps have excellent noise vs. supply current and bandwidth vs. supply current tradeoffs.  Examples of these parts are the AD620, AD8221, AD8226, and AD8429.

    - CMOS based inamps:  These parts are typically limited to single supply operation.  These designs typically use autozero/chopping technology to cancel out offset error, which gives them excellent offset and offset drift specifications.  Examples of these parts are the AD8553, AD8293, and AD8231.

    While prices can vary considerably in both categories, on average CMOS parts are typically less expensive than BJT based designs.

    What do you think of about my approach basically.

    Alternative 1

    Using 2 power sources +/- 10V and +5V. Feeding Bridge with 10V and op-amps with +/-10V. Using 5 volts to power DAC. Use +/-10V input type ADC.

    Alternative 2

    Using 5V only. Excite bridge with 5V. Use single supply instrumentation amplifier rail to rail type ( ad8553 ). Again rail to rail type op-amps. and DAC.

    If you can use a 5V design, you won't have to have so many power supplies and may make for a simpler design.  But you are correct that powering the bridge from a wider supply will generate a larger signal, and therefore lesson the requirements on the signal conditioning.

    I don't know which errors in your system will dominate, so I can't offer you much advice on the route to choose.   For example I don't know if the errors are dominated by the mechanical aspects of the system and the load cell, or whether the signal conditioning circuitry plays a significant role.  Assuming you had a perfect mechanical system and low bandwidth, alternative 1 might be better because autozero/chopper amps can have very low offset drift, even when measuring smaller signals.  If you bandwidth is high, alternative 2 might be better because at some point you will want the better noise performance that a BJT part (along with the larger voltage out of the bridge) can provide.

    If you do end up using a +/-10V power supply for the amplifiers, you can still use a 5V ADC if you wish.  We have parts like the AD8275 and AD8475 that are designed to convert large industrial signals into the range of modern single supply ADCs.

    Also for -10V, it is hard to make low noise low drift voltage sources. I have used lm317 before, but you set voltages with resistors, typical drift for such an source is 50ppm. Other alternative is using ref02 and boost my current with high current op-amps. But it sounds complicated a lot of components to use. I could not choose which one to try.

    Often instead of making a perfect supply, you can deal with variations in the supply by driving your ADC reference with a voltage that varies with the supply.  For example, you could use the AD8275 to sense the 20V supply across the bridge.  The AD8275 would attenuate this voltage to 4V, which you could then use as a reference for your ADC.

    As an example, let's say your power supply voltage dropped by by 5%.  This means the voltage that your in amp reads from the bridge will also drop by 5%, and this error will eventually get read by the ADC.  However since the ADC's reference voltage also dropped by 5%, the digital value coming out of the ADC will remain the same.

    You may also find this article or this reference helpful.

    Matt

Reply
  • Hi DC,

    As I check single supply op-amps, their spec are not good as dual supply op-amps right? Or they are more expensive compared to dual supplies. But with this design only 1 voltage source is necessary.

    Speaking very broadly, you can divide our in amp portfolio into two types of technologies:

    - BJT based inamps:  these parts can operate on wide supplies (+/-15).  Depending on the design chosen, they may or may not work well on single supply.    BJT based in amps have excellent noise vs. supply current and bandwidth vs. supply current tradeoffs.  Examples of these parts are the AD620, AD8221, AD8226, and AD8429.

    - CMOS based inamps:  These parts are typically limited to single supply operation.  These designs typically use autozero/chopping technology to cancel out offset error, which gives them excellent offset and offset drift specifications.  Examples of these parts are the AD8553, AD8293, and AD8231.

    While prices can vary considerably in both categories, on average CMOS parts are typically less expensive than BJT based designs.

    What do you think of about my approach basically.

    Alternative 1

    Using 2 power sources +/- 10V and +5V. Feeding Bridge with 10V and op-amps with +/-10V. Using 5 volts to power DAC. Use +/-10V input type ADC.

    Alternative 2

    Using 5V only. Excite bridge with 5V. Use single supply instrumentation amplifier rail to rail type ( ad8553 ). Again rail to rail type op-amps. and DAC.

    If you can use a 5V design, you won't have to have so many power supplies and may make for a simpler design.  But you are correct that powering the bridge from a wider supply will generate a larger signal, and therefore lesson the requirements on the signal conditioning.

    I don't know which errors in your system will dominate, so I can't offer you much advice on the route to choose.   For example I don't know if the errors are dominated by the mechanical aspects of the system and the load cell, or whether the signal conditioning circuitry plays a significant role.  Assuming you had a perfect mechanical system and low bandwidth, alternative 1 might be better because autozero/chopper amps can have very low offset drift, even when measuring smaller signals.  If you bandwidth is high, alternative 2 might be better because at some point you will want the better noise performance that a BJT part (along with the larger voltage out of the bridge) can provide.

    If you do end up using a +/-10V power supply for the amplifiers, you can still use a 5V ADC if you wish.  We have parts like the AD8275 and AD8475 that are designed to convert large industrial signals into the range of modern single supply ADCs.

    Also for -10V, it is hard to make low noise low drift voltage sources. I have used lm317 before, but you set voltages with resistors, typical drift for such an source is 50ppm. Other alternative is using ref02 and boost my current with high current op-amps. But it sounds complicated a lot of components to use. I could not choose which one to try.

    Often instead of making a perfect supply, you can deal with variations in the supply by driving your ADC reference with a voltage that varies with the supply.  For example, you could use the AD8275 to sense the 20V supply across the bridge.  The AD8275 would attenuate this voltage to 4V, which you could then use as a reference for your ADC.

    As an example, let's say your power supply voltage dropped by by 5%.  This means the voltage that your in amp reads from the bridge will also drop by 5%, and this error will eventually get read by the ADC.  However since the ADC's reference voltage also dropped by 5%, the digital value coming out of the ADC will remain the same.

    You may also find this article or this reference helpful.

    Matt

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