Load Cell Amplifier AD620

Question

I know AD620 is an old part but it is available at my place. I intend to develop a Load Cell amplifier. Load cell is 2mV/V. I am planing to use AD620 for it. Following are my calculations:

• Excitation Voltage for Load Cell = 5V
• Full Scale for load of 140 kg = 2mV X 5V = 10mV
• Gain = Vout/Vin = 5000mV/10mv = 500
• From formula of Rg we calculate it to be 98.99 Ohm

I have worked on Following schematic:

What is the problem?

1. If I leave the pin 5 open then Min Voltage is 1.9 and max is 4.2 volts. If pin5 is grounded then min voltage is 0.6 and max is 1.9 (strange)
2. the max voltage (4V) is reached at 6 mv Input. even gain calculation is done. But we have added 100 Ohm resistance instead of 98.99 Ohm.
3. Is there any difference between AD620AN, AD620AR and AD620ANZ ?
4. In some circuits i see that at sense + and sense - sometimes before feeding these to operational amplifiers some resistance is added. what is the purpose of adding these keeping in mind the voltage is already in mV
5. Can you suggest load cell amplifier other than INA125 because that is not available

Answer 1

As on any instrumentation amplifier (INA), of which the AD620 is an example, you can't just leave the REF pin (pin 5) floating. The output voltage is relative to REF, and you need to measure the difference in voltage between the two pins.

Also, although the AD620 can operate at low voltage (5V supply), it is not a rail-to-rail device, either on input or output. The +IN, –IN, REF and OUTPUT pins all need to be away from the supply rails by anywhere from 1.1 to 1.9 volts. In other words, you won't get a 5V output swing from a device powered with 5V.

Look at Figure 38 in the datasheet (I added a link to your question). It shows one recommended way to connect the output of the AD620 directly to an ADC. Note that the voltage divider establishes a 1V usable output range while using a power supply of 5V.

Answer 2

Dave covered the main part of your question, so I'll just answer some of your more minor questions:

Is there any difference between AD620AN, AD620AR and AD620ANZ ?

This is answered on one of the last pages of the datasheet:

So an "AN" part is a DIP, an "AR" is an SOIC (surface-mount part), and a part with a "Z" is RoHS-compliant.

In some circuits i see that at sense + and sense - sometimes before feeding these to operational amplifiers some resistance is added. what is the purpose of adding these keeping in mind the voltage is already in mV

You might be thinking of some of the other TI parts with "INA" part numbers. If I recall correctly, they use this prefix for high-side current sense amplifiers as well as straight-ahead instrumentation amps. In that case, the resistors allow the sensed voltage to be outside the supply rails of the amplifier itself.

Here's an example application for the INA210: