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Using AD627 with gain > 1000

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mzoller

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Hi,
We are using AD627 amplifiers for force sensors (Wheatstone resistor bridge). In some cases we would like to exceed a gain of 1000, which is the maximum gain according to the amplifier's data sheet.
I have spent a long time trying to find an alternative part with higher maximum gain, but the only other suitable amp that I can find is the INA122 (max gain 10'000), which has considerably worse CMRR.
The amplifier must work on +9V single supply and provide a rail-to-rail output signal - ie. the highest output voltage should be less than 200 mV below supply.

Since there doesn't seem to be a better amplifier, I wonder what could happen if the specified maximum gain is exceeded on the AD627.
From the simplified schematic in the datasheet it doesn't seem like there would be a risk of damaging the device with a lower Rgain, or is there?
And yet, usually datasheet specs are there for a reason, so I'm wondering what it might be in this case...

I have tested the device with a gain of 2000, using 1 uF buffer capacitors at Vcc and the output (pins 7 and 6). It seems to work well, and the drive strength of the output is still good at gain 2000: with an 1kOhm load resistor, the output voltage drops from 9V to 8.2V, at a current of 8.29mA. For comparison, the same test with the INA122 makes the output voltage drop to 2.42 Volts, and I measure only 2.45mA current. Of course, in the final application, the amplifier only needs to drive a high-impedance load.

Any inputs on what problems to expect or what other scenarios to test will be greatly appreciated!


Thanks,
Martin
 

I won't expect principle problem when operating AD627 at higher gain, except reduced gain accuracy and bandwidth.

I wonder if a "zero drift" (autozero) instrumentation amplifier isn't better suited for your application.
 

Try it at the gain you want and see what happens.

Alternately why not just add a rail-rail op amp gain stage at the output of the AD627?
 

There should be no functional problems. The external R represents negligible loading because of the internal resistors.

The problem is reduced bandwidth and increased impact of offset. I see input offset max of 200mv, that becomes 0.4V at gain of 2000.

A common solution is to have multiple stages. It preserves bandwidth and gives an opportunity for intermediate offset measurement/cancellation.
 

Thank you all for the helpful replies.
It makes sense that the gain limitation would be due to offset and/or bandwidth becoming an issue at high gains.
Low bandwidth MAY be a problem for us in some applications, but offset not so much, as we are calibrating each sensor to a defined offset voltage at the amplifier's output.
 

Low bandwidth MAY be a problem for us in some applications
That's why gain is distributed over more than one stage since, with lower stage gain, the bandwidth of each stage can be higher.
 

Thank you all for the helpful replies.
It makes sense that the gain limitation would be due to offset and/or bandwidth becoming an issue at high gains.
Low bandwidth MAY be a problem for us in some applications, but offset not so much, as we are calibrating each sensor to a defined offset voltage at the amplifier's output.

In extreme cases offset will start eating dynamic range. If you're running on +/-5V supplies and have to cancel 2V of offset at the output you've used up a big chunk of your dynamic range.
 

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