mzoller
Newbie level 2
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
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