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Low voltage amplifier choice
mbless:
I need to design a low voltage amplifier and wanted to get some feedback on the amplifier choice. It will mostly be used with thermocouples but can be other low voltage sensors that produce 0-40mV. The amplifier output will be 0-5V, so a gain of approximately 120, and the bandwidth needs to be >100 kHz. I'm thinking of powering it on 4x AA (6V) to eliminate potential noise sources, so the opamps likely need to be rail-to-rail.
My current direction is to use an instrumentation amplifier (e.g. AD8421, INA821), but I'm wondering if there is a better route to go. Also, is it recommended to make your own InAmp, or is it too difficult to match the resistors and individual amps like integrated InAmps?
Wimberleytech:
AD8241 looks like a good solution. I would not roll my own.
I have used the AD624 in several projects where I had dual supplies.
Kleinstein:
The AD8421 is a nice really high performance amplifier, but with 5 V min. supply . So the common mode range is rather limited at a low supply voltage. The output is also far from Rail to Rail.
For just thermocouples the question is if one really needs an instrumentation amplifier. A simpler zero drift OP may be sufficient, though problematic with high BW. Thermocouple and 100 kHz BW does not match well.
Wimberleytech:
--- Quote from: Kleinstein on February 10, 2020, 08:10:23 am ---The AD8421 is a nice really high performance amplifier, but with 5 V min. supply . So the common mode range is rather limited at a low supply voltage. The output is also far from Rail to Rail.
For just thermocouples the question is if one really needs an instrumentation amplifier. A simpler zero drift OP may be sufficient, though problematic with high BW. Thermocouple and 100 kHz BW does not match well.
--- End quote ---
Good eye.
Analog Devices has a nice web ap to generate diamond plots for their instrumentation amplifiers
https://tinyurl.com/qlzewzd
I would suggest more voltage and configure into a split supply. In the projects I did, I used two 9V batteries (+/- 9V)
mbless:
--- Quote from: Kleinstein on February 10, 2020, 08:10:23 am ---The AD8421 is a nice really high performance amplifier, but with 5 V min. supply . So the common mode range is rather limited at a low supply voltage. The output is also far from Rail to Rail.
--- End quote ---
Oops, I did not see that... I thought of solving the common mode issue by using pull-up and pull-down on the 2 inputs to center the common mode voltage. It would also serve as open connection detection. The INA821 has rail-to-rail output, but the minimum output of 150mV is still not good enough. It looks like a dual supply is the best option.
--- Quote from: Kleinstein on February 10, 2020, 08:10:23 am ---For just thermocouples the question is if one really needs an instrumentation amplifier. A simpler zero drift OP may be sufficient, though problematic with high BW. Thermocouple and 100 kHz BW does not match well.
--- End quote ---
An InAmp may not be necessary, but I don't know what environments the device will be exposed to. Since cost isn't an issue, I thought that using a higher quality device would prevent some headaches and troubleshooting later on.
Can you expand on "Thermocouple and 100 kHz BW does not match well."? If you mean thermocouples don't need that much bandwidth, the requirement was provided by the client, so necessary or not I'll provide it if I can.
--- Quote from: Wimberleytech on February 10, 2020, 01:38:54 pm ---
Analog Devices has a nice web ap to generate diamond plots for their instrumentation amplifiers
https://tinyurl.com/qlzewzd
I would suggest more voltage and configure into a split supply. In the projects I did, I used two 9V batteries (+/- 9V)
--- End quote ---
Thanks for the link and idea. A dual supply would solve some problems.
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