Electronics > Projects, Designs, and Technical Stuff
Diffrential amp design
Zero999:
What's your budget? How many are you making? Generally for small volumes, it often works out cheaper, to use more expensive components, due to reduced design and assembly times.
As I said above, the INA821 is cheaper, than the AD620 and better too.
SiliconWizard:
--- Quote from: Zero999 on January 28, 2020, 11:32:14 am ---What's your budget? How many are you making? Generally for small volumes, it often works out cheaper, to use more expensive components, due to reduced design and assembly times.
--- End quote ---
Agreed.
--- Quote from: Zero999 on January 28, 2020, 11:32:14 am ---As I said above, the INA821 is cheaper, than the AD620 and better too.
--- End quote ---
Yep, it's less than half the price, has better noise and BW specs, half the power consumption, lower input bias current... looks like a no-brainer here?
Marco:
INA821 seems like a no brainer, even with the BJT input it's not that far off from integrated JFET solutions. Cheap enough and you aren't going to do substantially better without going discrete.
ali_asadzadeh:
--- Quote ---What's your budget? How many are you making? Generally for small volumes, it often works out cheaper, to use more expensive components, due to reduced design and assembly times.
--- End quote ---
We will produce 20K units, so the lower the price the better.
Zero999:
--- Quote from: ali_asadzadeh on January 30, 2020, 06:28:36 am ---
--- Quote ---What's your budget? How many are you making? Generally for small volumes, it often works out cheaper, to use more expensive components, due to reduced design and assembly times.
--- End quote ---
We will produce 20K units, so the lower the price the better.
--- End quote ---
According to Digi-Key, the INA821 is $2.489 each, vs $5.60 for the AD620, in quantities of 2500.
https://www.digikey.com/product-detail/en/analog-devices-inc/AD620ARZ-REEL/AD620ARZ-REELTR-ND/617756
https://www.digikey.com/product-detail/en/texas-instruments/INA821IDR/296-INA821IDRTR-ND/10448327
Use the circuit I posted previously with the INA821, instead of the AD620. The op-amp doesn't have to be anything special, because the common mode rejection and raising the signal well above the op-amp's noise floor, is done by the INA821. If you have a higher voltage rail than 5V available or can live with the output being limited to +3V, the OP07 is a reasonable choice and is cheap too. The output can be helped up closer to +V by using a pull-up resistor or current source (this can be as crude as a single PNP transistor), at the cost of extra parts and current draw.
https://www.digikey.com/product-detail/en/texas-instruments/OP07CDR/296-14626-2-ND/555699
NOTE: The current source is pretty crappy, because it relies on the transistor's hFE which is not a well controlled characteristic and is temperature dependant, but that doesn't matter, since all that's required is a pull-up of around 1mA or so. I chose the BC857B over just the BC587, because its gain can only range from 180 to 460, rather than 120 to 800 and it was the cheapest PNP BJT on Digi-Key, at a cent each.
https://www.digikey.com/product-detail/en/rochester-electronics-llc/BC857B/2156-BC857B-ITTR-ND/11512248
Of course if you're buying in quantities of 20k, then you want to go directly to the manufacturer, rather than a distributor, such as Digi-Key.
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